The behavior of an explosion

The Cambrian "explosion," the enigmatic phenomenon in which many of the phyla existing on the planet appeared in a relatively short period of time (at least 20 million years), remains a difficult event to study. Fossils are rare, intricate, and often represent creatures that are difficult to fit into one group or another. There are fossils of earlier creatures (and there may have even been an earlier, Ediacaran "explosion"), but at present it is the Cambrian event that is the most famous radiation of diverse forms of life. A relatively recent paper published in the Annual Review of Earth and Planetary Sciences takes a look at recent hypotheses as to the causes of the Cambrian radiation, but deciphering what occurred about 530 million years ago is no easy task;

Part of the difficulty in developing an explanation for the Cambrian "explosion" also stems from the fact that it is an unfolding in history; each successive step not only flows from the conditions established in the previous steps but also is shaped by interactions at the current step of unfolding. So in some sense there cannot be a simple explanation for the Cambrian "explosion" that is fully satisfying; understanding the nature of causation of complex historical events is difficult.

Part of what makes the Cambrian radiation so difficult to study is the question of what was not preserved. Many of the animals indicative of the Cambrian had developed hard shells or other traits that allowed for better preservation (although in some of the most famous localities like Chengjiang and the Burgess Shale the way in which the creatures were killed and buried preserves soft anatomy, as well), hence earlier creatures that had not yet developed hard body parts might be entirely missing from the fossil record. The "roots" of the Cambrian radiation extend back before the age of the Chengjiang fauna, dated at about 520 million years old, but the details of the beginnings of the radiation that is more recognizable in the Cambrian are still mysterious. As in other areas of biology, there is a bit of a "rocks vs. clocks" debate being that researchers using molecular clocks have proposed and even earlier divergence of the first representatives of animal phyla, but such studies must be calibrated to the fossil record and disagreements will likely continue until better paleontological resolution can be achieved.

Although more is desired of the fossil record, it is still clear that within a short amount of time there was a fast radiation of forms, and while species diversity might not have been very high until later (i.e. the Ordovician), the disparity between phyla was already substantial. Disparity, simply speaking, can be simply illustrated by considering that you've got much more in common with a mouse than a moon jelly or a sea sponge, the latter two organisms representing vastly different forms (even in relation to each other) than the two mammals. The trigger for this rise in disparity is still in debate, but any explanation will have to account for ecology, development, genetics, and morphology (at least). At present it seems like the study of development may be the most promising avenue, and there's a theme that runs through many considerations of the Cambrian that in the past it was "easier" to give rise to diverse forms based upon changes to development that it is among extant organisms that are highly derived;

... as evolution proceeds, the developmental stages responsible for laying down the body plans are overlain with down-stream genetic pathways, making it progressively harder to modify those earlier pathways; the developmental stages when the phylum-level body plans are laid down become entrenched and the body plans become frozen in place. This is a very appealing idea, but we still have a lot to learn about how encumbered the gene networks responsible for morphogenesis really are, and it is quite possible that they are not as encumbered as one might think.

Indeed, the study of development may lend substantial clues as to how variation arose during/antecedent to the Cambrian, but why those changes should have become established still must be addressed. This is where an understanding of ecology can be crucial, especially since the Cambrian organisms were likely moving into new niche spaces that may not have previously been occupied. As organisms interacted with each other, predator and prey relationships would have become established and evolutionary "arms races" initiated, the filling of niches by new creatures thus presenting something of a block to the development of striking new body plans (new variations would not be given much freedom to develop and enter a niche space; such creatures may have been gobbled up or out-competed by creatures already existing). After the Cretaceous extinction, for instance, mammals were in a world wide-open to them and experienced a massive radiation as they could quickly enter many of the niches left open by dinosaurs. No new phylum or major group of land creatures appeared because (in addition to a likely lack of novelty) the spaces previously taken up were quickly being filled, so variation and ecology are constantly working together based open the contingencies of extinction and open niches that can be filled.

Unfortunately there are many more questions than answers about the Cambrian radiation at this point, and many creatures remain enigmatic. The author notes that presently there doesn't seem to be evidence that a greater number of phyla emerged but became extinct (covered in an image-rich, text-poor post over at the Panda's Thumb), but the inability of present researchers to confidently identify a number of Cambrian organisms (and earlier Ediacaran organisms) still has some bearings on the more philosophical debate about contingency and "directed" evolution.

The debate, which has cooled off a bit since the death of Stephen Jay Gould, is directly impacted by questions of why some phyla give rise to a greater diversification within its ranks and why some seem to have only a single representative, like the phylum Cycliophora which is represented only by the genus Symbion (a microscopic creature found on a Norway lobster a few years ago). While there has not been a radiation of phyla like what is recognized during the Cambrian, can we say for sure that phyla have emerged and become extinct in the past? It could very well be so, but we may never be able to be certain. Microscopic representatives of unknown phyla like Symbion may never have been preserved in the fossil record and may never be found, and in a world full of life it might be difficult for a new variety of organism to find an ecological foothold.

The Cambrian radiation, when looked at as a whole, represents a unique (but not isolated) evolutionary event that gave rise to the early members of living phyla and set up later radiations/diversifications of life like the one that occurred during the Ordovician. The reasons why disparity was more marked over diversity and why no new large-bodied phyla have emerged since the Cambrian are very important to our understanding of how life on earth evolved, and even though the resources might not always be what we might desire, the debate over the fossils will continue to be rich.

[Hat-tip to Evolving Thoughts]

Marshall, C.R. (2006). EXPLAINING THE CAMBRIAN "EXPLOSION" OF ANIMALS. Annual Review of Earth and Planetary Sciences, 34(1), 355-384. DOI: 10.1146/annurev.earth.33.031504.103001