“Challenging Fossil of a Little Fish”    (page 2)
   - May 30, 2000

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The Boston Globe  -  Health/Science

Challenging Fossil of a Little Fish  (continued). . .

Because new animal groups did not continue to appear after the Cambrian explosion 530 millions years ago he believes that a unique kind of evolution was going on in Cambrian seas. And, because his years of examining rocks from before the Cambrian period has not turned up viable ancestors for the Cambrian animal groups, he concludes that their evolution must have happened quickly, within a mere two or three million years.

According to Chen, the two main forces of evolution espoused by neo-Darwinism, natural selection (“survival of the fittest”) and random genetic mutation, cannot account for the sudden emergence of so many complex genetic forms.

“Harmony can be a driving force (of evolution), too,” Chen proposed at the Chengjiang conference.

As if to underscore the abruptness of Haikouella’s place in the fossil record, Chen pointed out the features that make Haikouella look so much more advanced than expected for an early Cambrian animal.

Biologists had been expecting to see something that would look like a primitive ancestor to the middle Cambrian animal called Pikaia, formerly promoted as the world’s earliest chordate. Rather than finding evidence that Pikaia had a less complex ancestor, Chen instead found a chordate that already displayed many vertebrate characteristics fifteen million years earlier.

And some of the over 300 fossil specimens Chen’s team has recovered are so well preserved that paleontologists practically swoon over them.

“They’re almost like a photograph of the anatomy of the animals,” said French paleontologist Philippe Janvier.

But all this newfound clarity only adds to the larger problem, framed succinctly by Holland of Scripps Institution: “Where the hell are you going to get an animal like that?” In his view, Haikouella’s high level of development makes it more difficult to explain the evolutionary steps that produced it.

The place to find steps to Cambrian animals, of course, should be among the Precambrian rocks. Darwin wrote that, if his theory is true, then the world must have been swarming with the ancestors of the Cambrian critters during long ages before them. He expected future generations to find them.

Today, paleontologists still lack viable ancestors for the Cambrian’s forty or more animal phyla. Most researchers explain this by assuming that Precambrian animals were simply too small or too soft to leave a fossil record, or that conditions were unfavorable to fossilization.

But for the last three years, Chen’s discoveries at Precambrian fossil sites with Taiwanese biologist Chia-Wei Li have magnified this mystery. While sifting through the debris of a phosphate mining site, Chen and Li eventually discovered the earliest clear fossils of multi-cellular animals. They found sponges and tiny sponge embryos by the thousands — but nothing resembling the fish-like Haikouella or forerunners of other Cambrian creatures such as trilobites.

When word of the discovery got out, Chen and Li suddenly found themselves in the international spotlight. But when the hoopla was over and their discovery established, they wondered what evolutionary problems they had actually solved.

In fact, the pair had failed to find any recognizable body plans showing steps along the way toward the complex Cambrian animals with their legs, antennae, eyes and other features.

What they had actually proved was that Chinese phosphate is fully capable of preserving whatever animals may have lived there in Precambrian times. Because they found sponges and sponge embryos in abundance, researchers are no longer so confident that Precambrian animals were too soft or too small to be preserved.

“I think this is a major mystery in paleontology,” said Chen. “Before the Cambrian, we should see a number of steps: differentiation of cells, differentiation of tissue, of dorsal and ventral, right and left. But we don’t have strong evidence for any of these.”

Taiwanese biologist Li was also direct: “No evolution theory can explain these kinds of phenomena.”

In Chen’s view, his evidence supports a history of life that runs opposite to the standard evolutionary tree diagrams, a progression he calls top-down evolution.

In the most published diagram in the history of evolutionary biology, Darwin illustrated what became the standard view of how new taxa, or animal categories, evolve. Beginning with small variations, evolving animals diverge farther from the original ancestor, eventually becoming new species, then new genera, new families, and the divergence continues until the highest taxa are reached, which are separated from one another by the greatest differences.

But the fossil record shows that story is not true, according to Chen. The differences appear dramatically in the early days, instead of coming at the top. Chen suggested that biologists need to seek new mechanisms to explain these evolutionary leaps.

Wherever the first chordates came from, Nicholas Holland of Scripps agreed that science should now take seriously the possibility that evolution can occur in relatively quick jumps.

That still leaves a great divide between Chen, Li and the Chinese media on one side and the mainstream Western view, in which scientists are reluctant to admit that the Cambrian explosion poses a difficult challenge.

But conferences such as the one in Chengjiang may be changing some views. One of the symposium organizers, paleontologist David Bottjer of the University of Southern California in Los Angeles, said he disagrees with the idea of rapid evolution, but he conceded, “The Cambrian Explosion is going to tell us something different about evolution, in the sense that it’s not the same story that we have always been taught.”

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