Of all the rapid evolutionary events we have explored in this blog , we have yet to look at the biggest and most consequential – the Cambrian Explosion. It is the big bang of evolutionary biology – the origin of animals. Within a period of 25 million years, all of the major body plans existing today (and many more that did not survive) evolved. Our view of the events occurring during the period from 555 to 530 million years ago is obscured by the fact that the shells and skeletons that make up most of the fossil record were only first evolving during that period. We can only infer what happened before hard-bodied fossils through genome comparisons of existing animals or from the occasional soft-body fossil bed, such as the Burgess Shale. A recent review of the topic estimated that before the explosion, there were a half dozen basic types of animal and by its conclusion there were five times as many. What is fascinating about this feverish period of experimentation in how to construct a multi-cellular organism is that it came to an end so quickly. All of today’s animals, including vertebrates have an ancestor with a similar body plan in the Cambrian. Another way of putting this is that in the last 500 million years there has been little innovation in the regulatory genes that knit multi-cellular organisms together from a fertilized egg.
A breath of fresh air
One of the reasons suggested for the rapid changes during the Cambrian period is an increase in oxygen. There is evidence of an increase in oxygen in the world’s oceans at this time. Though plant life had been producing oxygen for hundreds of millions of years by this time, something changed in its accumulation in the atmosphere – perhaps because plant matter was getting buried rather than decomposing, which consumes oxygen. There were also exposed mineral beds (such as iron ore) across the earth’s surface that could absorb any oxygen that the plants could produce. Once these were oxidized, the atmosphere and the oceans where life existed could change. Increased oxygen allows for an efficient metabolism for larger animals and the bursts of energy that are needed to be a predator. This is thought to have started an arms race where larger animals get better at catching prey and prey get good at protecting themselves with hard skeletons. Once this arms race had reached a certain intensity, it seems that evolution of the finer points of body structure (e.g. how many teeth, how thick a shell) continued but the opportunity to experiment with an entirely different body plan had been lost.
Louis Dollo, a 19th century paleontologist, proposed a law of irreversibility to help put this lost opportunity into context.
“an organism never returns exactly to a former state, even if it finds itself placed in conditions of existence identical to those in which it has previously lived … it always keeps some trace of the intermediate stages through which it has passed.”
I would like to add this rule to those that I proposed in my first post on this topic, which were:
- The McShea-Brandon Rule: Diversity tends to increase gradually over long periods of time.
- Wallace’s Rule: Diversity is concentrated in the most productive ecosystems, and
- Haldane’s Rule: Diversity is concentrated in particular groups of organisms.
To which I would add:
- Dollo’s Rule: Diversity is constrained by the structures and genes that evolution has to work with.
There are two aspects to this rule. The first is that it is statistically improbable that a random mutation will yield a physiologically viable individual, especially when it occurs in something as vital as the order of development in an embryo. The second is that the ecological context matters. During the transition to the Cambrian period, which was also the beginning of the Paleozoic era, there was less competition from existing body plans. Shallow seas may have been isolated from one another such that a new body plan could work well enough to survive and reproduce.
Perhaps without the development of the hard body fossilization that dominates the fossil record, the Cambrian period would not attract attention as a rapid evolutionary event. However, it is clear that the branching in the tree of life that ocurred during this time has left an indelible mark on what we understand as life on earth.