About two weeks ago I dropped in on a freshman Biology class on a college visit. The professor was giving a lecture on something called “The Cambrian Explosion”, something I’d never heard of. He described the “explosion” as an evolutionary arms race, a period in which the evolutionary process; the diversification of organisms accelerated from 0 to 60 in a “relative blink.” Needless to say, the lecture piqued my interest, and so I searched the web for some more insight on the curious “explosion.” I found a great article from the innovative science blog nautil.us by Brooke Borel called “The Greatest Animal War”. Borel’s article gave me a better idea of what the Cambrian Explosion was all about.
Pictured is a Cambrian Age fossil of Anomalocaris canadensis from the Burgess Shale Formation. The fossil preserves the mouthpiece of the organism in great detail, and the mouthpiece itself serves as evidence of useful trait acquired during the Cambrian Explosion.
Above is the grasping claw of the very same Anamolacris canadensis from the Burgess Shale. The claw is just another evolutionary tool used by the organism to predate and survive.
Scientists interpret the earliest fossil records (which date from about 580 million years ago) as evidence that early organisms were small, simple, and… soft. The fossil record shows these early life forms to be defenseless and apparently harmless, lacking the “claws, teeth, limbs, or brains” that are characteristic of competitive, modern organisms (organisms that require these features to survive.) Mysteriously, around roughly 542 million years ago evolution sped up, and so began what Borel describes as a “period of unprecedented one-upmanship.” Organisms developed all sorts of mechanisms from eyes; to spikes in order compete with their rapidly diversifying and evolving contemporaries. According to the nautilus article “most of today’s 30 to 40 animal phyla originated in the Cambrian, and have persisted through time with hundreds of variations on a theme.” Meaning, that the majority of the ancestors of modern organisms all developed in this 54 million year span.
Now where the University Professor and Borel diverged was on the answer to the questions of “what caused this evolutionary arms race? and “Why did it take an extra billion and a half years for the first eukaryotic organisms to arrive?” The professor was reluctant to commit to any theory that attempted to answer those questions, having to repeat the phrase “well, we just can’t know for sure,” in response to questions throughout the lecture. Borel however is more definite in her answer, stating “certainly, the environment around the time of the Cambrian encouraged the explosion as well. Oxygen had accumulated in the oceans after extreme ice ages… with plentiful oxygen, animals could grow large and absorb the air they needed to breathe through their skin.” The lack of a definitive answer as to why the Cambrian Explosion occurred hardly undermines its significance as an unprecedented explosion of diversity that has never been repeated in Earth’s history. The mystery shrouding its roots makes it all the more curious and exciting a topic to read about.