August 2, 2006

Walcott and the Cambrian Explosion


Tangled Bank # 59 is just itchin' to be read at Science and Reason.

My highly subjective pick of the bunch: A look at the Cambrian explosion from Rigor Vitae:

Everyone's talking about the Cambrian Explosion, it seems. While evolutionary biologists discuss the actual rate of adaptive radiation, a handful of creationists have hailed it as proof of...something...I forget exactly what. The Cambrian Explosion was, of course, that event from half a billion years ago, when suddenly—well, relatively suddenly—over the course of about 8 million years, a great diversity of metazoan life appeared on Earth, with representatives of most of the phyla that have ever turned up on the planet. For most of the preceding 3 ½ billion years, life on Earth had been unicellular. The cause of this explosion is poorly understood. Is it an illusion suggested by gaps in our knowledge of the fossil record? Was it caused by climatic or geological changes, or the evolution of Hox genes or sexual reproduction? Or was it simply a response to opportunities that could only be exploited by multicellular life?

I really just can't get enough of this stuff. By far, deep history is one of the most interesting topics in science precisely because of all the mystery. It's like putting a giant world-sized puzzle together that not only gives you the satisfaction of completion, but also a notion of where you and I came from.

Back in 1909, while riding along a mountain trail in British Columbia, then Secretary of Smithsonian Charles Doolittle Walcott (an ironic middle name to be sure) found a gold mine of paleontological evidence: an outcropping of rock filled to the brim with fossil organisms, some 60,000 organisms all told.

Some 140 different species of all shapes and sizes - some familiar, some completely alien - where excavated from the site, now known as the Burgess Shale, and ended up in the American Museum of Natural History. Walcott had lifetimes of research in his hands, one of the most significant discoveries in history; unfortunately, when the time came, he - and after his death, subsequent researchers in the 1970's - described many of the fossils inaccurately.

Stephen Jay Gould criticized Walcott for placing the Burgess fossils into modern groups, giving extant invertebrates ancestors that are over 500 million year old. He argued that the Burgess fossils showed how evolution was very different in the Cambrian, full of striking new body plans and "false starts."

For this assertion, Gould invoked the wrath of Richard Dawkins:

It is as though a gardener looked at an oak tree and remarked, wonderingly: 'Isn't it strange that no major new boughs have appeared on this tree for many years? These days all the new growth appears to be at the twig level.'

But, as science would have it, perceptions change with new evidence, and Walcott would not end up being so terribly wrong in the end.

Wishing to end the controversy, Richard Fortey went back through all of the Burgess fossils that were deemed unique phyla and compared them using modern cladistical analysis. It turns out that enormity of strange body plans was carelessly emphasized during research in the 1970's, to the point that some fossils were described upside-down.

As Fortey says in his book Life, "there are relatively few Cambrian designs that are wholly novel. Most often they turn out to be just interesting elaborations of well-established designs."

1 comment:

  1. Nice blog.
    Loving the burgess shale story.
    I linked to it at
    http://www.bayblab.blogspot.com
    thanks for the good stuff.

    ReplyDelete