Book Review: The Cambrian Explosion
By Douglas H. Erwin and James W. Valentine
The Cambrian Period, stretching from roughly 538 million years ago (Ma) to approximately 485 Ma when the Ordovician Period began, was a time of tremendous change for both the planet and the life upon it. Before the Cambrian, numerous clades of multicellular animals (metazoans) with a wide—and sometimes a very weird-to-our-eyes—variety of body types appeared during the Ediacaran (~635 Ma – 538 Ma), but unfortunately for us most of these creatures were soft-bodied and, thus, poorly preserved—if preserved at all—in the fossil record. Roughly 30 spots around the world have so far yielded Ediacaran fossils, including the eponymous Ediacara Hills in Australia, the Charnwood Forest in central England, the Yangtze valley in China and Mistaken Point in Newfoundland. Our picture of pre-Cambrian life is, thus, far from complete—which is, indeed, the case for most of geological history. However, enough fossils from enough places with enough variety from the Cambrian have been found that we recognize the burst of evolutionary radiation that began around 538 Ma and lasted for 13 to 25 million years as the Cambrian explosion. By the end of this period, almost all present-day animal phyla had appeared, and the planet itself was markedly changed by this life and its abundance.
Douglas H. Erwin and James W. Valentine, the authors of The Cambrian Explosion: The Construction of Animal Biodiversity, trace their book’s origin to a 2007 conference of molecular developmental biologists, at which they were asked to recommend a graduate-level text on the Cambrian explosion. There was, they realized, no such volume, and so they wrote one. Be forewarned: this is no light read but truly is a textbook, one that I read much of the time with the book in one hand and an iPad in the other googling my brains out to drive home the finer points of the sulfur cycle (oh, to have finished my last chemistry course four months ago rather than forty years ago!) or to remind myself for the twentieth time what the hell gastrulation is (I was a physics student, for god’s sake, not an embryologist). The authors, both paleontologists, approach their subject with as much breadth as depth, ranging from geochemistry to plate tectonics to paleoclimatology to morphology to the aforementioned embryology to paleoecology to comparative genomics to, yes, fossils. It is staggering to realize the expanse of interdisciplinary knowledge that 21st-century paleontologists must master in order to advance the field. It ain’t just digging up a bunch dead critters and sticking them in museums these days (okay, it never really was, but a khaki-clad dude toting a rock pick and a specimen bag through a landscape of dusty cliffs and boulders is what most of us think of when the word paleontologist pops into our brains—that or Indiana Jones, who was, I am at pains to point out, an archaeologist, not a paleontologist).
I was drawn to the book because I am, at heart, that khaki-clad dude wandering far and wide to dig up fossils and bring them home. There is something amazing about holding in your hand a thing that was alive hundreds of millions of years ago when the dry land you are standing on was the floor of some long-gone sea and knowing that you are the first person to see, to recognize this piece of rock as a traveler through deep time, come to you through so many years that our century-scaled brains cannot comprehend their number, their weight, their expanse. Every time I pull a Platystrophia with both its shells intact—sometimes even with growth lines visible—from a bunch of rubble along a road cut, I feel not just the thrill of the find but the thrill of connection, a life now to a life then, laced with a tincture of memento mori, though these bones will almost certainly not have the luck to make it to fossil. But beyond collecting, I want to understand what I have found: who the creature is, how it lived, where it lived. Which is why I find myself reading graduate-level textbooks for funsies. I cannot claim to have understood everything I read, but I grasped enough this time through to want to let it percolate on my brain’s backburner and pour myself another mug of paleogoodness in a few months.
That I failed to fully digest every element of the text speaks to my deficiencies as a student and not to any shortcoming of the authors as teachers. Erwin and Valentine are adept at presenting their subjects in clear, straightforward prose and provide a plethora of illustrations and tables that work hand-in-hand with the text to explicate everything from the carbon-13 isotopic record for the Ediacaran and Cambrian periods at eight key sites (41) to a phylogenetic tree illustrating potential relationships among clades (75) to a comparison of the occupation of ecological space in the Ediacaran and Cambrian (233) to a comparison of Hox gene complexes among different clades (262) that even I was able to follow. That last is found in “Chapter 8: The Evolution of the Metazoan Genome and the Cambrian Explosion,” a chapter I found astonishing. The Cambrian Explosion was published in 2013, and I had no idea genomics had advanced to the point that differences in gene regulatory networks across clades could be used with such accuracy to locate potential last common ancestors. I can only imagine that both the reach and the precision of this science have advanced exponentially in the nearly ten years since publication and—hint, hint, Drs. Erwin and Valentine—would love an updated version of the text that brings these findings to light. Another strength of the text is the evenhanded manner with which the authors put forward various hypotheses that have been proposed to explicate a particular problem; I almost always knew from the start which explanation they favored, but they nonetheless made the cases for the alternative explanations fully before explaining their support of one hypothesis over the others.
The final chapter, Chapter 10, is titled “Constructing the Cambrian,” and it is toward this end that the authors’ have built over the previous nine chapters. Here, they synthesize all the various disciplines and lines of evidence they have detailed into a thesis proposing that the groundwork for the explosion was laid long before the Cambrian during the Cryogenian (~720 Ma – ~635 Ma) when Metazoans originated and sponges and the stem eumetazoan lineages diverged before the Sturtian glaciation (717 Ma – 665 Ma). This was followed by a second phase during which the cnidarians emerged; the protostomes and deuterostomes diverged between the Sturtian and Marinoan glaciations (i.e., between 665 Ma and 645 Ma); and the major bilaterian clades emerged, likely in the late Cryogenian into the Ediacaran. With the powder so primed, the Cambrian exploded with the appearance of numerous crown bilaterian groups, many of which are still extant, a few of which are extinct (after all, there have been several mass extinction events in the 485 million years since the mass extinction event that marked the Cambrian’s end). If the preceding three sentences are just so much gobbledygook to you, here’s the takeaway: multicellular animals came on the scene long before the Cambrian and slowly diverged into a handful of different clades over a couple of hundred million years; core sets of genes evolved within these clades, and the biological activities of the animals themselves altered the physical environment, clarifying and oxygenating the waters in which they lived. This all set the stage for the Cambrian explosion, which the authors describe as “a very different sort of evolutionary diversification, one involving ecological feedback between multiple, evolutionarily independent lineages with both morphologic and taxic diversification within each of these different lineages” (341). Early sponges and primitive corals were joined by mollusks, brachiopods, comb jellies, sea anemone, early starfish, ancestral fish, and hosts of arthropods—especially trilobites, with hundreds of species emerging—laying the groundwork for much of the diversity of life we see today. The Cambrian Explosion is a difficult read for the layperson, but I’m a bit of a masochist and enjoyed the deep dive with Erwin, Valentine and the science they surfaced. Seriously, guys, second edition!
Learn more about Douglas H. Erwin and his work.
Learn more about James W. Valentine and his work.