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Biology means the study of living things, also called organisms. Living things - things like you, like cats, like trees and so on - are made up of tiny building-blocks called cells, much too small for you to see, which fit together like Lego, or like bricks in a wall. Some very small living things are made of just one cell, living on its own: these are called "single-celled organisms".
These cells, and all the fluids in your body like blood and spit, are made out of chemicals. Pure substances like water, salt, bicarbonate of soda and so on that you might find in the kitchen are chemicals, but some chemicals are much more complicated than that, and they can combine to make things which are more complicated still. Flour, for example, is made of dried, powdered cells from plants, and each of those cells is made from many different chemicals combined together. The study of the chemicals in living things is called Biochemistry.
N.B. Most of the chemicals in living things are what are called organic chemicals. They're called that because many of them are found in organisms, but what makes a chemical an organic chemical is actually the fact that it contains atoms of the element carbon, combined into molecules each containing carbon combined with other elements. An element is a pure indivisible substance, an atom is the smallest possible unit of that substance and a molecule is a slightly larger unit made up of two or more atoms linked together. In addition to organic chemicals all living things contain water, for example, and water is made up of lots and lots of tiny molecules each of which contains two atoms of an element called hydrogen and one of oxygen.
Living things need energy to keep on living: nearly all of them get this energy either from the sun or by breaking down the chemicals contained in other living things. They also need new chemicals with which to repair their own cells and grow new ones. When you eat an apple, for example, chemicals and cells in your insides "digest" the chemicals in the apple - that is, they break them down and use them for energy, or to make more cells for you so you can grow a bit bigger. The group of chemical processes which a living thing uses to feed and live and grow is called its metabolism (especially when considering how the organism processes food and other energy sources) or physiology.
Living things are able to have offspring (children, or puppies, or seeds, or eggs) - that is, to make new living things similar to themselves. This is called reproduction. [You may also hear this word used to describe modern copies of non-living things from the past, such as old paintings or antique furniture.]
In order to understand them better, we divide living things up into groups of creatures which are similar to each other and related to each other. These groups are arranged on a scale according to how similar the creatures are. This works like a house address - first there's your name, which is just for you; then the number of your house, which you share only with your family; then the name of your street, which you share with all the other people on your street; and so on right up to the name of your country, which you share with all the other people in your country.
At the house-number end of the scale for living things we have species and races. To understand these you need to know the words fertile and sterile. Fertile means able to breed (have offspring); sterile or infertile means not able to breed.
A species is a group whose members are fertile with each other (unless they have something wrong with them which stops them from breeding), and can produce fertile offspring with each other, but who can't breed with members of any other group. Members of different species which are very similar can sometimes have offspring, but their offspring will be sterile, and so the mixed family line will die out after one generation.
[A generation is a layer of reproduction in a family: you and any brothers and sisters you may have are one generation, then your parents are one generation before you, your grandparents are two generations before you and so on. Each generation "generates" the next one.]
So, cattle can breed with other cattle, and sheep can breed with other sheep, but sheep and cattle cannot breed with each other: they are different species. Horses can breed with horses, and donkeys can breed with donkeys; horses and donkeys can actually breed together as well but their offspring are nearly always sterile, although there have been a very few cases of fertile female mules [a mule is the offspring of a male donkey and a female horse; the offspring of a female donkey and a male horse is called a hinny]. Again, horses and donkeys are different species, but more closely related than sheep and cattle are.
On the other hand, although a Persian cat looks very different from a Siamese cat, they can breed together and their offspring will be fertile. Persians and Siamese are members of groups which look obviously different but which belong to the same species and can have fertile offspring together: this type of group is called a sub-species, race or breed. [Confusingly "to breed" is to have offspring, but "a breed" is a group of animals who have had their breeding directed by humans in order to make them all have a similar appearance.]
Generally-speaking sub-species or races are different-appearing groups which happen naturally within a species, while breeds are artificial races which humans make by separating animals into groups and controlling which pairs breed together, in order to make a particular characteristic more common in that group. For example, if you take a group of spaniels and encourage the ones with the longest noses and the curliest coats to have puppies together, whilst not allowing the ones with short noses and straight coats to breed at all, after several generations you get a poodle, which was once a new breed.
The word hybrid may be applied to any organism which is a cross between two obviously different types of parent, but it is usually used for crosses between species, or between sub-species so different from each other that they are almost separate species. Crosses between races or breeds rather than species are usually called cross-breeds. Hybrids between animal species are nearly always sterile, i.e. unable to have offspring, although the situation with plants is a bit fuzzier.
Note that the system of naming species is not perfect, or always consistent. In older books you may find dogs and wolves described as separate species, for example, even though they can have fully fertile puppies together and are really just races of the same species: this came about basically just because Victorian naturalists (scientists who studied nature, especially living things) didn't like to admit that their pet spaniel was really a short hairy wolf. Some sources say that lions and leopards produce fertile hybrids called leopons (other sources say that only female leopons are fertile): but even if it turns out that they can indeed produce fertile hybrids we'll probably still think of lions and leopards as separate species, because they look so different that we can't get used to the idea that they might just be extreme sub-species. And we still persist in calling Neanderthals and Denisovans distinct humanoid species more primitive than Homo sapiens, so-called "modern man", even though we now know that they interbred freely with H. sapiens and were therefore just odd racial types we don't have any more.
At the other end of the scale, equivalent to the "country" line in your address, we divide living things into three domains and then into kingdoms. The organisms in the different domains differ in very big ways in how their actual cells are made.
Two of the domains, Bacteria and Archaea, which are together called prokaryotes ("not having a cell membrane"), contain only single-celled organisms made of very simple cells: to us they mostly just look like soup and sludge, but they come in a wide range of types some of which have very different and weird metabolisms. The third domain, Eukaryota ("having a cell membrane"), is made up of organisms which have much more complicated structures called organelles inside their cells, including a cell-nucleus which contains their DNA (see below) and is separated from the rest of the cell by a very thin skin called a membrane. Modern stromatolites at low tide in Shark Bay, Australia, from Desmond Armstrong's stromatolites page Ever since the dawn of life, some prokaryotes have formed colonies called stromatolites which look like living rocks and can be fairly large. The stromatolite isn't an organism, just a group of separate cells who happen to be hanging around together. It's a kind of city, like a termite mound or a coral: it's the tiny little single cells in the stromatolite which are the organisms. Many eukaryotes, on the other hand, are "multi-celled": that is, creatures like us who are made from many tiny cells fitted together like building-blocks. All the organisms which are big enough for you to see as individuals, rather than as a mass of sludge or a rock-like, communal heap - trees, jellyfish, puffballs, your auntie, wiggly things that live under rocks - are eukaryotes. Each domain is divided into kingdoms. The living things in each kingdom differ from those in all the other kingdoms not just in shape but in very big ways to do with how their metabolism works and what they feed on. Within the domain Eukaryota the kingdoms are Plants, Animals (including us), also called Metazoa, and Fungi (mushrooms and so on), plus a jumble of simple, single-celled organisms collectively called Protists which nobody has sorted out yet. In older books you will find Algae (seaweeds, pond-scum etc.) listed as plants, but they are now considered to be protists - except for so-called "blue-green algae", which are really a type of prokaryote called cyanobacteria. Channel Wrack seaweed (Pelvetia canaliculata), a type of common brown algae, at Ferny Ness in East Lothian © M J Richardson at Geograph Algal seaweeds are considered to be colonies of single-celled protists, rather than multi-cellular organisms containing lots of cells, because the cells in a seaweed can survive on their own if they have to. Animals/Metazoa are more closely related to fungi than to plants, and include some organisms such as sponges which are on the cusp between being a "proper" multicelled organism and being a colony like a stromatolite. A sponge looks like a creature, but if you push it through a sieve and turn it into a mush of individual cells, those cells will sort themselves out and reassemble into a sponge again. This is not a million miles away from a type of fungus called a slime-mould which exists as independent single cells which occasionally come together, build themselves into a simple creature vaguely resembling a slug, crawl to a suitable location, re-assemble themselves as a basic mushroom, produce spores and then turn back into independent single cells again. The group of animals like us whose cells are fixed together in organised structures which can't just dissolve and wander off on their own is usually called Eumetazoa although, confusingly, some sources call only these fixed organisms Metazoa and consider sponges to be "animals, other than Metazoa". Plants (and some protists) live on sunlight and air and nutrients (chemicals which feed an organism and help its metabolism to work) in the soil, although a few supplement their nutrients by trapping and dissolving other organisms; animals live by eating organisms, especially plants and/or other animals; and fungi generally live by growing on plants and animals which have died and absorbing nutrients from them, although some fungi also grow on live plants and animals, such as between your toes if you don't keep them clean and dry. The branches of Biology which study plants, animals and fungi are called Botany, Zoology and Mycology. The study of tiny, usually single-celled organisms such as protists or bacteria is called Microbiology. The next stage below the kingdom is the phylum (plural: phyla). Phyla are groups of organisms which share a body-plan which is very different from anything else's body plan. For example within the Animal Kingdom the chordates - creatures like us with a spinal cord, and in nearly all cases a jointed skeleton inside - make up one phylum, and arthropods - creatures like insects and crabs with jointed skeletons on the outside - make up another phylum, and there are many other phyla for animals which don't have skeletons at all. Slow worm, from Jonas Bergsten at Wikipedia: Anguis fragilis In between the phylum and the species there are several different lines of "address" which tell you how closely different species are related. If you really want to know what all the lines of a species' address are called, there's a list at the bottom of the page, but the only other one you need to know about at this point is the genus, which is a group of species which are closely related and very similar. As well as their common name - the name which ordinary people call them - all known organisms are also given a scientific name which consists of their genus-name, with a capital letter, followed by their species name in lower-case, all in italics and usually in Latin or Greek. The scientific name of the legless lizard called a "slow worm", for example, is Anguis fragilis. The study of these addresses, of how living things can be grouped together, and the assigning of scientific names is called Taxonomy or biological classification. The cells from which living things are built contain a special, very complicated chemical called DNA (short for DeoxyriboNucleic Acid). DNA itself is made of a series of tiny little building-blocks called nucleotide bases which can be fitted together in different orders, like bits of a Meccano set, or like letters making up a word, in order to form a DNA molecule. A molecule is a little individual section of a chemical which relates to the chemical as a whole in about the same way that an individual grain of sugar relates to a bag of sugar. The order in which these "letters" are arranged in the DNA molecule acts as a blueprint for making many different versions of another similar chemical called RNA (RiboNucleic Acid). DNA stays in the centre of the cell, making RNA, and then the different shapes of RNA go to other bits of the cell and tell the cell how to arrange other chemicals in order to build more cells, and how to make the organism's metabolism work. Some very primitive single-celled organisms work directly from RNA, and don't have DNA at all. The DNA in an organism or a group of organisms can change slightly over time. Sometimes this is due to what we call mutation - little accidents which happen to the DNA when it is being copied as new cells are built, so that the copy ends up with slightly different "letters" from the original. Mutations are usually neutral or slightly harmful, but occasionally the change is for the better and produces something useful, such as resistance to a particular disease, in the same way that typing errors usually create misspellings, but occasionally change a word into another which is just as good or even better. "The waves were stinging" and "The waves were singing", for example, or "They walked together" and "They talked together", are equally sensible sentences. This 9-month-old Australian Labradoodle, a cross between a Labrador and a poodle, is a little bit like each of his parents and a little bit different from either of them - from Searchtempo at Wikipedia: Labradoodle Sometimes changes in an organism's DNA happen because bits of it get lost or mixed together differently when two organisms combine their DNA to have offspring. When an organism has different DNA from either of its parents (whether due to mutation or just because it includes DNA from the other parent), the organism itself will be slightly different, because DNA controls how the organism looks and how its metabolism works. There are failsafes built into cells which mean that under normal circumstances minor mutations may not change how the finished organism turns out, but if the organisms's situation changes - if, say, it lives in freshwater pools which are becoming salty - these failsafes cease to work and then even minor mutations produce a range of slightly different versions of the organism, giving a better chance of finding a version which can cope better with the new situation. The "letters" of DNA are usually group together into short "words" called genes which control particular characteristics, so you might have a gene for having curly hair, for example, or a tree might have a gene for having smooth bark. Each gene acts as a template for a particular shape of RNA molecule, which in turn acts as a template for the production of a particular chemical, which has an effect on how the organism turns out. These genes are linked together in long strings called chromosomes, which you can think of as like a sentence or a story, a string of gene-words acting together. When two organisms of the same species have offspring together, they mix their chromosomes and genes together and swap them around to form slightly different sentences, different arrangements of DNA "words", making an organism which gets half its characteristics from one parent, and half from the other, and so is slightly different from either of them. The study of how these characteristics are passed on is called Genetics. Four billion years ago the moon was closer to Earth, and powerful high tides weathered the rocks and washed minerals into the sea where they contributed to the development of life - from space artist Don Dixon\'s Cosmographica gallery A very long time ago, when the world was just a ball of rock which had only just cooled down and turned solid, there were no living things here. Nobody knows for sure how the first living things happened, but most scientists think that over millions of years the sun and rain and lightning cooked the chemicals in the sea until they began to form into an early type of RNA, and then into the first single-celled organisms. There are today still structures called viruses which are just a DNA or RNA molecule in a chemical shell, which can only exist by spending much of their time actually inside other organisms' cells and tricking those cells into replicating the virus's genes, and which are on the cusp between being organisms themselves, and being just complicated chemicals. Viruses probably happened when bits of more developed cells broke off and began to wander around on their own, rather than having formed at the very beginning of life, but they show how it's possible for a very complex chemical to also be a very simple organism. Over the years, certain types of organisms did better at surviving and having offspring in their particular environment (the particular bit of the world they lived in), and became more common. Those that did less well in that environment, became rarer, and many eventually died out. [The type of place where an organism lives, whether in a rain-forest or under a stone or halfway up a mountain, is called its habitat. Its environment is its habitat plus other factors such as what other things live there, what the weather is like and so on.] Over time, as the less effective organisms had fewer offspring, the more effective had more offspring and spread their genes through the group, which became better-suited to the circumstances it lived in. This is called natural selection. An organism which twenty generations ago was the most effective thing around and the best at breeding, might today be edged out by something even more efficient which came along because its parents' DNA made a new, better mix, or it had a useful new mutation. Some groups also became commoner or rarer due to good or bad luck. A group of organisms might be wiped out by a large volcanic eruption for example, or they might change to fit their environment, and then the environment itself might change so they no longer fitted it. Changes in DNA caused some groups of organisms to cease to be able to breed with their relatives. Once two groups can no longer interbreed they drift apart, because their characteristics are no longer being mingled by interbreeding. If the two groups find themselves in different situations which suit different types of metabolism or shape, natural selection causes them to become more and more different over time: this is called evolution. This is how the different levels of an organism's taxonomic "address" came about: at one end you have groups of sub-species or races which may have become distinct from each other only a few hundred generations ago, and at the other end you have the split between the oldest domains of the archaea and bacteria, which are thought to have diverged as much as three and a half billion years ago. In most cases when a new group forms, the older group it split off from is still there as well. These surviving older groups may be just as well-adapted as the new ones, but for a slightly different environment. Many of these older groups, and the new groups which evolved from them through many layers of change, are still around today. Bacteria and archaea first formed in those earliest days when the world was just bare rock and water, but if you got them all together and weighed them, the weight of all the bacteria still living in the world today would be equal to the weight of all the plants. Great Horsetail, Equisetum temateia, from Rro at Wikipedia: Equisetum Some groups which existed in the past are still hanging on but have become much less common, or are still common but much less varied than they used to be. There are only a few species of equine (horses, donkeys, zebras etc) in the world today; there are still a lot of individual animals, but far fewer different types of equine than there used to be. The same thing has happened to the primitive plants called horsetails, and to the dinosaurs, who still exist as birds but have lost all the big, four-legged forms we see in museums and picture-books. Other groups, from sub-species all the way up to whole phyla, have become extinct - that is, they have died out, and none of them are living today. All the different "taxonomic" groups - kingdoms, phyla and so on - formed in this way, by evolution. An evolutionary group consisting of a single ancestor-species and all the species which have evolved from it (like saying "Fred Bloggs, Victorian plumber, plus all his children, grandchildren etc. up to the present day" only with species instead of individuals) is called a clade, Fossil ammonites in Madagascan limestone, from Enter the Earth, a fossils-and-rocks shop in Asheville, North Carolina and so the study of how species are related to each other is called cladistics. We used to just try and work these relationships out from how similar or different organisms looked, based on studying their anatomy, that is, their visible, physical shape and internal layout: but nowadays we usually do it by examining their DNA in a laboratory. DNA analysis has shown that a lot of what we used to think about what species were related to which was wrong, and this is still being sorted out: this is why you will find that older textbooks and websites will often give you different information about kingdoms, domains etc. from the most recent ones, especially when it comes to single-celled organisms. Organisms which have lived in the past, including ones which have become extinct, are sometimes preserved in rocks and mud as stone casts called fossils: the study of fossils, and of what sort of organisms they used to be, is called Palaeontology. You will also find "Palaeo" being combined with the names of other areas of biology to indicate "in the deep past" - Palaeobotany, for example, is the study of ancient plants. Common octopus, from OpenCage at Wikimedia: Octopus vulgaris Eventually, single-celled organisms began to join together to form colonies, and then multi-celled organisms in which different cells did different jobs. An octopus, for example, isn't just a ball of cells which are all the same: some cells become eye-cells, some become suckers, some become skin or blood and so on. All these different cells start out as the same kind of cell and then special DNA codes tell them how to become different and to do different jobs. A new organism which is still inside its mother, or inside an egg, is called an embryo, and when it gets a little bigger but is still not born or hatched yet it's a foetus or [in American spelling] a fetus. The study of how DNA tells cells how to make the new organism - how to come over here and be an arm, go over there and be teeth - is called Embryology. Although all the cells in a new embryo start out the same and then differentiate (turn into different sorts of cells) as the embryo develops, both animals and plants seem to contain the remains of some bacterial cells as well, secretly living inside their own cells, and not as an infection but as part of the organism. There is a word symbiosis, or "living in sympathy with each other", which means that two different types of living things have adapted to depend on each other for help. Usually it's used to describe situations where two different animals or plants work together, such as when the African Honey Guide bird finds a hive of bees and then leads a Honey Badger (also known as a Ratel) to the hive to break it open so they can share the honey. The common lichen Parmelia saxatilis growing near Renton in West Dunbartonshire © Lairich Rig at Geograph Sometimes, though, symbiosis can happen between different types of single-celled or very simple multi-celled organisms, and the different cells can fuse together and form a type of combined organism. Lichens, for example, are really symbiotic groups of algae and fungi, working together. It appears that probably all animals, and all plants, also started out as symbiotic mixtures of this kind, in which bacteria came to live inside, and become one with, archaeon cells. Animals like us, for example, and most other eukaryote organisms, contain special, tiny, simplified cells called mitochondria which seem to have evolved from bacteria, and which are so small that they live inside the other cells which make up our bodies. It is these mitochondria which make it possible for us to use a chemical called oxygen which is in the air we breathe, and which is vital to our metabolism. Without oxygen, and without mitochondria, we could not get energy from the food we eat. In the same way, in addition to their own mitochondria plant cells also contain other, tiny symbiotic cells called chloroplasts which have evolved from bacteria, and which make it possible for plants to use the energy in sunlight. Animals also have special bacteria, more separate from us than mitochondria are but nearly as important, which live inside our guts and help us to digest food; plants often have similar bacteria which help their roots to absorb things they need from the earth. Each one of us is a city of different cells. Symbiosis isn't the only way species interact, of course. Organisms may work together but they may also feed on each other, or become parasites and live in or on each other, like tapeworms or fleas: animals which are parasites are still ultimately eating other animals or plants, because they feed on the remains of animals or plants which their host has already eaten and digested for them, either in the digestive tract or as nutrients in the host's blood. Commensals are organisms which live on or in another organism but don't actually take anything from it or give anything to it - such as the tiny jungle frogs which live inside little pools of water trapped inside flowers high up in the forest canopy. Some single-celled organisms become pathogens - things which cause infection and disease. We tend to think of bacteria especially as things which cause disease, although most of them just live in the soil, doing their own thing, and some live inside us and help our bodies to work. The way in which a group of animals interact as a group with the world around them, how they feed, how they breed, how they move about from season to season and so on, is called their ecology. For any particular species, the other species of living things around them are part of their world, their environment. To a squirrel, for example, the world is made up not just of other squirrels it can fight with or be friends with or have offspring with, but of other living things that it might eat or that might eat it, trees to climb and nest in, streams to drink from, even cars to avoid. Mexican banana bat (yes, really), Musonycteris harrisoni, pollinating a flower, from Rolex Award to Professor Rodrigo Medellin Many living things depend on each other in more complex ways than eating or being eaten. Plants, for example, usually mingle their DNA and reproduce using a powder called pollen, which carries DNA from a male plant to a female one so it can mix with her DNA and make seeds. Some plants spread their pollen on the wind, but many depend on insects or bats or hummingbirds to carry it to other plants for them. The plant produces a sweet syrup called nectar, the insect or bat or bird comes to drink the nectar and in so doing it gets pollen dusted onto its face or body, and it then unknowingly carries that pollen to the next flower it visits. Once the seeds have formed, again many plants rely on animals to spread them - often by eating a fruit, walking a long way away from the parent plant and then passing the seeds out in their droppings, which provide handy nutrients for the new plant. Animals depend on plants - even meat-eating ones like lions eat other animals which eat plants - but many plants also depend on animals. Living things affect each other's numbers in more ways than just spreading pollen. If a particular plant becomes more common, for example, then animals that eat it will be well-fed and so be able to rear more babies. Then the animals become more common, until they become so common they eat so much of the plant that it becomes rare again, and then the animals have to go back to having only a few babies. The whole complicated pattern of how the living things in an area affect each other, feeding on each other or making their nests on each other or helping each other to breed, is called the ecosystem - that is, the system of how the individual species' ecologies combine. The study of the ecosystem is itself called Ecology. Appendix: Biological classification ranks showing one example of a common animal species. The eight major ranks are Domain; Kingdom; Phylum; Class; Order; Family; Genus; Species. However, as taxonomists study DNA and learn how different groups of organisms are related and how they originally branched off from each other, they often feel the need for intermediate groups which they shoehorn in like mezzanine floors in a building, by adding terms such as "sub, "super" or "infra" to the ranks above or below them. A sample classification: Domain: Eukaryota - organisms with cell-nuclei contained in membranes Kingdom: Animalia or Metazoa - animals, i.e. multi-celled organisms which are usually self-propelled, have a fixed body plan, feed on plants and/or other animals and cannot photosynthesize (directly use sunlight for energy) the way plants and algae can Subkingdom: Eumetazoa - animals which are definitely fixed structures which cannot be puréed into individual cells and reassembled again without damage in the way that e.g. sponges can be Phylum: Chordata - animals with a spinal cord Subphylum: Vertebrata - animals with backbones and, usually, jointed internal skeletons Superclass: Gnathostomata - vertebrates which have jaws (as opposed to the Agnatha, Agnathostomata or Cyclostomata, jawless vertberates, which nowadays means hagfish and lampreys) Class: Mammalia - mammals (that is, animals which produce milk from "mammary glands" to feed their young) Infraclass: Eutheria - placental mammals (that is, mammals who carry their still-very-unformed foetuses inside a womb instead of a pouch) plus their closest fossil ancestors Subclass: Placentalia - placental mammals Magnorder: Epitheria - all placental mammals except armadillos, anteaters and tree sloths (who are called Xenarthra) Magnorder: Boreoeutheria - all Epitheria except aardvarks, elephants, manatees and their close relatives (who are called Afrotheria) Superorder: Euarchontoglires - primates (including humans), tree-shrews, colugos, rodents and lagomorphs (rabbits, hares etc.) Grandorder: Glires - rodents and lagomorphs Order: Rodentia - rodents Suborder: Myomorpha - rats, mice, gerbils, hamsters, lemmings and voles Family: Muridae - rats and mice Genus: Rattus - true rats Species: Rattus norvegicus - the common, Norway or brown rat Sub-Species: Rattus norvegicus domesticus - the pet, domestic or fancy rat Breed: Dumbo (with big, low-set ears and a wide head) Variety: Hooded or Japanese (i.e. with a white body, a solid-coloured head, neck and shoulders and a coloured stripe extending along the back) Individual: Babs Jordan
Ever since the dawn of life, some prokaryotes have formed colonies called stromatolites which look like living rocks and can be fairly large. The stromatolite isn't an organism, just a group of separate cells who happen to be hanging around together. It's a kind of city, like a termite mound or a coral: it's the tiny little single cells in the stromatolite which are the organisms.
Many eukaryotes, on the other hand, are "multi-celled": that is, creatures like us who are made from many tiny cells fitted together like building-blocks. All the organisms which are big enough for you to see as individuals, rather than as a mass of sludge or a rock-like, communal heap - trees, jellyfish, puffballs, your auntie, wiggly things that live under rocks - are eukaryotes.
Each domain is divided into kingdoms. The living things in each kingdom differ from those in all the other kingdoms not just in shape but in very big ways to do with how their metabolism works and what they feed on. Within the domain Eukaryota the kingdoms are Plants, Animals (including us), also called Metazoa, and Fungi (mushrooms and so on), plus a jumble of simple, single-celled organisms collectively called Protists which nobody has sorted out yet. In older books you will find Algae (seaweeds, pond-scum etc.) listed as plants, but they are now considered to be protists - except for so-called "blue-green algae", which are really a type of prokaryote called cyanobacteria. Channel Wrack seaweed (Pelvetia canaliculata), a type of common brown algae, at Ferny Ness in East Lothian © M J Richardson at Geograph Algal seaweeds are considered to be colonies of single-celled protists, rather than multi-cellular organisms containing lots of cells, because the cells in a seaweed can survive on their own if they have to. Animals/Metazoa are more closely related to fungi than to plants, and include some organisms such as sponges which are on the cusp between being a "proper" multicelled organism and being a colony like a stromatolite. A sponge looks like a creature, but if you push it through a sieve and turn it into a mush of individual cells, those cells will sort themselves out and reassemble into a sponge again. This is not a million miles away from a type of fungus called a slime-mould which exists as independent single cells which occasionally come together, build themselves into a simple creature vaguely resembling a slug, crawl to a suitable location, re-assemble themselves as a basic mushroom, produce spores and then turn back into independent single cells again. The group of animals like us whose cells are fixed together in organised structures which can't just dissolve and wander off on their own is usually called Eumetazoa although, confusingly, some sources call only these fixed organisms Metazoa and consider sponges to be "animals, other than Metazoa". Plants (and some protists) live on sunlight and air and nutrients (chemicals which feed an organism and help its metabolism to work) in the soil, although a few supplement their nutrients by trapping and dissolving other organisms; animals live by eating organisms, especially plants and/or other animals; and fungi generally live by growing on plants and animals which have died and absorbing nutrients from them, although some fungi also grow on live plants and animals, such as between your toes if you don't keep them clean and dry. The branches of Biology which study plants, animals and fungi are called Botany, Zoology and Mycology. The study of tiny, usually single-celled organisms such as protists or bacteria is called Microbiology. The next stage below the kingdom is the phylum (plural: phyla). Phyla are groups of organisms which share a body-plan which is very different from anything else's body plan. For example within the Animal Kingdom the chordates - creatures like us with a spinal cord, and in nearly all cases a jointed skeleton inside - make up one phylum, and arthropods - creatures like insects and crabs with jointed skeletons on the outside - make up another phylum, and there are many other phyla for animals which don't have skeletons at all. Slow worm, from Jonas Bergsten at Wikipedia: Anguis fragilis In between the phylum and the species there are several different lines of "address" which tell you how closely different species are related. If you really want to know what all the lines of a species' address are called, there's a list at the bottom of the page, but the only other one you need to know about at this point is the genus, which is a group of species which are closely related and very similar. As well as their common name - the name which ordinary people call them - all known organisms are also given a scientific name which consists of their genus-name, with a capital letter, followed by their species name in lower-case, all in italics and usually in Latin or Greek. The scientific name of the legless lizard called a "slow worm", for example, is Anguis fragilis. The study of these addresses, of how living things can be grouped together, and the assigning of scientific names is called Taxonomy or biological classification. The cells from which living things are built contain a special, very complicated chemical called DNA (short for DeoxyriboNucleic Acid). DNA itself is made of a series of tiny little building-blocks called nucleotide bases which can be fitted together in different orders, like bits of a Meccano set, or like letters making up a word, in order to form a DNA molecule. A molecule is a little individual section of a chemical which relates to the chemical as a whole in about the same way that an individual grain of sugar relates to a bag of sugar. The order in which these "letters" are arranged in the DNA molecule acts as a blueprint for making many different versions of another similar chemical called RNA (RiboNucleic Acid). DNA stays in the centre of the cell, making RNA, and then the different shapes of RNA go to other bits of the cell and tell the cell how to arrange other chemicals in order to build more cells, and how to make the organism's metabolism work. Some very primitive single-celled organisms work directly from RNA, and don't have DNA at all. The DNA in an organism or a group of organisms can change slightly over time. Sometimes this is due to what we call mutation - little accidents which happen to the DNA when it is being copied as new cells are built, so that the copy ends up with slightly different "letters" from the original. Mutations are usually neutral or slightly harmful, but occasionally the change is for the better and produces something useful, such as resistance to a particular disease, in the same way that typing errors usually create misspellings, but occasionally change a word into another which is just as good or even better. "The waves were stinging" and "The waves were singing", for example, or "They walked together" and "They talked together", are equally sensible sentences. This 9-month-old Australian Labradoodle, a cross between a Labrador and a poodle, is a little bit like each of his parents and a little bit different from either of them - from Searchtempo at Wikipedia: Labradoodle Sometimes changes in an organism's DNA happen because bits of it get lost or mixed together differently when two organisms combine their DNA to have offspring. When an organism has different DNA from either of its parents (whether due to mutation or just because it includes DNA from the other parent), the organism itself will be slightly different, because DNA controls how the organism looks and how its metabolism works. There are failsafes built into cells which mean that under normal circumstances minor mutations may not change how the finished organism turns out, but if the organisms's situation changes - if, say, it lives in freshwater pools which are becoming salty - these failsafes cease to work and then even minor mutations produce a range of slightly different versions of the organism, giving a better chance of finding a version which can cope better with the new situation. The "letters" of DNA are usually group together into short "words" called genes which control particular characteristics, so you might have a gene for having curly hair, for example, or a tree might have a gene for having smooth bark. Each gene acts as a template for a particular shape of RNA molecule, which in turn acts as a template for the production of a particular chemical, which has an effect on how the organism turns out. These genes are linked together in long strings called chromosomes, which you can think of as like a sentence or a story, a string of gene-words acting together. When two organisms of the same species have offspring together, they mix their chromosomes and genes together and swap them around to form slightly different sentences, different arrangements of DNA "words", making an organism which gets half its characteristics from one parent, and half from the other, and so is slightly different from either of them. The study of how these characteristics are passed on is called Genetics. Four billion years ago the moon was closer to Earth, and powerful high tides weathered the rocks and washed minerals into the sea where they contributed to the development of life - from space artist Don Dixon\'s Cosmographica gallery A very long time ago, when the world was just a ball of rock which had only just cooled down and turned solid, there were no living things here. Nobody knows for sure how the first living things happened, but most scientists think that over millions of years the sun and rain and lightning cooked the chemicals in the sea until they began to form into an early type of RNA, and then into the first single-celled organisms. There are today still structures called viruses which are just a DNA or RNA molecule in a chemical shell, which can only exist by spending much of their time actually inside other organisms' cells and tricking those cells into replicating the virus's genes, and which are on the cusp between being organisms themselves, and being just complicated chemicals. Viruses probably happened when bits of more developed cells broke off and began to wander around on their own, rather than having formed at the very beginning of life, but they show how it's possible for a very complex chemical to also be a very simple organism. Over the years, certain types of organisms did better at surviving and having offspring in their particular environment (the particular bit of the world they lived in), and became more common. Those that did less well in that environment, became rarer, and many eventually died out. [The type of place where an organism lives, whether in a rain-forest or under a stone or halfway up a mountain, is called its habitat. Its environment is its habitat plus other factors such as what other things live there, what the weather is like and so on.] Over time, as the less effective organisms had fewer offspring, the more effective had more offspring and spread their genes through the group, which became better-suited to the circumstances it lived in. This is called natural selection. An organism which twenty generations ago was the most effective thing around and the best at breeding, might today be edged out by something even more efficient which came along because its parents' DNA made a new, better mix, or it had a useful new mutation. Some groups also became commoner or rarer due to good or bad luck. A group of organisms might be wiped out by a large volcanic eruption for example, or they might change to fit their environment, and then the environment itself might change so they no longer fitted it. Changes in DNA caused some groups of organisms to cease to be able to breed with their relatives. Once two groups can no longer interbreed they drift apart, because their characteristics are no longer being mingled by interbreeding. If the two groups find themselves in different situations which suit different types of metabolism or shape, natural selection causes them to become more and more different over time: this is called evolution. This is how the different levels of an organism's taxonomic "address" came about: at one end you have groups of sub-species or races which may have become distinct from each other only a few hundred generations ago, and at the other end you have the split between the oldest domains of the archaea and bacteria, which are thought to have diverged as much as three and a half billion years ago. In most cases when a new group forms, the older group it split off from is still there as well. These surviving older groups may be just as well-adapted as the new ones, but for a slightly different environment. Many of these older groups, and the new groups which evolved from them through many layers of change, are still around today. Bacteria and archaea first formed in those earliest days when the world was just bare rock and water, but if you got them all together and weighed them, the weight of all the bacteria still living in the world today would be equal to the weight of all the plants. Great Horsetail, Equisetum temateia, from Rro at Wikipedia: Equisetum Some groups which existed in the past are still hanging on but have become much less common, or are still common but much less varied than they used to be. There are only a few species of equine (horses, donkeys, zebras etc) in the world today; there are still a lot of individual animals, but far fewer different types of equine than there used to be. The same thing has happened to the primitive plants called horsetails, and to the dinosaurs, who still exist as birds but have lost all the big, four-legged forms we see in museums and picture-books. Other groups, from sub-species all the way up to whole phyla, have become extinct - that is, they have died out, and none of them are living today. All the different "taxonomic" groups - kingdoms, phyla and so on - formed in this way, by evolution. An evolutionary group consisting of a single ancestor-species and all the species which have evolved from it (like saying "Fred Bloggs, Victorian plumber, plus all his children, grandchildren etc. up to the present day" only with species instead of individuals) is called a clade, Fossil ammonites in Madagascan limestone, from Enter the Earth, a fossils-and-rocks shop in Asheville, North Carolina and so the study of how species are related to each other is called cladistics. We used to just try and work these relationships out from how similar or different organisms looked, based on studying their anatomy, that is, their visible, physical shape and internal layout: but nowadays we usually do it by examining their DNA in a laboratory. DNA analysis has shown that a lot of what we used to think about what species were related to which was wrong, and this is still being sorted out: this is why you will find that older textbooks and websites will often give you different information about kingdoms, domains etc. from the most recent ones, especially when it comes to single-celled organisms. Organisms which have lived in the past, including ones which have become extinct, are sometimes preserved in rocks and mud as stone casts called fossils: the study of fossils, and of what sort of organisms they used to be, is called Palaeontology. You will also find "Palaeo" being combined with the names of other areas of biology to indicate "in the deep past" - Palaeobotany, for example, is the study of ancient plants. Common octopus, from OpenCage at Wikimedia: Octopus vulgaris Eventually, single-celled organisms began to join together to form colonies, and then multi-celled organisms in which different cells did different jobs. An octopus, for example, isn't just a ball of cells which are all the same: some cells become eye-cells, some become suckers, some become skin or blood and so on. All these different cells start out as the same kind of cell and then special DNA codes tell them how to become different and to do different jobs. A new organism which is still inside its mother, or inside an egg, is called an embryo, and when it gets a little bigger but is still not born or hatched yet it's a foetus or [in American spelling] a fetus. The study of how DNA tells cells how to make the new organism - how to come over here and be an arm, go over there and be teeth - is called Embryology. Although all the cells in a new embryo start out the same and then differentiate (turn into different sorts of cells) as the embryo develops, both animals and plants seem to contain the remains of some bacterial cells as well, secretly living inside their own cells, and not as an infection but as part of the organism. There is a word symbiosis, or "living in sympathy with each other", which means that two different types of living things have adapted to depend on each other for help. Usually it's used to describe situations where two different animals or plants work together, such as when the African Honey Guide bird finds a hive of bees and then leads a Honey Badger (also known as a Ratel) to the hive to break it open so they can share the honey. The common lichen Parmelia saxatilis growing near Renton in West Dunbartonshire © Lairich Rig at Geograph Sometimes, though, symbiosis can happen between different types of single-celled or very simple multi-celled organisms, and the different cells can fuse together and form a type of combined organism. Lichens, for example, are really symbiotic groups of algae and fungi, working together. It appears that probably all animals, and all plants, also started out as symbiotic mixtures of this kind, in which bacteria came to live inside, and become one with, archaeon cells. Animals like us, for example, and most other eukaryote organisms, contain special, tiny, simplified cells called mitochondria which seem to have evolved from bacteria, and which are so small that they live inside the other cells which make up our bodies. It is these mitochondria which make it possible for us to use a chemical called oxygen which is in the air we breathe, and which is vital to our metabolism. Without oxygen, and without mitochondria, we could not get energy from the food we eat. In the same way, in addition to their own mitochondria plant cells also contain other, tiny symbiotic cells called chloroplasts which have evolved from bacteria, and which make it possible for plants to use the energy in sunlight. Animals also have special bacteria, more separate from us than mitochondria are but nearly as important, which live inside our guts and help us to digest food; plants often have similar bacteria which help their roots to absorb things they need from the earth. Each one of us is a city of different cells. Symbiosis isn't the only way species interact, of course. Organisms may work together but they may also feed on each other, or become parasites and live in or on each other, like tapeworms or fleas: animals which are parasites are still ultimately eating other animals or plants, because they feed on the remains of animals or plants which their host has already eaten and digested for them, either in the digestive tract or as nutrients in the host's blood. Commensals are organisms which live on or in another organism but don't actually take anything from it or give anything to it - such as the tiny jungle frogs which live inside little pools of water trapped inside flowers high up in the forest canopy. Some single-celled organisms become pathogens - things which cause infection and disease. We tend to think of bacteria especially as things which cause disease, although most of them just live in the soil, doing their own thing, and some live inside us and help our bodies to work. The way in which a group of animals interact as a group with the world around them, how they feed, how they breed, how they move about from season to season and so on, is called their ecology. For any particular species, the other species of living things around them are part of their world, their environment. To a squirrel, for example, the world is made up not just of other squirrels it can fight with or be friends with or have offspring with, but of other living things that it might eat or that might eat it, trees to climb and nest in, streams to drink from, even cars to avoid. Mexican banana bat (yes, really), Musonycteris harrisoni, pollinating a flower, from Rolex Award to Professor Rodrigo Medellin Many living things depend on each other in more complex ways than eating or being eaten. Plants, for example, usually mingle their DNA and reproduce using a powder called pollen, which carries DNA from a male plant to a female one so it can mix with her DNA and make seeds. Some plants spread their pollen on the wind, but many depend on insects or bats or hummingbirds to carry it to other plants for them. The plant produces a sweet syrup called nectar, the insect or bat or bird comes to drink the nectar and in so doing it gets pollen dusted onto its face or body, and it then unknowingly carries that pollen to the next flower it visits. Once the seeds have formed, again many plants rely on animals to spread them - often by eating a fruit, walking a long way away from the parent plant and then passing the seeds out in their droppings, which provide handy nutrients for the new plant. Animals depend on plants - even meat-eating ones like lions eat other animals which eat plants - but many plants also depend on animals. Living things affect each other's numbers in more ways than just spreading pollen. If a particular plant becomes more common, for example, then animals that eat it will be well-fed and so be able to rear more babies. Then the animals become more common, until they become so common they eat so much of the plant that it becomes rare again, and then the animals have to go back to having only a few babies. The whole complicated pattern of how the living things in an area affect each other, feeding on each other or making their nests on each other or helping each other to breed, is called the ecosystem - that is, the system of how the individual species' ecologies combine. The study of the ecosystem is itself called Ecology. Appendix: Biological classification ranks showing one example of a common animal species. The eight major ranks are Domain; Kingdom; Phylum; Class; Order; Family; Genus; Species. However, as taxonomists study DNA and learn how different groups of organisms are related and how they originally branched off from each other, they often feel the need for intermediate groups which they shoehorn in like mezzanine floors in a building, by adding terms such as "sub, "super" or "infra" to the ranks above or below them. A sample classification: Domain: Eukaryota - organisms with cell-nuclei contained in membranes Kingdom: Animalia or Metazoa - animals, i.e. multi-celled organisms which are usually self-propelled, have a fixed body plan, feed on plants and/or other animals and cannot photosynthesize (directly use sunlight for energy) the way plants and algae can Subkingdom: Eumetazoa - animals which are definitely fixed structures which cannot be puréed into individual cells and reassembled again without damage in the way that e.g. sponges can be Phylum: Chordata - animals with a spinal cord Subphylum: Vertebrata - animals with backbones and, usually, jointed internal skeletons Superclass: Gnathostomata - vertebrates which have jaws (as opposed to the Agnatha, Agnathostomata or Cyclostomata, jawless vertberates, which nowadays means hagfish and lampreys) Class: Mammalia - mammals (that is, animals which produce milk from "mammary glands" to feed their young) Infraclass: Eutheria - placental mammals (that is, mammals who carry their still-very-unformed foetuses inside a womb instead of a pouch) plus their closest fossil ancestors Subclass: Placentalia - placental mammals Magnorder: Epitheria - all placental mammals except armadillos, anteaters and tree sloths (who are called Xenarthra) Magnorder: Boreoeutheria - all Epitheria except aardvarks, elephants, manatees and their close relatives (who are called Afrotheria) Superorder: Euarchontoglires - primates (including humans), tree-shrews, colugos, rodents and lagomorphs (rabbits, hares etc.) Grandorder: Glires - rodents and lagomorphs Order: Rodentia - rodents Suborder: Myomorpha - rats, mice, gerbils, hamsters, lemmings and voles Family: Muridae - rats and mice Genus: Rattus - true rats Species: Rattus norvegicus - the common, Norway or brown rat Sub-Species: Rattus norvegicus domesticus - the pet, domestic or fancy rat Breed: Dumbo (with big, low-set ears and a wide head) Variety: Hooded or Japanese (i.e. with a white body, a solid-coloured head, neck and shoulders and a coloured stripe extending along the back) Individual: Babs Jordan
Animals/Metazoa are more closely related to fungi than to plants, and include some organisms such as sponges which are on the cusp between being a "proper" multicelled organism and being a colony like a stromatolite. A sponge looks like a creature, but if you push it through a sieve and turn it into a mush of individual cells, those cells will sort themselves out and reassemble into a sponge again. This is not a million miles away from a type of fungus called a slime-mould which exists as independent single cells which occasionally come together, build themselves into a simple creature vaguely resembling a slug, crawl to a suitable location, re-assemble themselves as a basic mushroom, produce spores and then turn back into independent single cells again. The group of animals like us whose cells are fixed together in organised structures which can't just dissolve and wander off on their own is usually called Eumetazoa although, confusingly, some sources call only these fixed organisms Metazoa and consider sponges to be "animals, other than Metazoa".
Plants (and some protists) live on sunlight and air and nutrients (chemicals which feed an organism and help its metabolism to work) in the soil, although a few supplement their nutrients by trapping and dissolving other organisms; animals live by eating organisms, especially plants and/or other animals; and fungi generally live by growing on plants and animals which have died and absorbing nutrients from them, although some fungi also grow on live plants and animals, such as between your toes if you don't keep them clean and dry. The branches of Biology which study plants, animals and fungi are called Botany, Zoology and Mycology. The study of tiny, usually single-celled organisms such as protists or bacteria is called Microbiology.
The next stage below the kingdom is the phylum (plural: phyla). Phyla are groups of organisms which share a body-plan which is very different from anything else's body plan. For example within the Animal Kingdom the chordates - creatures like us with a spinal cord, and in nearly all cases a jointed skeleton inside - make up one phylum, and arthropods - creatures like insects and crabs with jointed skeletons on the outside - make up another phylum, and there are many other phyla for animals which don't have skeletons at all.
In between the phylum and the species there are several different lines of "address" which tell you how closely different species are related. If you really want to know what all the lines of a species' address are called, there's a list at the bottom of the page, but the only other one you need to know about at this point is the genus, which is a group of species which are closely related and very similar. As well as their common name - the name which ordinary people call them - all known organisms are also given a scientific name which consists of their genus-name, with a capital letter, followed by their species name in lower-case, all in italics and usually in Latin or Greek. The scientific name of the legless lizard called a "slow worm", for example, is Anguis fragilis. The study of these addresses, of how living things can be grouped together, and the assigning of scientific names is called Taxonomy or biological classification.
The cells from which living things are built contain a special, very complicated chemical called DNA (short for DeoxyriboNucleic Acid). DNA itself is made of a series of tiny little building-blocks called nucleotide bases which can be fitted together in different orders, like bits of a Meccano set, or like letters making up a word, in order to form a DNA molecule. A molecule is a little individual section of a chemical which relates to the chemical as a whole in about the same way that an individual grain of sugar relates to a bag of sugar.
The order in which these "letters" are arranged in the DNA molecule acts as a blueprint for making many different versions of another similar chemical called RNA (RiboNucleic Acid). DNA stays in the centre of the cell, making RNA, and then the different shapes of RNA go to other bits of the cell and tell the cell how to arrange other chemicals in order to build more cells, and how to make the organism's metabolism work. Some very primitive single-celled organisms work directly from RNA, and don't have DNA at all.
The DNA in an organism or a group of organisms can change slightly over time. Sometimes this is due to what we call mutation - little accidents which happen to the DNA when it is being copied as new cells are built, so that the copy ends up with slightly different "letters" from the original. Mutations are usually neutral or slightly harmful, but occasionally the change is for the better and produces something useful, such as resistance to a particular disease, in the same way that typing errors usually create misspellings, but occasionally change a word into another which is just as good or even better. "The waves were stinging" and "The waves were singing", for example, or "They walked together" and "They talked together", are equally sensible sentences. This 9-month-old Australian Labradoodle, a cross between a Labrador and a poodle, is a little bit like each of his parents and a little bit different from either of them - from Searchtempo at Wikipedia: Labradoodle Sometimes changes in an organism's DNA happen because bits of it get lost or mixed together differently when two organisms combine their DNA to have offspring. When an organism has different DNA from either of its parents (whether due to mutation or just because it includes DNA from the other parent), the organism itself will be slightly different, because DNA controls how the organism looks and how its metabolism works. There are failsafes built into cells which mean that under normal circumstances minor mutations may not change how the finished organism turns out, but if the organisms's situation changes - if, say, it lives in freshwater pools which are becoming salty - these failsafes cease to work and then even minor mutations produce a range of slightly different versions of the organism, giving a better chance of finding a version which can cope better with the new situation. The "letters" of DNA are usually group together into short "words" called genes which control particular characteristics, so you might have a gene for having curly hair, for example, or a tree might have a gene for having smooth bark. Each gene acts as a template for a particular shape of RNA molecule, which in turn acts as a template for the production of a particular chemical, which has an effect on how the organism turns out. These genes are linked together in long strings called chromosomes, which you can think of as like a sentence or a story, a string of gene-words acting together. When two organisms of the same species have offspring together, they mix their chromosomes and genes together and swap them around to form slightly different sentences, different arrangements of DNA "words", making an organism which gets half its characteristics from one parent, and half from the other, and so is slightly different from either of them. The study of how these characteristics are passed on is called Genetics.
Sometimes changes in an organism's DNA happen because bits of it get lost or mixed together differently when two organisms combine their DNA to have offspring. When an organism has different DNA from either of its parents (whether due to mutation or just because it includes DNA from the other parent), the organism itself will be slightly different, because DNA controls how the organism looks and how its metabolism works. There are failsafes built into cells which mean that under normal circumstances minor mutations may not change how the finished organism turns out, but if the organisms's situation changes - if, say, it lives in freshwater pools which are becoming salty - these failsafes cease to work and then even minor mutations produce a range of slightly different versions of the organism, giving a better chance of finding a version which can cope better with the new situation.
The "letters" of DNA are usually group together into short "words" called genes which control particular characteristics, so you might have a gene for having curly hair, for example, or a tree might have a gene for having smooth bark. Each gene acts as a template for a particular shape of RNA molecule, which in turn acts as a template for the production of a particular chemical, which has an effect on how the organism turns out. These genes are linked together in long strings called chromosomes, which you can think of as like a sentence or a story, a string of gene-words acting together. When two organisms of the same species have offspring together, they mix their chromosomes and genes together and swap them around to form slightly different sentences, different arrangements of DNA "words", making an organism which gets half its characteristics from one parent, and half from the other, and so is slightly different from either of them. The study of how these characteristics are passed on is called Genetics.
A very long time ago, when the world was just a ball of rock which had only just cooled down and turned solid, there were no living things here. Nobody knows for sure how the first living things happened, but most scientists think that over millions of years the sun and rain and lightning cooked the chemicals in the sea until they began to form into an early type of RNA, and then into the first single-celled organisms.
There are today still structures called viruses which are just a DNA or RNA molecule in a chemical shell, which can only exist by spending much of their time actually inside other organisms' cells and tricking those cells into replicating the virus's genes, and which are on the cusp between being organisms themselves, and being just complicated chemicals. Viruses probably happened when bits of more developed cells broke off and began to wander around on their own, rather than having formed at the very beginning of life, but they show how it's possible for a very complex chemical to also be a very simple organism.
Over the years, certain types of organisms did better at surviving and having offspring in their particular environment (the particular bit of the world they lived in), and became more common. Those that did less well in that environment, became rarer, and many eventually died out. [The type of place where an organism lives, whether in a rain-forest or under a stone or halfway up a mountain, is called its habitat. Its environment is its habitat plus other factors such as what other things live there, what the weather is like and so on.]
Over time, as the less effective organisms had fewer offspring, the more effective had more offspring and spread their genes through the group, which became better-suited to the circumstances it lived in. This is called natural selection. An organism which twenty generations ago was the most effective thing around and the best at breeding, might today be edged out by something even more efficient which came along because its parents' DNA made a new, better mix, or it had a useful new mutation. Some groups also became commoner or rarer due to good or bad luck. A group of organisms might be wiped out by a large volcanic eruption for example, or they might change to fit their environment, and then the environment itself might change so they no longer fitted it.
Changes in DNA caused some groups of organisms to cease to be able to breed with their relatives. Once two groups can no longer interbreed they drift apart, because their characteristics are no longer being mingled by interbreeding. If the two groups find themselves in different situations which suit different types of metabolism or shape, natural selection causes them to become more and more different over time: this is called evolution. This is how the different levels of an organism's taxonomic "address" came about: at one end you have groups of sub-species or races which may have become distinct from each other only a few hundred generations ago, and at the other end you have the split between the oldest domains of the archaea and bacteria, which are thought to have diverged as much as three and a half billion years ago.
In most cases when a new group forms, the older group it split off from is still there as well. These surviving older groups may be just as well-adapted as the new ones, but for a slightly different environment. Many of these older groups, and the new groups which evolved from them through many layers of change, are still around today. Bacteria and archaea first formed in those earliest days when the world was just bare rock and water, but if you got them all together and weighed them, the weight of all the bacteria still living in the world today would be equal to the weight of all the plants. Great Horsetail, Equisetum temateia, from Rro at Wikipedia: Equisetum Some groups which existed in the past are still hanging on but have become much less common, or are still common but much less varied than they used to be. There are only a few species of equine (horses, donkeys, zebras etc) in the world today; there are still a lot of individual animals, but far fewer different types of equine than there used to be. The same thing has happened to the primitive plants called horsetails, and to the dinosaurs, who still exist as birds but have lost all the big, four-legged forms we see in museums and picture-books. Other groups, from sub-species all the way up to whole phyla, have become extinct - that is, they have died out, and none of them are living today. All the different "taxonomic" groups - kingdoms, phyla and so on - formed in this way, by evolution. An evolutionary group consisting of a single ancestor-species and all the species which have evolved from it (like saying "Fred Bloggs, Victorian plumber, plus all his children, grandchildren etc. up to the present day" only with species instead of individuals) is called a clade, Fossil ammonites in Madagascan limestone, from Enter the Earth, a fossils-and-rocks shop in Asheville, North Carolina and so the study of how species are related to each other is called cladistics. We used to just try and work these relationships out from how similar or different organisms looked, based on studying their anatomy, that is, their visible, physical shape and internal layout: but nowadays we usually do it by examining their DNA in a laboratory. DNA analysis has shown that a lot of what we used to think about what species were related to which was wrong, and this is still being sorted out: this is why you will find that older textbooks and websites will often give you different information about kingdoms, domains etc. from the most recent ones, especially when it comes to single-celled organisms. Organisms which have lived in the past, including ones which have become extinct, are sometimes preserved in rocks and mud as stone casts called fossils: the study of fossils, and of what sort of organisms they used to be, is called Palaeontology. You will also find "Palaeo" being combined with the names of other areas of biology to indicate "in the deep past" - Palaeobotany, for example, is the study of ancient plants. Common octopus, from OpenCage at Wikimedia: Octopus vulgaris Eventually, single-celled organisms began to join together to form colonies, and then multi-celled organisms in which different cells did different jobs. An octopus, for example, isn't just a ball of cells which are all the same: some cells become eye-cells, some become suckers, some become skin or blood and so on. All these different cells start out as the same kind of cell and then special DNA codes tell them how to become different and to do different jobs. A new organism which is still inside its mother, or inside an egg, is called an embryo, and when it gets a little bigger but is still not born or hatched yet it's a foetus or [in American spelling] a fetus. The study of how DNA tells cells how to make the new organism - how to come over here and be an arm, go over there and be teeth - is called Embryology. Although all the cells in a new embryo start out the same and then differentiate (turn into different sorts of cells) as the embryo develops, both animals and plants seem to contain the remains of some bacterial cells as well, secretly living inside their own cells, and not as an infection but as part of the organism. There is a word symbiosis, or "living in sympathy with each other", which means that two different types of living things have adapted to depend on each other for help. Usually it's used to describe situations where two different animals or plants work together, such as when the African Honey Guide bird finds a hive of bees and then leads a Honey Badger (also known as a Ratel) to the hive to break it open so they can share the honey. The common lichen Parmelia saxatilis growing near Renton in West Dunbartonshire © Lairich Rig at Geograph Sometimes, though, symbiosis can happen between different types of single-celled or very simple multi-celled organisms, and the different cells can fuse together and form a type of combined organism. Lichens, for example, are really symbiotic groups of algae and fungi, working together. It appears that probably all animals, and all plants, also started out as symbiotic mixtures of this kind, in which bacteria came to live inside, and become one with, archaeon cells. Animals like us, for example, and most other eukaryote organisms, contain special, tiny, simplified cells called mitochondria which seem to have evolved from bacteria, and which are so small that they live inside the other cells which make up our bodies. It is these mitochondria which make it possible for us to use a chemical called oxygen which is in the air we breathe, and which is vital to our metabolism. Without oxygen, and without mitochondria, we could not get energy from the food we eat. In the same way, in addition to their own mitochondria plant cells also contain other, tiny symbiotic cells called chloroplasts which have evolved from bacteria, and which make it possible for plants to use the energy in sunlight. Animals also have special bacteria, more separate from us than mitochondria are but nearly as important, which live inside our guts and help us to digest food; plants often have similar bacteria which help their roots to absorb things they need from the earth. Each one of us is a city of different cells. Symbiosis isn't the only way species interact, of course. Organisms may work together but they may also feed on each other, or become parasites and live in or on each other, like tapeworms or fleas: animals which are parasites are still ultimately eating other animals or plants, because they feed on the remains of animals or plants which their host has already eaten and digested for them, either in the digestive tract or as nutrients in the host's blood. Commensals are organisms which live on or in another organism but don't actually take anything from it or give anything to it - such as the tiny jungle frogs which live inside little pools of water trapped inside flowers high up in the forest canopy. Some single-celled organisms become pathogens - things which cause infection and disease. We tend to think of bacteria especially as things which cause disease, although most of them just live in the soil, doing their own thing, and some live inside us and help our bodies to work. The way in which a group of animals interact as a group with the world around them, how they feed, how they breed, how they move about from season to season and so on, is called their ecology. For any particular species, the other species of living things around them are part of their world, their environment. To a squirrel, for example, the world is made up not just of other squirrels it can fight with or be friends with or have offspring with, but of other living things that it might eat or that might eat it, trees to climb and nest in, streams to drink from, even cars to avoid. Mexican banana bat (yes, really), Musonycteris harrisoni, pollinating a flower, from Rolex Award to Professor Rodrigo Medellin Many living things depend on each other in more complex ways than eating or being eaten. Plants, for example, usually mingle their DNA and reproduce using a powder called pollen, which carries DNA from a male plant to a female one so it can mix with her DNA and make seeds. Some plants spread their pollen on the wind, but many depend on insects or bats or hummingbirds to carry it to other plants for them. The plant produces a sweet syrup called nectar, the insect or bat or bird comes to drink the nectar and in so doing it gets pollen dusted onto its face or body, and it then unknowingly carries that pollen to the next flower it visits. Once the seeds have formed, again many plants rely on animals to spread them - often by eating a fruit, walking a long way away from the parent plant and then passing the seeds out in their droppings, which provide handy nutrients for the new plant. Animals depend on plants - even meat-eating ones like lions eat other animals which eat plants - but many plants also depend on animals. Living things affect each other's numbers in more ways than just spreading pollen. If a particular plant becomes more common, for example, then animals that eat it will be well-fed and so be able to rear more babies. Then the animals become more common, until they become so common they eat so much of the plant that it becomes rare again, and then the animals have to go back to having only a few babies. The whole complicated pattern of how the living things in an area affect each other, feeding on each other or making their nests on each other or helping each other to breed, is called the ecosystem - that is, the system of how the individual species' ecologies combine. The study of the ecosystem is itself called Ecology. Appendix: Biological classification ranks showing one example of a common animal species. The eight major ranks are Domain; Kingdom; Phylum; Class; Order; Family; Genus; Species. However, as taxonomists study DNA and learn how different groups of organisms are related and how they originally branched off from each other, they often feel the need for intermediate groups which they shoehorn in like mezzanine floors in a building, by adding terms such as "sub, "super" or "infra" to the ranks above or below them. A sample classification: Domain: Eukaryota - organisms with cell-nuclei contained in membranes Kingdom: Animalia or Metazoa - animals, i.e. multi-celled organisms which are usually self-propelled, have a fixed body plan, feed on plants and/or other animals and cannot photosynthesize (directly use sunlight for energy) the way plants and algae can Subkingdom: Eumetazoa - animals which are definitely fixed structures which cannot be puréed into individual cells and reassembled again without damage in the way that e.g. sponges can be Phylum: Chordata - animals with a spinal cord Subphylum: Vertebrata - animals with backbones and, usually, jointed internal skeletons Superclass: Gnathostomata - vertebrates which have jaws (as opposed to the Agnatha, Agnathostomata or Cyclostomata, jawless vertberates, which nowadays means hagfish and lampreys) Class: Mammalia - mammals (that is, animals which produce milk from "mammary glands" to feed their young) Infraclass: Eutheria - placental mammals (that is, mammals who carry their still-very-unformed foetuses inside a womb instead of a pouch) plus their closest fossil ancestors Subclass: Placentalia - placental mammals Magnorder: Epitheria - all placental mammals except armadillos, anteaters and tree sloths (who are called Xenarthra) Magnorder: Boreoeutheria - all Epitheria except aardvarks, elephants, manatees and their close relatives (who are called Afrotheria) Superorder: Euarchontoglires - primates (including humans), tree-shrews, colugos, rodents and lagomorphs (rabbits, hares etc.) Grandorder: Glires - rodents and lagomorphs Order: Rodentia - rodents Suborder: Myomorpha - rats, mice, gerbils, hamsters, lemmings and voles Family: Muridae - rats and mice Genus: Rattus - true rats Species: Rattus norvegicus - the common, Norway or brown rat Sub-Species: Rattus norvegicus domesticus - the pet, domestic or fancy rat Breed: Dumbo (with big, low-set ears and a wide head) Variety: Hooded or Japanese (i.e. with a white body, a solid-coloured head, neck and shoulders and a coloured stripe extending along the back) Individual: Babs Jordan
Some groups which existed in the past are still hanging on but have become much less common, or are still common but much less varied than they used to be. There are only a few species of equine (horses, donkeys, zebras etc) in the world today; there are still a lot of individual animals, but far fewer different types of equine than there used to be. The same thing has happened to the primitive plants called horsetails, and to the dinosaurs, who still exist as birds but have lost all the big, four-legged forms we see in museums and picture-books. Other groups, from sub-species all the way up to whole phyla, have become extinct - that is, they have died out, and none of them are living today.
All the different "taxonomic" groups - kingdoms, phyla and so on - formed in this way, by evolution. An evolutionary group consisting of a single ancestor-species and all the species which have evolved from it (like saying "Fred Bloggs, Victorian plumber, plus all his children, grandchildren etc. up to the present day" only with species instead of individuals) is called a clade, Fossil ammonites in Madagascan limestone, from Enter the Earth, a fossils-and-rocks shop in Asheville, North Carolina and so the study of how species are related to each other is called cladistics. We used to just try and work these relationships out from how similar or different organisms looked, based on studying their anatomy, that is, their visible, physical shape and internal layout: but nowadays we usually do it by examining their DNA in a laboratory. DNA analysis has shown that a lot of what we used to think about what species were related to which was wrong, and this is still being sorted out: this is why you will find that older textbooks and websites will often give you different information about kingdoms, domains etc. from the most recent ones, especially when it comes to single-celled organisms. Organisms which have lived in the past, including ones which have become extinct, are sometimes preserved in rocks and mud as stone casts called fossils: the study of fossils, and of what sort of organisms they used to be, is called Palaeontology. You will also find "Palaeo" being combined with the names of other areas of biology to indicate "in the deep past" - Palaeobotany, for example, is the study of ancient plants.
We used to just try and work these relationships out from how similar or different organisms looked, based on studying their anatomy, that is, their visible, physical shape and internal layout: but nowadays we usually do it by examining their DNA in a laboratory. DNA analysis has shown that a lot of what we used to think about what species were related to which was wrong, and this is still being sorted out: this is why you will find that older textbooks and websites will often give you different information about kingdoms, domains etc. from the most recent ones, especially when it comes to single-celled organisms.
Organisms which have lived in the past, including ones which have become extinct, are sometimes preserved in rocks and mud as stone casts called fossils: the study of fossils, and of what sort of organisms they used to be, is called Palaeontology. You will also find "Palaeo" being combined with the names of other areas of biology to indicate "in the deep past" - Palaeobotany, for example, is the study of ancient plants.
Eventually, single-celled organisms began to join together to form colonies, and then multi-celled organisms in which different cells did different jobs. An octopus, for example, isn't just a ball of cells which are all the same: some cells become eye-cells, some become suckers, some become skin or blood and so on. All these different cells start out as the same kind of cell and then special DNA codes tell them how to become different and to do different jobs. A new organism which is still inside its mother, or inside an egg, is called an embryo, and when it gets a little bigger but is still not born or hatched yet it's a foetus or [in American spelling] a fetus. The study of how DNA tells cells how to make the new organism - how to come over here and be an arm, go over there and be teeth - is called Embryology.
Although all the cells in a new embryo start out the same and then differentiate (turn into different sorts of cells) as the embryo develops, both animals and plants seem to contain the remains of some bacterial cells as well, secretly living inside their own cells, and not as an infection but as part of the organism.
There is a word symbiosis, or "living in sympathy with each other", which means that two different types of living things have adapted to depend on each other for help. Usually it's used to describe situations where two different animals or plants work together, such as when the African Honey Guide bird finds a hive of bees and then leads a Honey Badger (also known as a Ratel) to the hive to break it open so they can share the honey.
Sometimes, though, symbiosis can happen between different types of single-celled or very simple multi-celled organisms, and the different cells can fuse together and form a type of combined organism. Lichens, for example, are really symbiotic groups of algae and fungi, working together. It appears that probably all animals, and all plants, also started out as symbiotic mixtures of this kind, in which bacteria came to live inside, and become one with, archaeon cells.
Animals like us, for example, and most other eukaryote organisms, contain special, tiny, simplified cells called mitochondria which seem to have evolved from bacteria, and which are so small that they live inside the other cells which make up our bodies. It is these mitochondria which make it possible for us to use a chemical called oxygen which is in the air we breathe, and which is vital to our metabolism. Without oxygen, and without mitochondria, we could not get energy from the food we eat.
In the same way, in addition to their own mitochondria plant cells also contain other, tiny symbiotic cells called chloroplasts which have evolved from bacteria, and which make it possible for plants to use the energy in sunlight. Animals also have special bacteria, more separate from us than mitochondria are but nearly as important, which live inside our guts and help us to digest food; plants often have similar bacteria which help their roots to absorb things they need from the earth. Each one of us is a city of different cells.
Symbiosis isn't the only way species interact, of course. Organisms may work together but they may also feed on each other, or become parasites and live in or on each other, like tapeworms or fleas: animals which are parasites are still ultimately eating other animals or plants, because they feed on the remains of animals or plants which their host has already eaten and digested for them, either in the digestive tract or as nutrients in the host's blood. Commensals are organisms which live on or in another organism but don't actually take anything from it or give anything to it - such as the tiny jungle frogs which live inside little pools of water trapped inside flowers high up in the forest canopy. Some single-celled organisms become pathogens - things which cause infection and disease. We tend to think of bacteria especially as things which cause disease, although most of them just live in the soil, doing their own thing, and some live inside us and help our bodies to work.
The way in which a group of animals interact as a group with the world around them, how they feed, how they breed, how they move about from season to season and so on, is called their ecology.
For any particular species, the other species of living things around them are part of their world, their environment. To a squirrel, for example, the world is made up not just of other squirrels it can fight with or be friends with or have offspring with, but of other living things that it might eat or that might eat it, trees to climb and nest in, streams to drink from, even cars to avoid.
Many living things depend on each other in more complex ways than eating or being eaten. Plants, for example, usually mingle their DNA and reproduce using a powder called pollen, which carries DNA from a male plant to a female one so it can mix with her DNA and make seeds. Some plants spread their pollen on the wind, but many depend on insects or bats or hummingbirds to carry it to other plants for them. The plant produces a sweet syrup called nectar, the insect or bat or bird comes to drink the nectar and in so doing it gets pollen dusted onto its face or body, and it then unknowingly carries that pollen to the next flower it visits. Once the seeds have formed, again many plants rely on animals to spread them - often by eating a fruit, walking a long way away from the parent plant and then passing the seeds out in their droppings, which provide handy nutrients for the new plant. Animals depend on plants - even meat-eating ones like lions eat other animals which eat plants - but many plants also depend on animals.
Living things affect each other's numbers in more ways than just spreading pollen. If a particular plant becomes more common, for example, then animals that eat it will be well-fed and so be able to rear more babies. Then the animals become more common, until they become so common they eat so much of the plant that it becomes rare again, and then the animals have to go back to having only a few babies. The whole complicated pattern of how the living things in an area affect each other, feeding on each other or making their nests on each other or helping each other to breed, is called the ecosystem - that is, the system of how the individual species' ecologies combine. The study of the ecosystem is itself called Ecology.
Domain: Eukaryota - organisms with cell-nuclei contained in membranes
Kingdom: Animalia or Metazoa - animals, i.e. multi-celled organisms which are usually self-propelled, have a fixed body plan, feed on plants and/or other animals and cannot photosynthesize (directly use sunlight for energy) the way plants and algae can
Subkingdom: Eumetazoa - animals which are definitely fixed structures which cannot be puréed into individual cells and reassembled again without damage in the way that e.g. sponges can be
Phylum: Chordata - animals with a spinal cord
Subphylum: Vertebrata - animals with backbones and, usually, jointed internal skeletons
Superclass: Gnathostomata - vertebrates which have jaws (as opposed to the Agnatha, Agnathostomata or Cyclostomata, jawless vertberates, which nowadays means hagfish and lampreys)
Class: Mammalia - mammals (that is, animals which produce milk from "mammary glands" to feed their young)
Infraclass: Eutheria - placental mammals (that is, mammals who carry their still-very-unformed foetuses inside a womb instead of a pouch) plus their closest fossil ancestors
Subclass: Placentalia - placental mammals
Magnorder: Epitheria - all placental mammals except armadillos, anteaters and tree sloths (who are called Xenarthra)
Magnorder: Boreoeutheria - all Epitheria except aardvarks, elephants, manatees and their close relatives (who are called Afrotheria)
Superorder: Euarchontoglires - primates (including humans), tree-shrews, colugos, rodents and lagomorphs (rabbits, hares etc.) Grandorder: Glires - rodents and lagomorphs Order: Rodentia - rodents Suborder: Myomorpha - rats, mice, gerbils, hamsters, lemmings and voles Family: Muridae - rats and mice Genus: Rattus - true rats Species: Rattus norvegicus - the common, Norway or brown rat Sub-Species: Rattus norvegicus domesticus - the pet, domestic or fancy rat Breed: Dumbo (with big, low-set ears and a wide head) Variety: Hooded or Japanese (i.e. with a white body, a solid-coloured head, neck and shoulders and a coloured stripe extending along the back) Individual: Babs Jordan
Grandorder: Glires - rodents and lagomorphs
Order: Rodentia - rodents
Suborder: Myomorpha - rats, mice, gerbils, hamsters, lemmings and voles
Family: Muridae - rats and mice
Genus: Rattus - true rats
Species: Rattus norvegicus - the common, Norway or brown rat
Sub-Species: Rattus norvegicus domesticus - the pet, domestic or fancy rat
Breed: Dumbo (with big, low-set ears and a wide head)
Variety: Hooded or Japanese (i.e. with a white body, a solid-coloured head, neck and shoulders and a coloured stripe extending along the back)
Individual: Babs Jordan
