The Cambrian “Explosion”, Part 6
This is the final part in a series by Keith Miller (earlier parts can be found on the sidebar). It is an updated extension of Miller and Campbell's 2003 essay “The ‘Cambrian explosion’: A challenge to evolutionary theory?” from the book Perspectives on an Evolving Creation: Grand Rapids, and it coincides with our Question, "Does the Cambrian Explosion pose a challenge to evolution?". A downloadable version of Miller's full paper can be found here.
Possible Causes of the Cambrian Radiation: What Lit the Fuse?
Numerous hypotheses exist for the geologically rapid diversification of invertebrates in the Cambrian, proposing various key evolutionary innovations or environmental triggers. Critical levels of ecological or behavioral complexity may also have stimulated diversification. At the molecular level, organisms may have reached a key threshold of genetic organization or evolved a key gene.
A number of important environmental changes occurred in the late Precambrian and in the early Cambrian that likely had important consequences for the early evolution of metazoans. Near the end of the Precambrian there were several episodes of nearly global glaciation in which sea ice and continental glaciation extended to the equatorial regions. The last of these “snowball Earth” episodes was about 635 million years ago.1 This time just precedes the earliest fossil evidence of metazoans. The major changes in ocean temperature and chemistry associated with the transition from a snowball Earth to a greenhouse world would likely have had profound effects on life. In particular, isotopic data indicates that the oceans became increasingly oxygenated after the end of the last of the global glaciations.2 Higher oxygen levels would have been critical for aerobic respiration and the evolution of larger body sizes.
The advent of mineralized hard parts was an important part of the Cambrian “explosion.” The ability of organisms to secrete hard parts had important consequences for both metazoan evolution, and for the preservability of these organisms in the fossil record. Much of the rapid increase in fossil diversity during the early Cambrian is among organisms with resistant hard parts. Changes in seawater chemistry may have played an important role in permitting or stimulating mineral precipitation by marine organisms. With the right concentrations of certain ions, normal physiological processes, such as respiration or photosynthesis, may cause precipitation. Such biomineralization could then be modified through natural selection. In addition, hard parts represent a handy way to store useful ions, or remove toxic ones. Carbonate and phosphate ions, present in most skeletons, are also good buffers against pH changes. Recent work on seawater chemistry during the latest Proterozoic and early Cambrian has indicated a major change in calcium ion concentrations between 544 and 515 million years.3 This time interval coincides with the onset of widespread biomineralization in the fossil record.
The rise of hard parts would likely have had important behavioral consequences. Hard skeletons provide firm attachments for muscles, enabling various activities and motions not otherwise possible, and skeletons would have helped to support larger and more complex bodies. Hard parts also would provide a protective armor against predators, and evidence for predation is found almost as early as the first skeletal elements appear in the fossil record. Predator-prey interactions seem particularly effective at producing an evolutionary escalation, with the prey evolving defenses and the predator evolving ways to overcome them. Animals with mineralized armor would promote selection for harder jaws and claws in the predators. The more effective predators would in turn increase selective pressure for more resistant skeletons in the prey.
Changes in animal behavior can also change the physical environment. A major environmental change in the early Cambrian came as a result of increased complexity and intensity of bioturbation (burrowing, digging, or other moving and mixing of the sediment by organisms). Burrowing can be a response to escaping predation or seeking out food resources. These evolving behaviors also disrupted the existing seafloor habitat. For much of the Precambrian and into the early Cambrian, microbial and algal mats largely covered the seafloor. These mats provided a stable base for sessile animals and kept mud out of the water, making it easy for filter feeders to obtain relatively high amounts of food and low amounts of sediment. The advent of algal grazers, extensive burrowing and other bioturbation disrupted these mats. This created problems for animals adapted to the old seafloor pattern, but provided a new habitat of muddy seafloors.4 Additionally, the constant burrowing unearthed buried nutrients, making them accessible to animals at the surface of the sediment.
Available food resources and ecological roles were also altered with the appearance of planktonic or swimming metazoans in the early Cambrian. Prior to the Early Cambrian, there is no evidence for macroscopic zooplankton or swimming animals. However, in the Cambrian several actively swimming, plankton-feeding animals appeared. At the same time, many kinds of planktonic algae became extinct and the surviving forms were much smaller. Evolution of swimming and plankton-feeding ability leads to the diversification of plankton feeders, but it also affects the bottom-dwelling organisms.5 Both the fecal material and the carcasses of these animals would have fallen to the bottom, moving large quantities of nutrients from the water column, where they were previously inaccessible to animals, to the sea floor. Even today, most of the nutrients in the deep sea come from these sources.
This brief survey of possible factors in the Cambrian explosion illustrates how ocean chemistry, environment, ecology and animal behavior are complexly intertwined. Complex positive and negative feedbacks make it very difficult to tease out which factor was most critical to the rapid diversification of metazoan life at the end of the Precambrian and early Cambrian. However, evidence from multiple sources strongly suggests that several significant changes in the world’s ocean environment conspired to light the fuse of evolutionary innovation.
Given our current, and continually growing, knowledge of the deep past, it is increasingly clear that the rise of multicellular animals is not an impenetrable mystery. While there is much that is not known, and will never be known, there is also much that has been discovered, and much excitement for what will yet be learned. The animals of the Cambrian did not appear in all their modern complexity out of a void, but rather provide pointers to their common ancestry. Despite the claims of evolution skeptics, the fossil record provides multiple examples of organisms displaying transitional anatomies. The anatomical characters that define the body plans of the major living animal phyla, can be seen to have been acquired piecemeal during the early evolution of the metazoa. Just as with all other taxonomic groups (e.g. classes, orders, families, genera, species), the divisions between phyla break down as we move closer to their times of origin from common ancestors. The tree of life continues to stand tall.
1. Hoffman, P.F. and D.P. Schrag, 2002, “The snowball Earth hypothesis: testing the limits of global change,” Terra Nova 14: 129-155.
2. Fike, D.A., J.P. Grotzinger, L.M. Pratt, and R.E. Summons, 2006, “Oxidation of the Ediacaran ocean,” Nature 444: 744-747.
3. Brennan, S.T., T.K. Lowenstein, and J. Horita, 2004, “Seawater chemistry and the advent of biocalcification,” Geology 32: 473-476.
4. Bottjer, D., J. Hagadorn, and S. Dornbos, 2000, “The Cambrian Substrate Revolution,” GSA Today 10 (9): 1-7. Dornbos, S.Q. and D.J. Bottjer, 2000, “Evolutionary paleoecology of the earliest echinoderms: Helicoplacoids and the Cambrian substrate revolution,” Geology 28: 839-842.