Mike Huckabee, a current presidential hopeful, apparently believes that humans and all other living species on earth were created individually by God. He should take the journey to discover his inner fish. Then he would soon learn that a generous part of the blueprint of our own design is shared with that of a shark. The common language of the genes can be read by the developing embryos of utterly different-looking species that separated on the evolutionary tree hundreds of millions of years ago. Whether we find it flattering or not, we are one with the fish.

Or with the worm. Somehow, Your Inner Worm is not quite so appealing as a book title, though it’s no less apposite. We can climb down the steps on the evolutionary tree even as far as the first living cell and find there a legacy within the biochemistry of our own bodies linked with the twists and turns of evolution. Contrariwise, there is a famous Punch cartoon showing Charles Darwin emerging from original chaos via the worm, which gets legs and crawls on to land, before climbing upwards by way of various apes – eventually arriving at its acme in the person of the great biologist himself. “Man is but a worm,” the caption reads – and so he is, to a surprising degree. We cannot shake off the millions of years of our history of descent. It’s still wired into the way we grow.

Neil Shubin has undertaken expeditions into the high Arctic in search of fossils that bridge the Devonian transition between fish and land-living animals almost 400m years ago. This is one of the defining eras in life’s spread over our planet. Scientifically, it is now also one of the best-known of evolutionary thresholds: we can see the transition from fin to leg almost bone by bone, spelt out in fossils prised from hard sandstone. A lumbering fish with legs might seem a rather unglamorous ancestor. Yet such cumbersome creatures lay at the base of alligators, frogs, birds, bats, whales, gazelles – and ourselves – a wonderful testament to the creative power of evolution. It is not so long ago that philosophers mused over the impossibility of making a hand. Now we know exactly when and how this organ arose.

This is a great time to be a palaeontologist because the discovery of how genes work is permitting us to see the why and when of evolution. Fossils preserve for all time intermediate stages in the development of new organs. Discovery of feathered dinosaurs shows how an apparently specialised structure that evolved for one purpose may be hijacked for quite a different one: in this case from insulation into flight. Birds live on as dinosaur survivors from the age of Jurassic Park. Without our knowledge of fossils, the timing and origin of our fauna and flora would be mostly guesswork – but now we are getting towards a history of physiology itself.

This is a new synthesis of what used to be anathema to every medical student – classical anatomy – with dusty old palaeontology and state-of-the-art genetics. We are approaching a new theory of everything for the history of life. Ironically, this is happening just when human rapacity is putting life itself in peril. Shubin is a clear and readable guide through this brave new world of homeobox genes, and fruitflies with extra wings. Few scientists would be capable of such a comprehensive view. By the end, the reader will have a much better idea of how we are put together, and why. Every one of our body bits has a decipherable history.

Yet Shubin also shows that we are cobbled together in a far from perfect fashion.

This is because a development process established during growth of the embryo must follow a predestined path, even though there might be a much better way of assembling organs if we were starting all over again. Oddly enough, one of the best proofs of evolution is precisely this ritual of cell fates dating back to our history in those old fish days. We are put together by a rather inefficient committee – or by the expression of developmental genes, to put it correctly. Nerves take apparently inexplicable doubling-back routes that betray a distinct legacy of our sharkish origins. Any organism is a compromise between what is needed to survive and what it is possible to do with our inherited genetic machinery.

Our evolutionary history is even linked with health problems. “Men’s tendency to develop hernias is a trade-off between our fish ancestry and our mammal present,” says our guide. Not a lot of people know this. So next time you are banged up with this complaint in a doctor’s waiting room, try saying “damn that ancestral shark!” as a line to elicit sympathy. Apparently, it’s all because our testes need to be in the right temperature to maximise the production of sperm; the equivalent genitals in the shark are tucked safely inside the body. Your inner fish will make you think about your organs in ways you have never considered before.

Your inner fish: A Journey into the 3.5 Billion-Year History of the Human Body by Neil Shubin
Allen Lane £20 pp237

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