In 1665 the recently founded Royal Society of London (for the Improvement of Natural Knowledge) published the Extraordinary Micrographia by Robert Hooke as its inaugural book. The Micrographia contains Hooke’s excellent drawings and detailed written observations of diverse substances (silk, Muscovy glass, sand); of objects (point of needle); parts of plants (poppy seeds, stings of nettle); and of animals (insects, sponges, mollusks’ teeth, feathers); All of them made observations in extraordinary and unprecedented detail using their magnifying glasses and microscopes.
Hooke’s most famous image, of a flea 300 times larger than its natural size, revealed for the first time to human eyes the astonishing complexity that characterizes the body of even such a small thing. Hooke was able to describe the details of the flea’s body, ‘adorned with a curious polished suit of sable armour, neatly laced up, and studded with many sharp pins, the shape of which was almost like the quills of a porcupine, or the bright conical steel-bodkins’. He found that the minute perfection and complexity he could see in this divinely created animal was incomparably better than that quintessence of man-made precision – a needle – the point of which his microscope showed ‘a broad, blunt and very irregular end’.
What Hook’s new instruments revealed was that, up close, even the smallest of the millions of animal species is a surprisingly complex animal. And this has proven true for every animal, ‘Are they even animals?’, from sea sponges and sea squirts to octopuses and dragonflies; From fleas and vinegar insects to Nobel Prize winners and humans who created magic flutes.
(Author’s note for excerpt: The theory of evolution tells us that the first animals must have evolved from something simpler, something much smaller that was not yet an animal. Our closest relatives on the giant tree of life are a group of tiny organisms called choanoflagellates, which, unlike animals, spend their lives as a single small cell. Choanoflagellates look like a ping-pong ball with a collar (this is its cell’s main Part is) attached – The ‘Choano’ part of their name means collar. In the middle of the collar is a whip-like structure similar to the tail of a sperm. This is called a flagellum or whip, and this is where the second part of the name ‘choano-flagellate’ comes from.
The close relationship between complex, large animals and simple, small choanoflagellates is a big clue that tells us where we can look for the origins of animals. Of course, we can’t interrogate our ancestors directly, they probably last breathed 700 million years ago, but by looking at these closest living non-animal relatives, the choanoflagellates, we should be able to pick up some clues.)
Before we put our ordinary relatives under the microscope, it’s worth thinking about what it is we’re trying to explain. What is it about animals that is so special? The biggest innovation, simply put, is multicellularity – for example, your own body contains tens of trillions of cells; A blue whale may have 10 quadrillion. Around this single, central fact, everything that seems most remarkable about animal biology circulates. What the groups of cells that formed the first animals had to do is fairly clear. First and foremost, they had to devise a way to stick together after dividing (by which I mean evolve): a new gene was needed to make the cells stick together, or perhaps an existing gene was repurposed (from a gene we can trace in our single-celled relatives).
To function as a single organism rather than a group of individuals, animal cells had to develop ways to talk to each other, coordinate their actions, and influence each other in useful ways. Related to this, and perhaps most interesting, an animal’s many cells have to perform different and specific roles and then behave altruistically, cooperating – you go shopping, I’ll cook and he can do the laundry.
Animal cells have become extremely diverse: we have muscle cells and nerve cells, cells that line our stomachs and secrete digestive enzymes, cells that detect light or sound or temperature or motion or pressure or taste, cilia-covered cells that filter particles out of our lungs, blood cells that carry oxygen, lymphocytes that destroy bacteria and more. A thousand others. These types of specialists cooperate, each contributing to the common goal by expertly executing their own allotted work, like workers on a production line. As the best example of their altruism, and unlike any single-celled organism, almost every cell in an animal’s body has given up any possibility of passing its genes to the next generation. This extraordinary privilege is reserved for special reproductive cells: the egg and the sperm.
With the evolution of many different types of cells came the need to organize them into larger structures: tissues (muscle, bone, blood, nerves) and organs (brain, kidneys, stomach). Organizing cells allows them to work efficiently; A body with muscles, nerves and digestive cells in disarray would be a destructive monster. Your muscles are concentrations of billions of individual muscle cells, and muscles work only because the individual cells have been carefully arranged side by side so that they all pull in the same direction. Your kidneys are made up of millions of tiny filtering structures (called glomeruli), each of which contributes a drop of urine to the overall outflow of waste; And each tiny glomerulus itself is made up of many different types of cells, each arranged to sit in the right place according to its particular role.
Finally, animal cells are arranged to form a body of a certain shape. Our tissues and organs organize themselves to work together. Different parts of your body need to be scaled to the correct size: left leg the same length as the right leg; The heart is neither too big nor too small; A brain that is shaped and sized to fill your cranium; Your circulatory system has organized to supply food and oxygen to each of your cells. These higher levels of organization required the invention of new genes to regulate embryonic development. As Hooke’s discovery of the complexities of a tiny flea showed, all this new complexity (and by implication new genes) needed to be invented just to create a tiny insect. Everything else in animal evolution can be thought of as more or less subtle variations on this wonderful new subject.
To understand the roots of these animal innovations, we are looking for parallels and precursors in our closest single-celled relatives. The first is the ability of some of our close neighbors to form small colonies of genetically similar individuals. Like an animal, there is an initial cell that divides several times and results in new cells that stick together. These cell clusters are formed by cell division rather than the aggregation of groups of unrelated individuals; And this means that the cells of the mini colony are genetically identical and therefore ready to cooperate with each other. From the genes’ point of view, cooperating with other cells in such a colony would be to help make an exact copy of itself.
Different species of these ‘single-celled’ relatives grow in small colonies with different morphologies, and the shape they adopt depends not on the size of individual cells but on how these cells are arranged. Some species form chains of cells, some are joined in branched chains, some form a ball of cells on a stalk and some form small rosettes. Some of these tiny colonies look like the early stages of animal embryos growing all over the world.
A slightly rarer, though more striking animal-like trait found in at least some species of choanoflagellates is the ability of their cells to change their appearance, becoming different shapes and having different functions. This is a pretty amazing trick – like animals, they can use one set of genes to make more than one type of cell: it’s like using the same ingredients to make pancakes or Yorkshire pudding. Under certain environmental conditions, choanoflagellates can absorb their distinctive collar structure and transform themselves into a blob-like cell (no collar, no flagellum, no oval cell body), and, instead of swimming like sperm, this other cell type moves like amoeba. This appears to be the beginning of animal-like flexibility in the forms their cells can take.
Another way to compare animals with choanoflagellates is to think about the genes they might have in common, and choanoflagellates have recently been shown to have a healthy number of genes that were previously thought of as quintessential components of a multicellular animal’s toolkit. There are genes that animals use to stick cells together, there are genes that are used to form embryos, there are genes that are involved in communication between cells, and so on.
At this point we should avoid the temptation to conclude that our unicellular ancestor was preparing itself to become an animal. There is no capacity for planning in development; The genes that co-opted to create animals may have had important (and perhaps different) roles in our non-animal ancestors.
,Author’s note for excerpt: The simple step from a single-celled ancestor to the first animal can be seen in the fossil record where animals appeared about 600 million years ago. The evolution of the multicellular body was a small change in itself, but it was to have the most extraordinary, unexpected consequences. This, 600 million years later, would continue to generate the extraordinary diversity of animal life that exists today – humans, whales and giraffes; beetles, insects and butterflies; Snails and giant squid. The final consequences of the invention of the first animal appear to me to be like those unexpected effects on human history which were produced by the invention of writing or the wheel.)
(Excerpted with permission from The Tree of Life: Solving Science’s Greatest Puzzles by Max Telford, published by John Murray, 2025)
