The Book of the Week: Your Inner Fish

Michael Benton muses on our slippery past.

February 7, 2008

Comparative anatomy was very much a Victorian subject. Sir Richard Owen and Thomas Henry Huxley were the great scientists of their day and both were comparative anatomists. Owen famously trained on animal carcasses from London Zoo and whenever anything died - whether it was a chimpanzee, an elephant, a giant squid or a shark - he dissected it and wrote a descriptive monograph.

The subject lies at the heart of our understanding of the diversity of life and evolution (although Owen famously resisted evolution), and yet it might be seen as rather a dry one in which all the key observations have been made. This is far from the case.

Comparative anatomy hit the headlines repeatedly throughout the 20th century as fossil discoveries filled important gaps in evolution. But a new theme has raised the stakes in the past 20 years: developmental genetics, or evo-devo as it is known to aficionados.

The key theme of Neil Shubin's book is profound and yet simple: a study of our body shows us the common features of other mammals, other vertebrates, even fruit flies and sponges. After some introductory material, the book runs through key organ systems, from teeth to noses, body patterning to ears, showing how insights from fossils and genes all make sense of the broad patterns as well as the more ludicrous aspects of human anatomy.

Why, for example, does the vas deferens, the duct that delivers sperm, loop up from the human testicle into the gut area, over the pelvic bones, and then back down to the penis? Surely this duct could be a tenth of the length and thus function much more efficiently? The reason for the extra length is that the gonads in sharks and other vertebrates are located deep in the body, close to the liver. Indeed, that is their location in the early human embryo, and the testis migrates from beneath the embryonic armpit to plop into the scrotum. No wonder males still suffer hernias in this area - it's all down to a weakness in the abdominal wall that reflects our ancestry.

Other indications of ancestry are locked in the genes. Anatomists have long understood that the human head is a curious amalgam of precursor head and neck structures. In particular, the four "gill pouches" in the neck region of the human embryo are the same as those in the shark embryo. In the shark, these embryonic pouches indeed become gills, necessary for breathing underwater. In humans, the first arch supplies bones to the upper and lower jaws and two middle ear ossicles (malleus and incus), as well as associated vessels and muscles. The second arch supplies the third small ear bone (the stapes), a tiny throat bone and most of the muscles that control facial expressions. The third arch gives rise to bones, muscles and nerves deeper in the throat, and the fourth arch forms structures yet deeper in the throat, including the larynx.

These matches between embryonic structures in shark and human have been known to embryologists and anatomists for a long time. Further, the two-arch origin of the three ear ossicles, while sounding rather odd, can be explained from the fossils. The stapes is, in fact, the ancestral hearing bone, still found today in fishes, amphibians and reptiles. The other two (the malleus and incus) are the reptilian jaw joint, encapsulated in the middle ear during the Triassic some 200 million years ago, when the reptilian jaw gave way to a new mammalian jaw joint: numerous transitional fossils show this remarkable changeover.

Now, studies of the genes fill out the picture further. Biologists can perform manipulations with developing fish and frog embryos to show that each of the four gill pouches is instructed by the so-called Hox genes. These interact to control the development of particular structures in each species. Experimenters can turn off particular genes and show which ones do what. Such experiments have long been done with fruit flies, worms and even sponges to show that all animals have similar Hox genes, and these provide positional information, such as determining which end of the early embryo becomes the tail and which the head, how many segments there are along the body and what they become, where limbs are located, and indeed how each limb sprouts from the initial limb bud and differentiates into the joints and digits.

Some of this new stuff has been in other popular science books, but Shubin describes it uniquely, by linking the new evo-devo findings to the reader's own body and its strange characteristics. He tells the story with verve, recounting his excitement when he finds fossils in the field in North America and the Arctic. He tells us who discovered what and how: this is important so that non-scientists realise that scientists are (generally) normal people who are driven by the thrill of the chase. The illustrations are novel and excellent.

Some topics, such as radiometric dating and the principles of geology, are skated over, but there is so much to introduce to the non-expert reader that it is probably as well to leave out some basic themes that can be discovered elsewhere.

The final chapter that links all the genes, embryos and fossils into a narrative of the grand inclusive hierarchy of the tree of life is perfect. Nowhere does Shubin apologise or target the creationists/intelligent designers, but the final chapter is a masterstroke in answering them head on: why, other than by classic Darwinian evolution, would such an inclusive hierarchy exist in nature, and why would the human body be such a mish-mash of compromise at all levels, genetic to anatomical?

THE AUTHOR

Neil Shubin, a palaeontologist in the tradition of Ernst Mayr and Stephen Jay Gould, has discovered fossils that change the way we think about key transitions in evolution.

In 2006, Science announced that Shubin and colleagues had discovered Tiktaalik roseae, a 350-million-year-old fossil that blurred "the boundary between fish and land-living animals both in terms of anatomy and way of life".

Shubin, 45, grew up in Philadelphia and trained at Columbia, Harvard and Berkeley. He has developed and led expeditionary research programmes in North America, Africa, Asia and Greenland.

He remains captivated by discovery. "The world is filled with puzzles, all kinds of interesting questions, and it's our challenge to figure them out," he has said. "It's not just a static filing cabinet of things that human beings know."

Named provost of The Field Museum of Natural History in 2006, he is professor of anatomy at the University of Chicago and associate dean of the medical school.

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

By Neil Shubin
Allen Lane
240pp
£20.00
ISBN 9780713999358
Published 31 January 2008

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