The discovery of Tiktaalik (Tiktaalik Roseae), a 375 million year old fossil, generated headlines in 2006. The supposed ‘missing link’ between fish and land-vertebrates was popularly described as ‘the fish that could do a push up’ for having the shoulder and forelimb bones usually associated with land vertebrates like early amphibians, while also clearly being a fish. One of the aims of Your Inner Fish is to pull the general reader away from the media hype and provide the facts and science of Tiktaalik and its relevance. Written by Neil Shubin, one of the co-discoverers of Tiktaalik, instead of diminishing the excitement of the discovery, the science actually magnifies its importance and significance of the discovery.
In fact, most of Your Inner Fish is not about Tiktaalik directly, but about the science that gives the discovery of Tiktaalik context, that shows its important place in the history of life on this planet and for the eventual evolution of terrestrial animals including humans. It was all of this additional information that made Your Inner Fish a joy to read and easy to recommend.
The first thing Shubin wants the reader to appreciate is how unlikely it is for animal remains to form fossils to begin with, the difficulty of finding the ‘right’ fossil and why the Canadian Arctic is the best place to search for fossils for the fish-land-animal transition. Since one of the main things that make the Tiktaalik find important is what it shows about its stage in the development of limbs, Shubin spends a couple of chapters on limb development. He does this both from the perspective of fossil finds – of those fish that lived before Tiktaalik and those early amphibians that came after, showing Tiktaalik’s place in the transition – and from the perspective of genetics and embryology and experiments done in the 90’s on the ‘zone of polarising activity’ (ZPA) on embryonic limb buds.
By now, not even half-way through the book, a few things were becoming clear. Although it is not one of the aims of Your Inner Fish to convince the reader of the fact of evolution, by going through the areas of fossil study, comparative anatomy, genetics, embryology, etc; it is covering much of the same ground as books that do have that aim like Jerry Coyne’s Why Evolution is True or Richard Dawkins’ The Greatest Show on Earth. A sceptic or fence-sitter reading this book would be taken by Shubin through the main lines of evidence almost by stealth.
The order of fossils in the world’s rocks is powerful evidence of our connections to the rest of life. If, digging in 600-million-year-old rocks, we found the earliest jellyfish lying next to the skeleton of a woodchuck, then we would have to rewrite our texts. That woodchuck would have appeared earlier in the fossil record than the first mammal, reptile or even fish – before even the first worm. Moreover, our ancient woodchuck would tell us that much of what we think we know about the history of the earth on life on it is wrong. Despite more than 150 years or people looking for fossils – on every continent of earth and in virtually every rock layer that is accessible – this observation has never been made.
Secondly, the reader can appreciate that the discovery of Tiktaalik coincided with a lot of other important discoveries in other fields occurring around the same time, from the experiments on the ZPA to the discovery of the role of hox genes for building a body plan. By explaining these areas, Shubin is not just providing context that enlarges the discovery of Tiktaalik, he’s sharing the relevance of his co-discovery with the work of others.
Remarkably, work in chickens, frogs, and mice was telling us the same thing. The DNA recipe to build upper arms, forearms, wrists and digits is virtually identical in every creature that has limbs.
Thirdly, even for a reader like me – not a scientist, but somewhat scientifically-literate and a fan of books on evolution and the history of life – this book contains much to enjoy, even if you’re are familiar with some of it. Also, the nature of science is that it requires an appreciation of nuance and contrary evidence; something that is established accepted fact today might be discarded as disproven tomorrow. Despite this, Shubin shows he is not afraid to show where he stands on some key questions – Why would Tiktaalik need to do push ups? Why did it live in shallow water? Which evolved first, bones or teeth? Why and when did sensory organs – for sight, smell and hearing – develop? How did the step from single-celled to multi-celled life happen?
Debates usually begin as either-or scenarios. Over time, all-or-nothing positions give way to a more realistic approach.
One minor complaint I have about this book is on that last point – the transition from single-celled to multi-celled life. This chapter I felt was a little rushed and lacked the clarity of earlier ones.
Your Inner Fish has a long finish. As well as summarising his main points, he shares examples of poor design and the legacy of our ancestors that still affects humans today. The edition I read (published 2009) also contains an afterword where he shares a few things that have been discovered since the first edition was published. There is also an epilogue where Shubin shares his thoughts on his love of science and its importance. It told me that Your Inner Fish is worth reading solely for how science-affirming it is.
Shubin says that the question he is most often asked is whether Tiktaalik is a ‘missing link’. His answer, which could also serve as the message of his book, is no – it is one of a series of found links.