This is a remarkable book, one that might well escape the attention of interested readers due to its title, which is not likely to appeal to a non-professional readership. But it's nothing less than a milestone in evolutionary theory, written by one of our greatest living evolutionary biologists, James W. Valentine, of the University of California's pioneering Department of Integrative Biology. I believe it’s a scientist’s testament, too – an effort to summarize the sum product of a lifetime’s work and the central lessons learned. For all of these reasons and many more, I believe this book is one of the greatest scientific works written in the opening years of the 21st century, and I am certain that it will be regarded as such by those who follow in our footsteps. This is a book that belongs on the shelf of anyone who has wondered how animal life originated – a book to savor, and return to, as one’s understanding grows.
The title, admittedly, is off-putting. It’s a Rorschach test, in a sense; how many would-be book buyers would immediately see, in the title, a paraphrase of Charles Darwin’s On the Origin of Species? But that is expressly the author’s intent – namely, to write a book that rivals, or at least speaks to, Darwin’s claims in On the Origin of Species (1859). The author sets himself quite a hurdle to overcome, given that Darwin’s book is rightly regarded as among the finest scientific works ever written – if not the finest. If you haven’t read it, you should.
But Darwin was well aware of his magnificent book’s limitations. So far as Darwin knew, species originated by means of “descent with modification,” brought about by natural selection. Speciation, in Darwin’s framework, takes time – lots of time. Darwin was well aware that most of the extant animal phyla appeared quite suddenly, at the base of the Cambrian. In a chapter devoted to the subject, Darwin argued that there must have been a very long period, prior to the Cambrian and as yet undiscovered, in which the predecessors of these phyla evolved. If the phyla indeed emerged out of the blue, as the fossil evidence suggests, Darwin – in his honesty -- conceded that this evidence posed a formidable challenge to his theory. Unless future research could demonstrate a very lengthy evolutionary prelude to the Cambrian Explosion, Darwin conceded, there would be good grounds for doubting his theory.
The trouble is, there is still no incontrovertible evidence of animal evolution prior to the dawn of the Cambrian. Darwin would have taken solace in “molecular clock” studies, which trace beginnings of animal differentiation to the Cryogenian, deep with the Neoproterozoic (about 750 to 800 ma). Still, there is no generally accepted fossil evidence to support these studies. Beginning about 560 ma, to be sure, there is evidence aplenty of puzzling life forms that appear to be animal-like (the Ediacaran or Vendian fauna), but the jury is still out on their affinities to today’s animal world. But there is plenty of evidence of increasing oxygenation of the world’s oceans at the Cambrian’s dawn, followed shortly by the appearance of the first undoubted animal fossils. Within the short span of another twenty to forty million years – a mere heartbeat in geological time – there is abundant fossil evidence of animals that had already developed the thirty-odd body plans evident in today’s extant phyla. Given the short span in which this astounding development took place, it is clear that Darwin’s question – how do species originate? – falls short of the mark. We need to ask, how do phyla originate? And that is precisely the point of this book.
What is a phylum, exactly? Valentine tackles this murky point at the onset, pointing out that nobody knows, really. Defined ecumenically, a phylum describes a group of animals that share a distinctive body plan – a blueprint, of sorts, for organizing an organism in order to successfully grow, eat, fend off predators, and reproduce (if not necessarily in that order). Beginning with Linnaeus’ effort to impose order on the world of living creatures by means of hierarchical categories, phyla have been created to sum up biologists’ views, in the end, of distinctiveness and relatedness. A worm found in the ocean might be distinct in its color and structure, but further investigation – including a study of its ontogeny, or means of development – might reveal a connection – presumably, an ancestral connection -- to other, seemingly dissimilar beings. Underlying the Echinodermata, for example, is a rather sweeping commitment to pentaradial symmetry, often achieved only at the cost of a wrenching transformation as larvae become adults. But the number, always, is five. Contrast this with the ctenaphora, who move their iridescent bodies through the waters of the sea by means of rows of beating cilia. The number of rows, always, is eight… or, as fossils ctenophores suggest, some multiple of eight. Just how many phyla exist is, to this day, a matter of controversy, but the consensus number is in the mid-thirties. Why did they all appear so suddenly – and why have so few appeared since? What is more, does their very existence attest to the operation of evolutionary processes on the level not merely of species, as Darwinian evolutionists insist, but more broadly, on the much deeper and more broadly shared body plans that manifest themselves into today's extant phyla?
The pages of On the Origin of Phyla walk the reader through the best of today’s knowledge concerning the vexing questions raised by the evolution of animals, beginning with a convincing argument that the origin of phyla is indeed a problem worth explaining. The second section of the book examines each of the extant phyla in detail, explaining their body plans and critically examining the various strands of evidence, molecular and fossil, for their relatedness and differentiation. The third section of the book capably addresses the competing (and conflicting) perspectives offered to explain the seemingly sudden origin of phyla. In the end, this book does not provide definitive answers to the questions it poses, for no such answer can be given as yet. But nowhere else will the interested reader find a more objective, trustworthy guide.
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