If you read about natural history, even just popular accounts, you know about the epic mass extinctions visited on the living world throughout life's tenure on Earth. Words like 'devastating' and 'catastrophic' barely capture their scale—the most thorough purge is called The Great Dying and extinguished at least 95% of the species on the planet. And this isn't just animals dying. It's whole lineages dying. As our host writes, these things are "extremely nasty and most definitely to be avoided."
The third myth in From Extraterrestrials to Animal Minds: Six Myths of Evolution by Simon Conway Morris, the subject of Chapter 3, is "The Myth of Mass Extinctions." The chapter is long and detailed, clogged with taxonomic jargon in places, sparkling with typical mischief in others. (Page 76: "Not for a moment am I suggesting that the early Triassic is a preferred destination for the time traveler. Your tour group would be greeted by shocking scenes, but the cognoscenti would see which way the wind was blowing.") By now we know we have to ask: what exactly is the myth? Conway Morris isn't questioning the occurrence of mass extinctions, nor does he doubt their apocalyptic power. He's attacking a somewhat specific view of their effects on the biosphere.
That view is most commonly associated with Stephen Jay Gould and is linked (inextricably, in my opinion) to Gould's preference for the contingent and the "random" as major forces in the unfolding of life. The Gouldian view is something like this: mass extinctions redirect evolution, pushing life in new directions that would not have been pursued without the cataclysmic erasure of lineages and ecosystems. This is the overly famous "replay the tape of life" metaphor, now focused on what Gould saw as random reboots caused by catastrophes. And the upshot that matters most to Conway Morris is this one: if the asteroid zigs instead of zags and misses our blue planet, then the dinosaurs still "rule the earth" while the mammals cower in the bushes and get eaten. And that means the mammals don't take over and evolve along lineages that lead to humans. Put more precisely and technically: major radiations of new species/lineages, like the one that led to us, wouldn't have happened without the cataclysms.
My retelling doesn't do justice to this position, which is not a silly fable but a hard-nosed scientific view based, at least until fairly recently, on an informed reading of natural history. Conway Morris acknowledges this, though perhaps not clearly enough. His view is that yes, there were great dyings, and yes, these erased lineages and whole ecosystems, and yes, these dyings were followed by radiations of new lineages. But these radiations were going to happen anyway, and once the smoke cleared and a bazillion dead animals returned to the dust, the effect of the cataclysm was to speed up the process.
Conway Morris marshals considerable evidence making this case. The chapter is convincing, partly because it is well organized, partly because there really is a myth to be opposed and corrected, and (I suspect) partly because this is the author's area of legendary expertise. Versions of the myth, by the way, cause annoyance and distraction in other paleontologists as well.
I do think there is a weakness in his argument: a lineage that has been erased is a lineage whose prior accomplishments no longer count. This matters because Conway Morris rests his case on the fact that innovations that occurred after a cataclysm, during a post-apocalyptic radiation, nearly always existed before the apocalypse. I agree with him that this suggests that the Gouldian view is largely wrong and that dyings lead to birthings and expansions that are not "random," but we have to acknowledge that a complete extinction of a vast lineage can determine winners and losers by destroying whole clubs of future potential winners.
But I liked the chapter a lot and was much less grumpy about the money I spent on the book when I finished Chapter 3. The biology is fascinating, the story is compelling, and the need for correction and rebuttal (of Gouldian replaying of tapes and such) is clear. Plus, the author's cheekiness works well for the first time in the book. Here's a favorite passage, on the end-Cretaceous extinction event that ended the dinos:
Some groups seemed to have looked up and said, "Oh, another mass exctinction?" and carried on. Spiders, for example, continued to spin and lice to infest. Meanwhile out in the oceans, the pelagic fish swam on unconcerned.
Humor in mass extinction? I like it, even if Gary Larson was doing it decades ago.
Image credit: Artistic impression of the asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatán Peninsula in what is today Southeast Mexico. From Wikipedia.
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