The Cambrian Explosion: Tooth and Claw
Scientists ponder the causes of the Cambrian explosion.
Something quite bizarre happened at the end of the Precambrian Era. Rocks from that time show evidence of an astounding variety of multicelled and hard-shelled life forms that seemingly appeared all at once. Scientists have long pondered the causes of this sudden appearance of new life forms, known as the Cambrian explosion. One widely held explanation is that dramatic geologic and climatic events created new environmental niches for life to explore with evolutionary adaptations.
|A Cambrian seafloor in what is now Field, British Columbia. The shell-shaped, blue organisms at far left are Eldonia. Credit: Univ. of Michigan Exhitbit Museum of Natural History|
Many Earth scientists back the idea that the Earth was repeatedly encased in ice and, in between, melted by catastrophic greenhouse episodes, in the time period from 750 to about 600 million years ago. The freeze periods are often referred to as snowball Earth" episodes. It has been suggested that the last of these freeze-thaw cycles may have spurred the development of the profusion of new body forms seen in the Cambrian explosion. But Canadian paleontologist Nicholas Butterfield doesn’t think that geology and climate alone could have done the trick. According to Butterfield, the bony and shelly quality that life took on, and life’s new-found ability to crawl and burrow, were the direct result of two other developments: sex and violence. Until sexual reproduction and violence (in the form of predation) appeared, Butterfield says, life was constrained to a very simple world, defined largely by the physical environment. That’s the Precambrian world." Butterfield argues against direct effects of the so-called snowball Earth scenario in setting the stage for diverse species with a variety of body plans. Instead, he says, it was the inevitable encounters of the predator-prey relationship that provided an inexhaustible source of new environments." At the advent of the Cambrian, which led to most of the major groups of animals still around today, the fossil record suddenly reveals wonderful beasts, with claws and shells," Butterfield says. These defining features all [have to] do with other organisms." Suddenly, to survive, creatures have to move, have to see – what they would like to eat, and what would like to eat them." Such variation, Butterfield says, cannot begin to exist without selective pressure.
"I’ve made the claim that [complex life] started with the onset of sexual reproduction," Butterfield continues. I’ve got some 1200 million year old fossils of red algae. It’s a very clear cut case of the first fossil evidence of sex on Earth," (Paleobiology, vol. 26, p. 386, 2000). He says the Cambrian explosion is simply the exponential part of a curve whose origin is at the advent of this humble form of sexual life. The case for sex in the algae is made by two distinct types of the plants presented by the record: one producing lots of small spores, the males, and one producing a few, larger spores, the females. "The details of the cell division are very similar to that seen in the living relative Bangia," Butterfield says. "I don’t know what caused sex in the first place," he adds, "but one possible explanation for why it persisted was that it allowed the evolution of multicellular organisms. There was a "niche" in the Proterzoic world for large multicellular organisms that had yet to be exploited." And when the multicellular organisms appear on the stage, "this sets up a positive feedback loop whereby organisms respond to new environments" presented by other organisms, "by evolving new morphologies, and those new morphologies introduce new environments which induce newer morphologies.
Multicellular organisms can do this," Butterfield says, because there is an essentially inexhaustible supply of new morphology: size, shape, and, by extension, behavior. Asexual eukaryotes and prokaryotes simply can’t play this game, because they can’t build differentiated multicellular structures."
|A Cambrian eurypterid chases trilobites. Beneath, the legged worm-like animal is Aysheaia pedunculata, an onycophoran. Credit: Univ. of Michigan Exhitbit Museum of Natural History|
The last episode of snowball Earth occurred 50 million years before the Cambrian explosion, says Paul Hoffman of Harvard, a proponent of the snowball Earth hypothesis. Hoffman agrees that, by themselves, snowball Earth conditions and the thaw that followed are not an adequate explanation" for the Cambrian’s burgeoning life. But, he says, the earlier appearance of multicelled soft-bodied life, 575 million years ago, could be linked to the last of the deep freezes. The most recent of the snowball glaciations, at 590 to 580 million years ago, is a pretty close conjunction, in the broad sweep of Earth history," to the appearance of the so-called Ediacarans in the fossil record, 575 million years ago. Interactions among these creatures could then have led to the Cambrian explosion, Hoffman says.
Bruce Runnegar, director of the Center for Astrobiology at UCLA and an expert on the fossil record at the Cambrian threshold, says that Butterfield’s argument, that ecological complexity in and of itself fueled further complexity, is a hard hypothesis to disagree with, but how would you test it? We don’t understand the phenomenon well enough to have one unique explanation."
|Paul F. Hoffman and Daniel P. Schrag. Credit: Paul F. Hoffman and Daniel P. Schrag|
Runnegar notes that mass extinction of the ruling reptiles removed the constraint on mammalian radiation: mammals could not diversify until the dinosaurs were cleared from the scene. Perhaps, he suggests, just prior to the Cambrian explosion, there was some constraint on early life that was lifted as well.
Butterfield has collected more hard data than anyone else, but it’s still a rather small [sample] of the biosphere, because the fossil record is so incomplete," Runnegar says. Runnegar and colleagues are approaching the problem from a different angle, working to date key nodes in the tree of life, using molecular clocks and the fossil record, as well as models of early Earth’s climate and carbon cycles. Butterfield emphasizes that molecular clocks and the fossil record give conflicting dates for divergence of species. Resolving that conflict and improving our understanding of the conditions that existed on early Earth would aid in "understanding the processes by which [the Ediacaran fossils] came to be preserved simply because that will help to resolve the nature of their tissues and overall constitution," says Butterfield.
There is far too much story-telling going on based on shapes in rocks," he says. Geologists need a better understanding of how these imprints were formed. There are advances being made on this front, and it is this kind of understanding that will eventually lead to their correct interpretation."
Too many geoscientists, he adds, still leave good old fashioned Darwinian evolution out of the equation, and I think there is much to be learned by applying what we know about ecosystem structure, theory and assembly" to interpreting the hard records of Earth’s early history.