Old Fossils Contradict Explosion-of-Species Theory
Newly discovered fossils from China seem to be the oldest multi-celled animals yet found that were complex enough to have a two-sided body plan rather than a round one.
The report on the creatures, published online Friday by the journal Science, is the latest in a series of fossil finds that downplays a supposed surge of species and diversity in body styles believed to have happened rather suddenly 540 million years ago.
The episode, known as the Cambrian explosion, has been generating controversy for more than a century because it marked the point in pre-history where fossils for virtually all of the 40-some major animal body plans suddenly appeared.
Before then, the evidence of life amounted to rare fossilized bacteria and colonies of bacteria, puzzling scientists who were trying to understand the pace of evolution. Even Charles Darwin conceded in his “Origin of Species” that the oldest fossil records didn’t fit his theory of gradual, orderly evolution over time.
Many researchers now think that evolution is more episodic, influenced by mass extinctions brought about by volcanic eruptions, asteroid impacts, ice ages or other events that prune the tree of life so that it can then grow in different directions.
But many scientists also felt that fossils from pre-Cambrian times were there to be found, if only they discovered old enough rock in the right places. One of those places, it appears, is an open phosphorus mine in southern China where small, relatively soft creatures have been preserved in ancient ocean sediment called the Doushantuo Formation.
Jun-Yuan Chen of the Nanjing Institute of Geology and Paleontology has been studying specimens from the region for nearly a decade, including early sponges and eggs from those animals.
In the latest project, working with several American paleontologists to comb through tens of thousands of microfossils, Chen found 10 that clearly had bilateral shapes _ a top and a bottom, with left and right sides that are nearly identical.
Although the organisms were microscopic in size _ less than a fifth of a millimeter long _ the scientists identified several internal organs of the animals, which were shaped like flattened turtle shells. Among the structures are a mouth, a pharynx, a gut and three-layers of skin or soft shell.
They named it a Small Spring Animal (Vernanimalcula guizhouena) because the sediment was laid down on top of glacial deposits left behind by what some researchers think was a period of near-global ice cover, dubbed “Snowball Earth.”
That helps date the animals from 580 million to 600 million years old, well before the Cambrian period began, suggesting that the changes of that time had deep roots and had tiny prototypes some 40 million to 55 million years earlier.
“The genetic toolkit and pattern formation mechanisms required for bilaterian development … long before the Cambrian,” the researchers wrote. “The diversification of body plans in the Early Cambrian followed from the varied deployment of these mechanisms once conditions permitted, not from their sudden appearance just before the Cambrian boundary.”
On the Net: www.sciencemag.org
(Contact Lee Bowman at BowmanL(at)shns.com or online at http://www.shns.com)
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