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Re: The Drunken Sailor's Stumble (was Re: March of "progress")




Tim,
Well, I think there can be some element of "dumb luck" in what survives and what doesn't, but Gould just overstates that as well. There was probably some element of dumb luck in what survived the K-T extinction. There were probably birds in North America that could have survived if the asteroid had smashed into Southern Africa instead. But most of the bird survivors were apparently in the far south, just because the asteroid impact was far enough away to spare them (so there was some element of dumb luck---how much is debatable), but the bird extinction would have been devastating even down there (perhaps fewer than a dozen bird species worldwide?).
There may well have been some kind of extinction event at the beginning of the Cambrian as well (I'm not saying that it was a total nonevent), but I still believe most of extant Metazoan phyla were present, and the explosion would be better termed a radiation of survivors (similar to K-T). In other words, the "explosion" or radiation would be at class, order and families levels, but most of the phyla were already in place (and not sure that there were that many phyla that went extinct near the beginning of the Cambrian). Even the extinct Phylum Petalonamae made it into the Cambrian.
Lots of vertebrate clades made it through K-T, and it looks like a lot of metazoan phyla made it through whatever happened at the beginning of the Cambrian. We are just less certain whether the early Cambrian event was caused by a fairly quick physical event (like K-T) or a more drawn-out biological event (like an arms race of hard parts), or a bit of both.
-------Ken
P.S. I would guess that even Simon Conway Morris would agree that there was some element of dumb luck----but that Gould overstated just *how* different things would have turned out if we "reran the tape". Gould has good ideas, but he just tends to exaggerate some factors and therefore somewhat neglecting others. On the other hand, I fear Woese not only exaggerates some ideas, but that such exaggerations are having a far more damaging impact on science (and Gould's influence does not worry me anywhere near that much). After all, misunderstanding the phylogeny of extinct Cambrian animals isn't going to have anywhere near the repercussions that could come from misunderstandings of living organisms like bacteria.
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The most troublesome is that the so-called Cambrian "explosion" was supposedly a phyletic explosion of body-plans, when it is more realistically regarded as an explosion of hard parts scattered here and there on an already well-diversified Metazoan tree.

I respectfully disagree: I think the Cambrian explosion was real, not an artifact of preservation. But Gould (in "Wonderful Life") tends to overstate the diversity of Cambrian forms; some "unique" forms are now believed to be members of hitherto known forms (e.g. _Hallucogenia_ is likely to be a spine-backed onychophoran, not a "phylum" unto itself.) Also, Gould's notion that certain Cambrian lineages survived while others didn't was due to chance (Dumb Luck) is a little difficult to swallow, in my very humble opinion.



Tim
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