Thursday, October 03, 2013

Contingency vs. constraint

Well, hello. Who's this? Funky little animal. Yes, an animal, in spite of that stalk, which is more foot than root. It lived in mid-Cambrian seas, about 500 million years ago. Nothing quite like it exists today (click to enlarge).

Herpetogaster collinsi. Heretogaster means "creeping stomach." That's pretty much what the animal is. A mouth, a stomach, and an anus. The elaborate tentacles are possibly used to bring food to the mouth, but who knows.

Let me say at once that this beautiful imagining of the animal from its fossils is by the artist Quade Paul, and is reproduced in a 7 June review in Science of Douglas Erwin and James Valentine's The Cambrian Explosion: The Construction of Animal Diversity, which Paul illustrated. The beasty was small enough to hold in the palm of your hand, if you don't mind holding a creeping stomach.

The Cambrian was that period in Earth history when quite suddenly (in geologic time) a burgeoning of multi-celled animals appeared, with diverse body plans, including ancestors of all species extant today. No one is quite sure what caused the "explosion," but there you have it. Not all of the Cambrian novelty gave rise to present-day descendants. Survival or extinction was a bit of a crap shoot.

How much of a crap shoot recalls the divergence of opinion some years ago between Stephen Jay Gould and Simon Conway Morris. Gould emphasized the iffyness of evolution, and suggested that if the tape of life on Earth were replayed, the world would look very different today, including the likelihood that we would not be here. Morris chose to stress the constraints of environment and the ubiquity of evolutionary convergence to drive natural selection along a narrowly constricted path, leading in every replaying of the tape to a world broadly similar to the one we inhabit today, including (perhaps) a mouth-stomach-anus that is bipedal, impressively intelligent, self-conscious and zips about in jet airplanes. Read Gould's Wonderful Life and Morris's The Crucible of Creation and decide for yourself.