In what may rank as one of the most important fossil discoveries in decades, researchers have found exquisitely preserved remains of tiny creatures, opening a long-hidden window on early animal life.
Ranging from 570 million to 580 million years old, the microscopic animal fossils reveal their secrets in surprising detail - down to the level of individual cells. This is raising hopes that paleontologists will be able to document, at the most basic biological levels, key stages in the evolution of animals as they shifted from soft-bodied, microscopic forms to those visible to the naked eye.
In particular, researchers are keenly interested in finding fossil evidence for the evolutionary fuse that led to the Cambrian explosion - a burst in the diversity of life unmatched in Earth's history.
The explosion, which began roughly 550 million years ago and lasted a few tens of millions of years, gave rise to virtually every body plan seen in the animal kingdom today.
"This is going to trigger a flurry of activity," says Sean Carroll, a developmental biologist at the Howard Hughes Medical Institute and the University of Wisconsin at Madison, referring to the latest discoveries. "There is tremendous interest in the Cambrian explosion, when there was a huge increase in the size and diversity of animals. Before the Cambrian, we do not see that level of innovation," he says. "People thought that some of it was there in small, poorly preserved animals, or maybe it wasn't there at all."
Two groups reported the fossil finds independently this week.
One research team, based in Taiwan and the People's Republic of China, looked at phosphate deposits in southern China and found highly detailed remains of early sponges. They are reporting their findings in tomorrow's issue of the journal Science.
Meanwhile, Shuhai Xiao and Andrew Knoll of Harvard University, in Cambridge, Mass., and Yun Zhang of Beijing University analyzed microfossils taken from a phosphate mine also in southern China. They describe their work in today's issue of the journal Nature.
Combing the phosphate deposits, part of a 57-square-kilometer (22-square-mile) formation known as the Doushantuo phosphorites, they uncovered several specimens of multicelled algae, whose structure and reproductive features also appear in modern marine algae. They also discovered tiny globules that, after Shuhai Xiao's detective work, appear to be embryos from the same kind of organism. The embryos show different stages of division.
"These look much like the first 10 days of your own life looked like," Dr. Knoll says. "These embryos show at least the beginnings of bilaterally symmetrical animals, which include everything from worms to insects to ourselves."
The ability to see these cleavage patterns in embryos is significant, because they yield vital clues about the type of animal involved, according to James Valentine, professor of integrative biology at the University of California at Berkeley and curator of the university's Museum of Paleontology. By tracking these patterns, "You could be able to figure out when major animal groups diverged."
Until recently, hopes that such microscopic animal fossils could be recovered from Precambrian rocks were slim, because the creatures' remains were thought to be too fragile to survive the process of becoming fossils.
Faced with a dearth of fossils, researchers have had to turn to developmental biologists to suggest the forms such early life might have taken, while molecular biologists have tried to estimate when different animal groups began to diverge. One technique, for example, uses the slow rate of change in DNA molecules as biological clocks. This method has led some researchers to suggest that some level of divergence among multicelled animals may have occurred as far back as 1.2 billion years ago. More recent estimates put the time at 670 million years ago.
Then, last year, Stefan Bengtson, a researcher with the Swedish Museum of Natural History in Stockholm, and Yue Zhao, of the Chinese Academy of Science in Beijing, reported finding tiny globules from the Cambrian era that held the remains of embryos. The well-preserved remains were found in phosphate deposits in China and Siberia, boosting hopes that older microfossils, equally well-preserved, might be found, Knoll says.
Indeed, he and his graduate students now are combing samples dating back 700 million to 800 million years, checking for additional microfossils and their state of preservation.
Although the specific implications of these and subsequent fossil finds for animal evolution remain to be sorted out, Knoll adds that the discovery and the clues it gives raise an intriguing question that scientists are still striving to answer: If the early ancestors of bilaterians were present 570 million years ago or earlier, why did it take another 30 million years for them to begin their explosive diversification?