Metamorphosis: Unmasking the Mystery of How Life Transforms

Metamorphosis is the stuff of myth. On the face of it, a multicellular organism undergoing a complete change of form is no more remarkable than each individual's initial development from a single fertilised egg. But it seems so. A caterpillar dissolving into a cellular soup to generate a butterfly, a tadpole abandoning gills and tail for lungs and legs, or any number of sea creatures turning from swimming larvae into more sedentary adults: all evoke wonder and a sense of mysterious forces at work.

Science has no truck with mysterious forces, but the sense that these transformations demand an explanation that goes beyond conventional theory can persist. Frank Ryan frames his overview of the investigation of metamorphosis with a very detailed account of the work of British biologist Donald Williamson, who has proposed that radically different forms arise in one organism's lifetime because two genomes hybridised in their evolutionary past.

Initially, Williamson suggested that this was the only way to explain the many oddities of larval and adult form in the marine world of sea urchins, starfish and sea squirts. Natural selection working on mutations would never produce the total transformations of body plan - often shifting from bilateral to radial symmetry - that he was cataloguing. They must, he argued, result from the transfer of larval forms between unrelated species.

This challenged Darwinian orthodoxy, as Ryan emphasises. He also describes clearly how Williamson's methods, observing organisms in tanks, are those of an earlier tradition, honed in the days when marine biological laboratories at Naples in Italy or Woods Hole in the US were the best places to study developmental biology.

His own experiments, mixing eggs and sperm from different species, seem to have produced some unexpected hybrids, at least in a few cases. But others find them difficult or impossible to replicate. And attempts to apply modern, molecular genetic methods to his specimens failed to reveal anything unusual in their DNA.

Ryan weaves this close-up narrative of Williamson's work into a broader story about studies of metamorphosis, taking in Jean Henri Fabre's classic studies of insects, foundational work on insect physiology by Vincent Wigglesworth, and more recent elucidation of hormone action and control of genetic switches in development in insects and amphibians.

This is all fascinating work, although as with Williamson's marine invertebrates the detail may overwhelm the non-specialist. But it seems to point clearly to the possibility of a unified account of metamorphosis, seen as post-embryonic development, and other developmental changes, from the normal development of the embryo through to, for example, human adolescence.

All appear to be under the control of cascades of genes, linked in regulatory networks that respond to hormonal signals and to a variety of environmental triggers.

So it comes as a surprise that we return to Williamson at the end of the book, and find that he has now extended his cross-species hybridisation hypothesis to pretty well all examples of metamorphosis.

His most widely criticised offering to date was a paper in 2009 that suggested that caterpillars and butterflies must have separate, now hybridised, ancestral genomes.

Ryan alludes to the ensuing controversy, but only in passing, presumably because it suggests that his featured theorist is a man who has fallen so in love with his own idea he has taken it far beyond his own specialist area of zoology, and beyond the evidence.

Ryan ends by suggesting that it is at least possible that hybridisation, à la Williamson, might play an uncredited part in evolution of the variety of body patterns. Is he right? I cannot say. But glossing over the fact that his ideas have garnered virtually no support from other biologists makes this book seem a work of advocacy presented as a more straightforward popularisation.