The Why Files The Why Files --

Evolution: The never-ending story

Terrific trilobites

Evolutionary biologists believe that the processes of evolution must explain not just changes within organisms, but also major trends in the history of life. So what's with the "Cambrian explosion," when, about 520 million years ago, multicellular animals appeared in the fossil record and quickly diversified into most of the major body patterns seen today?

Mark Webster, assistant professor of paleobiology at the University of Chicago, has long been fascinated with trilobites, three-sectioned animals that persisted for 270 million years before finally kicking the bucket during the gigantic wave of extinction 251 million years ago.

The fossil looks like a large black tick carved in relief. Webster has glasses and a goatee.The fossil looks like a large black tick carved in relief. Webster has glasses and a goatee.
Photo: Dan Dry
Mark Webster of the University of Chicago with a 500-million-year-old trilobite from Nevada. Webster has found that early species of trilobites contained more diversity than later species.

Trilobites made a living as scavengers and predators, inhabiting both the sea floor and the ocean above. Although no freshwater species are known, evolution gave the 20,000 species of trilobites many ways to cover the marine waterfront.

Trilobites also made good and abundant fossils, Webster adds, making them an ideal vehicle for studying long-term, large-scale evolutionary trends.

Paleontologists thought that trilobites evolved rapidly at first, Webster says. "It's been known for a long time that trilobites seemed to evolve and diversify rapidly early in their history," with higher rates of evolution and speciation.

This makes sense, Webster says, because "Variation within a species is the raw material that natural selection operates on." A species that starts with more variation will be better prepared for the changing challenges of its environment, Webster says. "If some individuals are quite distinct in shape, the proverbial eggs of that species don't all lie in one basket, and in the face of selective pressure, some individual may be able to adapt, so it can evolve more rapidly."

A man with glasses holds plastic models of the shelled trilobite and the spiky wiwaxia side-by-side.
Photo: Fermilab
"I shall not be lunch!" says wiwaxia, right. Physicist David Boehnlein with plastic models of two prehistoric creatures, trilobite and wiwaxia. Wiwaxia developed a spiny back to resist predation from trilobites more than 500 million years ago.

Is it true?

But plausible does not equal true, and it turned out that nobody had looked systematically at the question of within-species diversity, so Webster measured the amount of variation within individual species by poring over reports on almost 1,000 trilobite species. "I found, yes, the degree of variation within species in the Cambrian period was significantly higher than it was later on" Webster told us (see #2 in the bibliography).

Webster suggests two possible causes for this "bottom-heavy" diversity among individual species:

The ecospace hypothesis: Early in a group's evolutionary history, competition for resources was probably low, so a species could survive even if some individuals were not perfectly tuned to the niche. "But through time, as the ecospace filled, and competition for resources increased, it had to be increasingly fine tuned to its particular niche, and the variation within that species declined because the variants could not compete so well" with more specialized organisms, Webster says. (The ecospace hypothesis explains adaptive radiation, in which many species descend from a single ancestor after it occupies an empty ecospace.)

The genomic hypothesis: As primitive animals develop from egg into adult, "the genetics that control development might not be as tightly integrated, so all body parts are growing essentially independently from each other," says Webster. But as organisms specialize to survive greater competition, "their developmental and genetic systems might become more tightly integrated, and it becomes much harder to change one part of the body without changing others," so evolutionary change would slow down.

Understanding the variation of trilobite diversity could shed light on larger evolutionary changes, Webster says, because trilobites are "the Cambrian explosion in miniature: rapid initial diversification, very high rates of evolution. If we can work out why trilobites had this high diversity, we can probably work out by extrapolation why it happened for other animals."

Chatting with Charles: What would you say to Darwin?

Mark Webster:

"Support for evolution only becomes stronger with time. One of the big problems for Darwin was the Cambrian explosion, where we went from having essentially no animals to having representatives of most or all major animal groups in a very short time. How did this massive explosion of body plans arise? We are beginning to get a handle on this, in terms of bottom-heavy diversification. I think in the next few years we will be close to having a solution to one of Darwin's dilemmas."


Did whales once paddle with their feet?

Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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