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Animal evolution: a new view of an old tree

By Peter Holland

The metaphor of the ‘evolutionary tree’ is powerful. Closely related species, such as octopus and squid, can be pictured as twigs sitting near each other on a small branch, in turn connected to larger and larger branches, each representing more distant evolutionary relationships. Every animal species, past and present, is a twig somewhere on the vast tree of life. But what is the shape of this metaphorical tree? Can we find the correct place for all the twigs, or perhaps even just the largest branches? In short, who is related to whom? To solve this would be to reconstruct the history of animal life on our planet.

Solving the puzzle is not trivial. Even with just a hundred species, there are more possible trees than there are protons in the universe. And of course there are millions of animal species alive today, so the number of possible evolutionary trees is simply unimaginable. Yet remarkable progress has been made.

In 1857, Charles Darwin wrote to his friend Thomas Henry Huxley: “The time will come, I believe, though I shall not live to see it, when we shall have fairly true genealogical trees of each great kingdom of nature.” Darwin did not live to see it. Through most of the twentieth century, biologists argued fervently about the tree of animal life, with every expert having different opinions. For example, many felt that ‘segmented worms’ such as earthworms and leeches must be close relatives of other segmented animals, such as insects and spiders. Perhaps simple-looking animals, such as flatworms and parasitic flukes, were on a branch emerging near the base of the animal tree of life. These were commonly held views, and are still found in many textbooks. They are, however, wrong.

In the 1990s, a new source of data emerged that has changed our view of animal evolution theory. There is a set of genes used by all animal cells and these genes accumulate mutations to their DNA sequences over time. The more closely related two species are, the more similar their DNA sequences. With new technologies it is possible to find the DNA sequence of hundreds of genes, from hundreds of species, and amass vast data sets for comparison between species. In the light of this new information, many of the old arguments have melted away. And the DNA sequences give a remarkably consistent picture. It seems we can now describe the “fairly true genealogical tree” of animal evolution, stretching back over half a billion years. We can deduce that soon after the origin of the first animals, most likely simple balls of cells, several major evolutionary branches separated. One branch lead to sponges, one to comb jellies, one to a little-known group called placozoans, one to jellyfish and sea anemones, and one to the first ‘bilaterians’. You and I are bilaterians, as are worms, snails, insects, and millions more: these are the animals with front and back, top and bottom, and left and right. The bilaterian part of the animal kingdom then split into three huge branches: the Lophotrochozoa (including snails, segmented worms, and many more), the Ecdysozoa (including insects, spiders, nematodes and more) and the Deuterostomia (for example, starfish, sea urchins, and vertebrates). The vertebrates branched and branched again; giving ever smaller groups of closely related species, until eventually we found our own place in the great tree of life. Nestled among the apes, monkeys and other primates, we sit on a mammalian branch along with, perhaps surprisingly, the rats, mice, and rabbits.

Why does knowing the tree of life matter? There are practical applications, because knowing which animals are closely related helps if we wish to extrapolate findings between species, for medical research, for instance.  But there is a wider, more fundamental reason. Having the tree of life provides the essential framework for understanding biology. We can now compare anatomy, physiology, behaviour, ecology, and development between animal species in a more meaningful way than ever before. We can see how characters changed along each branch of the tree of life. In short, we can now start to build a picture of the pattern and process of animal evolution.

Peter Holland is the author of The Animal Kingdom: A Very Short Introduction. He is Linacre Professor of Zoology and Head of the Department of Zoology at the University of Oxford, and a Fellow of Merton College, Oxford. After a degree in Zoology and a PhD in Genetics he has spent the last 20 years undertaking research into the evolution of the animal kingdom, focussing primarily on the genetic and developmental differences between animal groups. He has published over 150 research papers on animal development and evolution.

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Image credits: Charles Darwin’s 1837 sketch, his first diagram of an evolutionary tree from his First Notebook on Transmutation of Species (1837): public domain via Wikimedia CommonsVertebrates image by Bob the Wikimedian: creative commons licence via Wikimedia Commons.

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