Pharyngula

The Cambrian as an evolutionary exemplar

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I’ve been reading Valentine’s On the Origin of Phyla(amzn/b&n/abe/pwll) lately, and I have to tell you, it’s a hard slog. This is one of those extremely information-dense science texts that rather gracelessly hammers you with the data and difficult concepts on page after page. I am convinced that James W. Valentine is ten times smarter than I am and knows ten thousand times as much, and it’s a struggle to squeeze that volume of knowledge into my miniscule brain pan.

One thing I would like to greatly condense and simplify is his discussion of the Cambrian ‘explosion’. Misinterpretation of the Cambrian is one of the many prongs of the creationist assault on science; both old school Biblical creationists and the new stealth creationists of the ID movement have seized upon it as evidence of an abrupt creation—that a Designer poofed the precursors to all modern forms into existence suddenly, and without precursors, and that this observation contradicts evolutionary theory.

It doesn’t. Valentine has an excellent diagram that shows how wrong the creationists are.

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Let’s look at this from bottom to top, from oldest to youngest. There are two lessons here: one is that the Cambrian was a real transition event, but the other is that it looks remarkably natural and progressive—something best explained by material phenomena and not unsupported speculation about mysterious and invisible Designers.

Roughly 570-600 million years ago, fossils are sparse, but they include the phosphatized embryos of the Doushantuo formation in China and a scattering of trace fossils. Trace fossils are the remains of trackways and burrows, not the animals themselves, and tell us that there were small soft-bodied and multicellular animals living on the substrate; we even have a few fossils of more elaborate, bilaterally-symmetric animals, comparable to flatworms.

Here are some of these early trace fossils; they are small squiggles in the sediment, the faintest signs of living creatures once having crawled there.

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Near the end of the Neoproterozoic, the larger and more complex and enigmatic Vendian and Ediacaran fossils turn up. There are also more and more complicated trace fossils. Animals are getting larger and making more substantial trackways; in addition, they’re beginning to burrow down into the sediment. We begin to see signs of a phenomenon called bioturbation, where the substrate is stirred and turned over by animal activity, which was absent before.

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Another important feature begins to make its appearance: the small shelly fossils. These are little guys, only about 1 or 2mm across. The kings of creation at this time were scattered beasties the size of a baby’s toenail, but still, it was a step upward in size and durability from what had come before.

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The Cambrian itself begins 543 million years ago, and is broken up into periods several millions of years in length with their own distribution of fossils. The oldest, the Manykaian, is marked by more trace fossils, and a greater diversity of the small shelly fossils; the diagnostic fossil whose appearance is used to mark the beginning of the period is a trace fossil, the relatively large burrows of Treptichnus pedum. In the Tommotian, 530 million years ago, the first recognizable brachiopods and molluscs are found, and there are trace fossils that indicate something with many legs scurried by—the first arthropods. The first actual fossils of arthropods and echinoderms are found millions of years later.

It’s more than ten million years later that the spectacular and strange animals of the Burgess Shale make their appearance. It’s during the Middle Cambrian that we can say most of the modern phyla are present, although of course the representatives of those phyla don’t look much at all like their modern members.

One message from these data is that the Cambrian ‘explosion’ was real. It isn’t an artifact of poor sampling of ancient rocks—we have a range of good fossils from the period before, and it’s clear that the pre-Cambrian world was a different place than the post-Cambrian.

But another important lesson, and one that creationists like to hide, is that while this was a sudden event in a geological sense, it wasn’t actually all that rapid in human terms. The evolution of the canonical Cambrian forms was drawn out over tens of millions of years. They didn’t just come out of nowhere, either; while individual lineages are cryptic, we see a slow aggregate increase in the complexity of multicellular animals in the fossil record that culminated in the flowering of large-animal diversity in the Cambrian.

I’ve had many creationists try to use the Argument from the Cambrian Explosion as a fait accompli against evolution (most recently, just this week). It’s actually an argument from ignorance, though, since the data certainly does not fit a sudden creation by divine or alien fiat. It does fit with the idea of the appearance of these animals as a product of prior history, though…even though there are many mysteries about the details, the big picture does not require miracles or the supernatural.

Comments

  1. #1 Mytho
    January 27, 2008

    Biology is not my fiel of expertise, sometimes it’s a hard subject on it’s own merit, although very interesting nonetheless. After reading this exemplary article, I can only conclude that I’ve missed the best part of nature by neglecting a throughout study of this science in particular.

    Interesting, revealing, marvelous. What else can I say?

  2. #2 Mytho
    January 27, 2008

    Biology is not my fiel of expertise, sometimes it’s a hard subject on it’s own merit, although very interesting nonetheless. After reading this exemplary article, I can only conclude that I’ve missed the best part of nature by neglecting a throughout study of this science in particular.

    Interesting, revealing, marvelous. What else can I say?

  3. #3 Johannes Lochmann
    March 3, 2008

    Another very interesting one of my professors tended to stress is that before there could happen cellular (and multicellular, by that matter) evolution a phase of chemical and molecular evulutionary phase was required. So his guess was that at least 2 of the 3 billion years before the first known fossil traces where certainly free of cellular life and filled with all sorts of molecules engaged in strictly darwinian evolution in the sense of “how do I protect myself from reacting with my fellow molecular comrades and, after some time, that evil oxygen thingy!”, as he used to put it.

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