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