An Explosion Defused?

Discover, December 1, 1996

From Darwin’s time to today, one group of fossils–those of the early Cambrian Period, beginning 540 million years ago–have baffled researchers. Before the Cambrian, fossils tell us, there existed only a few types of simple, blobby animals. Then, within just 10 million years, almost every major group, or phylum, of animals appeared. Almost all the major body plans seem to have sprung into existence–including phyla such as arthropods (which came to include insects and crustaceans), annelids (leeches and worms), mollusks (squid and clams), echinoderms (starfish and sea urchins), and chordates (fish and humans). Now dubbed the Cambrian explosion, it’s looked upon as the great event in animal evolution–and, in its suddenness, one of evolution’s greatest mysteries.

Yet some scientists have long found such an explosion hard to believe. They share Darwin’s own suspicion that, for some reason, we haven’t yet found fossils of pre-Cambrian chordates, arthropods, and other phyla that are still hiding in the ground. And those skeptics will find some encouragement in a new study. After examining the genes of animals with lineages supposedly originating in the Cambrian explosion, a team of biologists now concludes that there was no explosion at all.

Jeffrey Levinton, a marine ecologist at the State University of New York at Stony Brook, working with Gregory Wray and Leo Shapiro, both also from Stony Brook, compared seven genes from mollusks, annelids, arthropods, echinoderms, and chordates. The degree of similarity in the dna sequences can tell researchers just how closely one phylum is related to another.

A few such studies have been done in the past 15 years, but they all used single genes because computers were so slow, and so few genes had been sequenced. These days, though, labs are speed-reading genes and promptly putting their results on the Internet. You don’t have to have any friends to get the sequences now–you just download them from the databases, says Levinton.

The researchers compared the sequences of each gene, calculating how far those of one phylum had diverged from the others. Not surprisingly, they found that the echinoderms and chordates were more closely related to each other than to the other phyla–a result that agrees with decades of research into anatomy. What was surprising was just how closely related the chordates and echinoderms were compared with the other three.

Their closeness was brought home when the researchers constructed a molecular clock. They built it by looking at the fossil record for the oldest member of certain living groups of animals. Then they looked at how much variation exists in a gene among each group, which gave them a good sense of how much that gene changed per million years. (They mainly used vertebrate fossils, since vertebrates have such a good fossil record.) When they used this clock on the splitting of the phyla, they found that chordates and echinoderms branched away from arthopods, annelids, and mollusks around 1.2 billion years ago. Then, 1 billion years ago, echinoderms and chordates went their separate ways. In other words, the genes suggest that a leisurely paced and comfortably spaced course of evolution was well under way half a billion years before the putative Cambrian explosion.

There’s a quick riposte to such work: genetic clocks are very unreliable. But Levinton turns this weakness into a virtue. It’s not a Rolex or a Timex, he says. I think of it more like a sundial on a cloudy day. If the Cambrian explosion happened in a flash, then the branching of the different phyla would have been lost in the fuzziness of this analysis. In other words, the phyla wouldn’t have clumped together so consistently the way they do. Only if they took hundreds of millions of years to diverge could such a result be possible.

While some paleontologists might despair over these results, to Levinton they point to hundreds of millions of years’ worth of fossils yet to be discovered: I’ve been around long enough to know that amazing things get discovered; I’m not troubled by the fact that no one’s discovered them yet.