The Tree of Life: Read an exclusive excerpt from a new book by Max Telford
In 1665 the recently founded Royal Society of London (for Improving of Natural Knowledge) published as its inaugural book the extraordinary Micrographia by Robert Hooke. Micrographia contains Hooke’s exquisite drawings and detailed written observations of diverse substances (silk, Muscovy glass, sand); of objects (the point of a needle); of parts of plants (poppy seeds, nettle stings); and of animals (insects, sponges, the teeth of a snail, feathers); all of them observed in extraordinary and unprecedented detail using his magnifying glasses and microscopes.
Hooke’s most famous image, of a flea magnified to 300 times its natural size, revealed to human eyes for the first time the astounding intricacy that characterises the body of even something so tiny. Hooke was able to describe the details of the flea’s body, ‘adorn’d with a curiously polish’d suit of sable Armour, neatly jointed, and beset with multitudes of sharp pinns, shap’d almost like Porcupine’s Quills, or bright conical Steel-bodkins’. He found the minuscule perfection and complexity he could see in this divinely created animal to be incomparably finer than that quintessence of man-made precision – a needle – whose point his microscope revealed to have ‘a broad, blunt, and very irregular end’.
What Hooke’s new instruments had revealed was that, close up, even the tiniest of the millions of animal species is an amazingly complex beast. And this has turned out to be true of every animal, from the ‘are they even animals?’, sea sponges and sea squirts, to octopuses and dragonflies; from fleas and vinegar worms to Nobel Prize-winning, Magic Flute-composing humans.
(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 that, unlike animals, spend their life as a single tiny cell. Choanoflagellates look like a ping-pong ball (this is the main part of its cell) with a collar attached – the ‘choano’ part of their name means collar. Wiggling away 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, tiny choanoflagellates is a big clue telling us where we might look to find out about the origins of animals. We can’t, of course, interrogate our ancestors directly, they breathed their last perhaps 700 million years ago, but looking at these closest surviving non-animal relatives, the choanoflagellates, we should be able to pick up some clues.)
Before we put our simple relatives under the microscope, it is worth thinking about what it is that we are even trying to explain. What is it about animals that is so special? The biggest novelty is, simply, multi-cellularity – your own body contains tens of trillions of cells, for example; a blue whale may have 10 quadrillion. Around this single, central fact, everything else that appears most remarkable about animal biology radiates. Some of what the clique of cells that made the earliest animals had to do is rather obvious. First and foremost, they had to invent (by which I mean evolve) a way to remain stuck together after they had divided: a new gene was needed to glue the cells together, or perhaps an existing gene was repurposed (from a gene that we can search for in our single-celled relatives).
To function as an organism rather than as a collection of individuals, animal cells had to evolve ways to talk to each other, to coordinate their actions and to influence each other in useful ways. Related to this, and perhaps most interesting of all, an animal’s many cells had to take on different and specific roles and then to behave altruistically, to cooperate – you go shopping, I’ll cook and he can do the washing up.
Animal cells have become super 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 movement or pressure or taste, cells covered in cilia that waft particles out of our lungs, blood cells that carry oxygen, lymphocytes that destroy bacteria and a thousand others besides. These many kinds of specialists cooperate, each contributing to the common goal by expertly performing their own allotted task, like workers on a production line. As the ultimate example of their altruism, and unlike any single-celled organism, almost every cell in the body of an animal has given up any possibility of passing its genes on to the next generation. This extraordinary privilege is reserved for the specialised germ cells: eggs and sperm.
With the evolution of multiple different kinds of cell there came a need to order them into larger structures: tissues (muscle, bone, blood, nerves) and organs (brain, kidney, stomach). Organising cells allows them to work efficiently; a body that jumbled up muscle, nerve and digestive cells would be a disastrous monster. Your muscles are concentrations of billions of individual muscle cells, and muscles work only because the individual cells have been carefully organised to lie side by side so that they all pull in the same direction. Your kidneys are made of millions of tiny filtering structures (called glomeruli), each contributing a droplet of urine to the overall outflow of waste; and each tiny glomerulus is itself composed of several different cell types each organised to sit in the right place according to its special role.
Finally, animal cells are organised to make a body with a certain shape. Our tissues and organs themselves are arranged just so in order to function together. Different bits of your body have to be scaled to the right size: left leg same length as right leg; heart not too big and not too small; a brain that is of a shape and size to fill your cranium; your circulatory system arranged to supply every one of your cells with food and oxygen. These higher levels of organisation required the invention of new genes to regulate embryonic development. As Hooke’s discovery of the intricacies of a tiny flea showed, all of this new complexity (and the new genes this implies) needed to be invented simply to make a tiny insect. Everything else in animal evolution could be thought of as more or less subtle variations on this wonderful new theme.
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 near neighbours to form small colonies of genetically identical individuals. Just like an animal, there is an initial cell that divides multiple times and the new cells that result then remain stuck together. These cell clusters are produced by cell division rather than by the gathering up of a bunch of unrelated individuals; and this means the cells of the mini colony are genetically identical and so are primed to cooperate with each other. From a gene’s point of view, cooperating with other cells in such a colony would be helping an exact copy of yourself.
Different species of these ‘single-celled’ relatives even grow into tiny colonies with different morphologies, and the shapes these adopt depend not on the shapes of individual cells but on how these cells are arranged. Some species form chains of cells, some are linked in branched chains, others make a ball of cells on a stalk and others make tiny rosettes. Some of these mini colonies look for all the world like the earliest stages of a growing animal embryo.
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 what they look like, taking on different shapes and different functions. This is a pretty amazing trick – like animals, they can use one set of genes to build more than one kind of cell: it’s like using the same ingredients to make a pancake or a Yorkshire pudding. Under certain environmental conditions, choanoflagellates can absorb their characteristic collar structure and transform themselves into a blob-like cell (no collar, no flagellum, no oval cell body), and, rather than swimming like a sperm, this second cell type moves by flowing like an amoeba. This seems to suggest the beginnings of an animal-like flexibility in the forms their cells can adopt.
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 the genes that had previously been thought of as quintessential components of a multicellular animal’s toolkit. There are genes animals use to glue cells together, genes used in making embryos, genes involved in communication between cells, and so on.
At this point we must resist the temptation to conclude that our single-celled ancestor had been preparing itself to become an animal. Evolution has no ability to plan; the genes that ended up being co-opted for making animals must have had important (and probably rather different) roles in our non-animal ancestor.
(Author’s note for excerpt: The simple step from a single celled ancestor to the first of the animals can be seen in the fossil record where animals appear roughly 600 million years ago. The evolution of the many celled body was in itself rather a tiny change but it was one that was going to have the most extraordinary, unanticipated consequences. It would go on, 600 million years later, to produce the extraordinary diversity of animal life that exists today – humans, whales and giraffes; beetles, bugs and butterflies; snails and giant squid. The ultimate consequences of the invention of the first animal seem to me to be like the unanticipated impacts on human history that rippled out from the invention of writing or the wheel.)
(Excerpted with permission from The Tree of Life: Solving Science’s Greatest Puzzle by Max Telford, published by John Murray; 2025)
