What Caused the Cambrian Explosion?

Last week we looked at some of the reasons why creationists can’t stop talking about the Cambrian explosion. Today let’s look at a new paper published in Science1 that explores possible causes of the Cambrian explosion, the seminal diversification of animal life that began about 530 million years ago. As we’ll see, the way scientists think about the causes of the Cambrian explosion has little in common with the way creationists think about it.

animals from the burgess shale

Paul Smith and David Harper (of Oxford and Durham Universities, respectively) outline a number of ideas about why Cambrian animals changed so much during a geologically short period of time. They reasonably conclude that we should consider multiple causes acting simultaneously, rather than any single stand-alone process, and that we should consider both abiotic and biotic factors.

Noting that “it is unlikely that any single causal mechanism can explain the Cambrian explosion,” and that such a complicated event likely involved the “complex interaction of abiotic and biotic processes,” Smith and Harper point to early Cambrian sea level rise as a major factor. Geologists have long known that worldwide sea level surged during the Cambrian, drowning low-lying continental areas to create shallow seas of the kind that, in today’s environment, teem with life. That may be just a coincidence--or it may be one of the key causes.

What would a rise in sea level during the Cambrian accomplish? Smith and Harper note that “sea-level rise would have generated a very large habitable area lying between the base of wave turbulence.” Additionally, Cambrian flooding would release nutrients such as phosphate and calcium, which would aid the formation of hard shells.

Smith and Harper describe the Cambrian explosion as a series of “interacting processes generat[ing] an evolutionary cascade that led to the rapid rise in diversity.” Some of these processes are biotic: an arms race sparked by “near-simultaneous appearance of both predatory and defensive hard tissues across a wide range of animal groups.” Some of these processes are abiotic: “Calcium concentrations in seawater increased almost three-fold in the early Cambrian, and this input may have facilitated the origin of biomineralization.”

This multifaceted complexity is how scientists think about the causes of the Cambrian explosion. While there is disagreement and controversy among scientists about the relative importance of the factors involved in the Cambrian explosion, and much research remains to be done in the Ediacaran fossils that predate the Cambrian, there is essentially no disagreement among scientists that evolution was responsible for the diversification of Cambrian animals.

Some outside the field of Cambrian paleontology disagree. Subscribers to the Proceedings of the Biological Society of Washington were perhaps puzzled to open their August 2004 issue to find an article by Stephen Meyer of the Discovery Institute, titled “The origin of biological information and the higher taxonomic categories.” This review paper argued that an “information-based analysis of the Cambrian explosion will support the claim...that the mechanism of selection and genetic mutation does not constitute an adequate causal explanation of the origination of biological form in the higher taxonomic groups.”

Meyer’s paper, which was subsequently withdrawn by the journal, goes on to conclude that what “natural selection lacks, intelligent-selection—purposive or goal-directed design—provides.” The “information” of Cambrian organisms, in this view, could only have originated from an intelligent designer working outside of natural processes.

These ideas were revisited in Illustra Media’s 2009 DVD Darwin’s Dilemma: The Mystery of the Cambrian Fossil Record. This film argues that the Cambrian is a problem for evolution to explain, and that even “Darwin was deeply troubled by the Cambrian explosion.” Employing sweeping cinematography and sweeping errors about Darwin’s early geological experiences, this movie highlights interviews with several Discovery Institute fellows, including Paul Nelson, who opines:

I think to build an animal, the kind of process that the evidence requires is a process that can look into the future and bring everything together, to actualize something like a trilobite or a chordate or a mollusk or the other different forms that we see in the Cambrian explosion. It’s going to be a process that has foresight. It’s going to be a process that can visualize complexity. It’s going to be a process indistinguishable from intelligence. That’s not natural selection. That’s design.

And Stephen Meyer adds:

But from the standpoint of intelligent design, it’s not mysterious at all because we know that intelligent agents can bring things into existence that didn’t exist before because they had an idea. They had a blueprint in their minds that they realized in their creative activity. There’s no need to tinker through millions of years of evolutionary history if you can actualize a plan at a discrete moment in time. And that’s exactly what appears to have occurred in the Cambrian explosion.

How different this rhetoric sounds from Smith and Harper’s recent paper in Science. No mention of physical processes such as sea level rise and changing ion concentrations in seawater facilitating biomineralization. No mention of an arms race of escalating predation and defense. Instead, what we hear is “information” and “foresight” and “blueprint.” It sounds like plans for a new shopping mall rather than an explanation of the fossil record. Such self-referential pontifications about information yield no new information to help our understanding of one of the most important periods in the history of life.

Though Smith and Harper’s work helps us understand what might have happened, we may never know the full story of what caused the Cambrian explosion, in part because few representatives of these half billion year-old rocks survive. Perhaps marvelously preserved fossils once existed that would have answered many of our scientific questions, but long ago they turned to sand in the cold shadows of some lonely, unseen cliff. Think of Walcott’s tiny quarry in the Burgess Shale, clinging to a steep slope high in the Canadian Rockies, and imagine how many equivalents have been exposed, basked in the sun for a time, and then crumbled to dust.


1 Smith, P.M., and Harper, D.A.T. Causes of the Cambrian Explosion. Science 20 September 2013: Vol. 341 no. 6152 pp. 1355-1356 DOI: 10.1126/science.1239450

Short Bio

Steve Newton is a former Programs and Policy Director at NCSE.