Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

by Neil Shubin

Allen Lane £20, pp229

There has never been a shortage of jokes about the moral inadequacies of the legal profession. Take this golden offering: why do sharks not eat lawyers? Professional courtesy, of course. Thus, barristers are equated with savage, underwater killers and distanced, albeit jokingly, from humanity.

And why not? you might ask. Lawyers scarcely deserve a reputation for being lovable or cuddly. Yet fossil expert Neil Shubin will have no lampooning of the legal trade. We are all sharks under the skin, he says or, to put it more precisely: 'We're all modified sharks - or worse, there is a lawyer inside each of us.'

It is a disturbing notion. Nevertheless, it is based on sound science. As Shubin makes clear, evolution does not proceed in mighty anatomical jumps but through a process of gradual change, by transforming - very slightly - a gene, cell or bone so that it acts to a new purpose. In this way, a new species is eventually created, albeit one that still carries the hallmarks of its evolutionary predecessors, an inner connection that can - and often does - stretch over the aeons, from fish to humans.

You can see these biological stigmata today, says Shubin. 'Our hands resemble fossil fins, our heads are organised like those of long-extinct jawless fish and major parts of our genomes still look and function like those of worms and bacteria.' We are all shark siblings, in short.

The thesis is intriguing and although not entirely new, it is made fresh and accessible by Shubin's laidback, rather chatty style, not to mention his admirable sense of the ridiculous and his catalogue of dotty anecdotes: an Arctic fossil dig turned to panic at the sight of approaching polar bears that, on closer inspection, were revealed to be large Arctic rabbits, and the marooning of his team on a remote Canadian river bank where they promptly found, by fluke, the fossil of a tritheledont, an extraordinarily rare, 200-million-year-old 'part mammal, part reptile'.

Through this narrative Shubin, head of Chicago University's anatomy school, outlines his case for existence of 'the fish within us': fossil amphibian fins that demonstrate a structural affinity with human hands; teeth, first discovered in ancient jawless fish, that evolved into modern mammary and sweat glands; and genes, which control our eyes and ears, that correspond directly to DNA found in primitive jellyfish. If nothing else, the scope of Your Inner Fish is ambitious, and if Shubin occasionally falters in his storytelling, trapping himself in the complexities of his material, he rarely loses the plot for long.

In any case, the real value in Shubin's book goes beyond biology. In demonstrating how anatomical features are co-opted in the natural selection of species, the author takes a well-aimed swipe at intelligent design. The latter notion, brainchild of fundamental Christians, argues that the breadth and complexity of life on Earth cannot be explained without recourse to the work of an intelligent designer, in other words God. How else can we explain the complexity of the eye, the structure of DNA or the wiring of the brain? Only a super-smart entity could have done that.

Well, from the pages of Your Inner Fish, it is clear that if a supreme being were responsible for creating life on Earth, from bacteria to humans, He or She displayed little intelligence. Far from being the perfectly crafted handiwork of a deity, our bodies are jerry-rigged patchworks of old bones, cells and genes bolted on to old frameworks that creak and groan at every opportunity. Men suffer hernias because their spermatic cords, inherited from ancient fish ancestors, leave them susceptible to gut tissue spilling through muscle walls, for example, while the evolution of the voice box has left us vulnerable to all sorts of breathing and swallowing ailments.

Or consider hiccups. Spasms in our diaphragms, hiccups are triggered by electric signals generated in the brain stem. Amphibian brain stems emit similar signals, which control the regular motion of their gills. Our brain stems, inherited from amphibian ancestors, still spurt out odd signals producing hiccups that are, according to Shubin, essentially the same phenomenon as gill breathing. Similarly, modern lifestyles leave us vulnerable to predispositions to obesity, heart attacks and haemorrhoids because we have the genes of hunter-gatherers who lived active, not sedentary, lives.

Then there is our sense of smell. Three per cent of our DNA is devoted to receptors that give us a sense of smell. The same is true for all other mammals, including rats, dogs and cats, except that in humans, a third of this material, the equivalent of about 300 genes, has been rendered useless by recent mutations. We no longer rely on our sense of smell, so its genetic roots have atrophied.

But if we are created in His image, why fit us out with hundreds of useless, redundant smell-receptor genes in the first place? Scarcely the handiwork of a very intelligent designer, it would seem. Certainly, for an infallible deity, God appears to have made an awful bodge of the bodies of men and women. Nor should we be surprised, says Shubin: 'We were not designed rationally, but are products of a convoluted history.' And that is putting it mildly.