About 540 million years ago, in the beginning of the Cambrian, life on Earth suddenly exploded with diversity. Within less than 30 million years, most phyla that exist today emerged. The question whether it was a rapid, unprecedented phenomenon or a more drawn out process of evolution rooted in the Precambrian is a subject of controversy among scientists.
The answer to this question may lie with a late Precambrian fauna, the Ediacara. Fossils of the Ediacara, soft-bodied and relatively complex organisms, appear in late Precambrian rock but are absent from Cambrian sediments. "The question is," says Samuel Bowring, Associate Professor of Earth, Atmospheric, and Planetary Sciences at MIT, "did they die off or were they simply not preserved [in the fossil record]?"
Most fossils from the Cambrian (543 million to 490 million years ago) had either hard shells or skeletons, which contributed to their preservation. In most environments, soft-bodied organisms like the Ediacara decayed too quickly to leave fossils.
Molecular biologists have postulated that the Cambrian explosion of life should have taken place as early as one billion years ago. But "where are the fossils?" asks Bowring. The known fossil record does not indicate massive radiation of life before the Cambrian. "But then," muses Bowring, "do we even know what to look for?" The fossils could simply have not been found yet, or might not have been preserved because they lacked hard shells or skeletons. "If we could trace [the Ediacaran] into the Cambrian," says Bowring, "I suspect it would be a major breakthrough. Most people think that they do not continue into the Cambrian."
The Ediacaran fossils are similar to those found in Burgess Shale, a famous fossil bed from the Middle Cambrian (510 million to about 500 million years ago) in the Canadian Rockies. "There are people who think that there are relatives of Ediacara in the Burgess Shale," says Bowring, "but that's very controversial." The Burgess Shale fossils, also soft-bodied, survived because they were buried in fine grained mud impervious to oxygen.
Little is known about the classification of the Burgess Shale fossils. It is not known whether they were animals; it has even been suggested that they are lichen. "Lichen they do not look like," grins Bowring, "I've never seen bilaterally symmetrical lichen."
Using zircon dating, Bowring's team has marked the end of the Precambrian -- and the latest known Ediacara -- at 543 million years ago, and that of the beginning of the Cambrian explosion at about 540 million years ago. Not all fossils can be dated using this --> technique, however; the fossils in the Ediacara Hills in Australia, for instance, lack the necessary volcanic ash.
Zircon dating uses the rate of decay of uranium in zircon crystals to date volcanic ash from that period to within 200,000 to 300,000 years.
Fossils at the Cheng Jiang site in southwestern China could provide a link between the Ediacara and the Burgess Shale fossils. "It's just like the Burgess Shale, but older," Bowring says. He plans to start work there within a few years.
In the meantime, Bowring plans to continue studying the Ediacara fossils in Namibia and Newfoundland, and to start work at a White Sea site abundant in Ediacara. "There are literally tens of thousands of specimens there," says Bowring, "some of them are a meter in length."