Taxonomy: Ruling the Kingdoms of Life
The best bit of biology is studying how different creatures are related to each other. This is called “taxonomy'” the study of living things and putting them into categories. We all have some idea about how it works – you’ve got reptiles, mammals, birds, etc. But it goes so much deeper. How do viruses and fungi fit in? What did the animals in between mammals and reptiles look like? Are dinosaurs actually birds? Are bananas actually herbs? Nothing excites the five-year-old child in us, more than these questions. Let’s dive in, to the classification of life.
Types of Life: The Great Kingdoms
Life on Earth is broadly divided into six kingdoms – Animals, Plants, Fungi, Protists, Bacteria and Archaea.
Plants and animals are exactly what you think they are. Plants are the green things that use the sun to produce energy, such as ferns, grasses and trees. Many plants have roots, stems, leaves, trunks, and flowers. Animals are the complex multicellular things that move around breathe oxygen. They include fish, amphibians and mammals – a lot the creatures we know really well.
Fungi are actually really different from both animals and plants. They include a really wide variety of creatures, both single and multicellular, the most commonly known are mushrooms and yeasts. Some fungi are enormous, the size of forests and thousands of years old others are tiny molds that live and die very quickly eating leftover gunk in your bathroom. They don’t photosynthesize like plants do, instead, they consume decaying organic matter (think a mushroom growing out of a rotten log in a forest). However, they are clearly pretty different from animals as well – they don’t have animal body plans like skeletons or skin – they grow in a much more plant-like way.
The other kingdoms, Archaea, Bacteria and Protists are a bit weirder. Protists are microscopic single-celled creatures that have complex cellular structures. These include creatures like amoebas and parameciums. While they are still generally microscopic, they are a lot bigger than bacteria. Protists often actually hunt and eat bacteria, and can move quite fast.
Bacteria are simpler single-celled creatures that eat basic organic compounds. They often infect other animals, like us when we get sick, but also just live in the ground, eating rotting organic matter. Some bacteria are actually quite like plants, such as blue-green algae, they photosynthesize and produce most of the world’s oxygen (see our post here).
Archaea are the oldest of them all the kingdoms. They are very simple creatures and look a lot like bacteria. However, they are chemically quite different, they have a different type of cell wall. They were only recently discovered, are not as well researched and much harder to understand. However, it is likely they were the first organisms alive on Earth.
All of these kingdoms then divide into small groups called “Phylum”. Phylum then split further into groups such as “Classes”, “Orders”, “Families”, “Genuises” and eventually “Species”.
For example, humans are classified in the animal kingdom as chordates (we have a backbone). Then our class is mammals, we have fur and raise our young on milk. Our phylum is primates, along with other apes such as chimpanzees. How genus is homo, we stand upright, no other homo species survive. And finally, our species is sapiens.
However, it is not just animals that separate into these different subcategories – all of the kingdoms of life do. For example, plants separate into lots of different categories at different stages – vascular and non-vascular plants, flowering vs non-flowering plants (see our plants post here). Fungi separate broadly into mushrooms and single-celled fungi such as yeast. Each kingdom has its own beautiful structure (see more – our timeline here). In fact, if you look at the Wikipedia page on any creature, it will tell you how it is classified, what its phylum, class, order, family, genus and species.
Taxonomy is all a bit more complicated than that. Over time, we have found more creatures and tried to fit them into the system and create new categories to make it all work (like ‘super-orders’ or ‘sub-orders’). This has meant over time the system has gotten even more complicated. Even simple categories of animals that we talk about every day don’t always have a nice neat scientific definition. Fish, for example, scientifically would be “non-tetrapodian vertebrates”: animals with a backbone that don’t have four legs. Vertebrates are actually a subphylum because there are a few strange animals that have a spinal column, but not a backbone (mostly weird sea-worms) that you probably want to exclude from being called fish. So clearly, it gets pretty complicated pretty quickly.
Animal Classification: Some things you need to know
Having a system talking about living things suddenly spits out some incredible facts about the animals on our planet. These include:
- most animals people know are ‘vertebrates’, animals with a backbone. However, there are only 50,000 vertebrates. There are over a million invertebrates (insects, snails, etc.) – vastly more of them;
- the majority of species are insects – they are by far the largest group, with over 80,000 species of just wevils;
- crabs have evolved five separate times – there is a specific scientific term for ‘an animal evolving into a crab’ (carcinization) and an entire area of science trying to explain why it keeps happening;
- we have only classified about 10% of species – 90% remain to be classified;
- these unclassified species are all around us – we often think unknown species are only in unexplored places like jungles and rainforrests. However, to prove a point a biologist once spent three decades of her life observing only her ordinary garden in the English midlands. She identified 2,673 different species including 474 plants, 1,997 insects, 138 other invertebrates such as spiders and woodlice, and 64 vertebrates including 54 species of birds and seven mammals, in an area of less than a square kilometre. It is virtually guaranteed you have seen hundreds of unknown species in your life; and
- octopuses are even weirder than you think – they are actually more closely related to snails than they are any type of fish and have evolved brains, eyes and all their features completely separately to most other animals.
Animal Evolution: A story in embryos
One of the most amazing things about animal evolution is the history animals carry in their embryos. As all life is connected by evolution, embryos can tell us a lot about whether two animals are related to each other.
When an animal grows, it starts out very small and simple – as a single cell in an egg or a womb. The animal then grows its more complex structures (skeleton, limbs etc) when still inside the egg or womb, so it is mostly developed once it is hatched or born. However, if you think about how evolution works, it never starts from scratch, redesigning a new creature. Instead, evolution exaggerates the already existing characteristics of an organism, slowly developing one species from another. For example, a land mammal slowly spending more time in the water, losing its limbs and evolving fins, as the ancestors of whales did.
This means that embryos store the evolutionary history of a species. For instance:
– whale embryos still have prominent back legs, where adult whales do not;
– hoofed animals like horses have separate toes for a while in their embryos, which adult horses do not; and
– chicken embryos have teeth.
These characteristics are called ‘vestigial traits’ – traits an animal has in its embryo form that it has lost in its adult form.
If you look at a human embryo:
– there is a significant period where human embryo’s have tails – leftover from our monkey ancestors;
– we have a psuedo-gill-like structure for an embryonic stage, leftover from when we were fish; and
– we have webbed feet and hands (fingers and toes don’t actually grow separately, we grow flippers then the webbing dies until we get hands).
It is a cliche to say, but the evolution of life on earth is really is one continuous story. All life on earth is fundamentally connected and comes from the same source. We see echoes of our evolutionary ancestors in our own young. In the end, we are all just different branches of the same tree. This is what the science of taxonomy, has to teach us. It is a truly wonderful pursuit.
