The rocks of the Doushantuo Formation, in China's Guizhou Province, are sprinkled with tiny, ancient fossils. They are no more than a millimeter in length. The 600-million-year-old organisms are preserved with such detail that the fossils, when freed from the rock in a chemical bath and scanned with X-rays, reveal not only individual cells but possible cell nuclei. Some fossils are jagged and round, like wizened Koosh balls. Other orbs, among the most intriguing specimens, are split by Y-shaped seams.
When viewed from the side, the cellular clusters look a bit like pie sliced in thirds. When viewed by biologists, the clusters also look a bit like animal embryos, frozen in time.
“If they are animals, they’d be the oldest animals in the fossil records,” said John Cunningham, a paleobiologist at the University of Bristol in Britain. Cunningham is not convinced that the organisms, known as the Weng'an biota, are indeed ancient animals. In a report published Wednesday in the Journal of the Geological Society, Cunningham and his co-authors examined the best evidence for and against the case that the fossils are from animals. Their answer is: They don't know.
Ancient trilobites and other scattered specimens provide convincing evidence that animals lived 530 million years ago. Given what scientists know about the rate at which organisms branch off from common ancestors, the first animals should appear in the fossil record about 70 million years or so before those creatures — right around the time of the Weng'an biota.
Timing alone, though, isn't sufficient evidence. Nor are the Y-shaped seams. Algae cells also grow Y junctions, noted the authors of the new report. “It seems that none of the characteristics which have been used are unique to any kind of animals,” Cunningham said.
“This is a good study that summarizes the current 'state of the art' on Weng'an fossils,” said David Bottjer, a paleontologist at the University of Southern California who has been studying the region's fossils since 1998. When paleontologists first described the fossils in the late 1990s, they suggested that the Weng'an biota were animals. “It is fair to say that this has been disputed since then, as no 'smoking gun' for animal fossils has emerged,” said Bottjer, who was not involved with the new paper.
Shuhai Xiao, a professor of paleobiology at Virginia Tech who was among the first wave of researchers to study the Weng'an biota, expressed doubt about the algae explanation. There’s no good evidence that these fossils are photosynthetic, Xiao said, in the way that algae are. Though some species of algae grow in balls, these clumps are hollow; cells caught in the center wouldn't get enough sunlight to survive. The clusters of Weng'an biota are solid, full of cells.
Xiao maintains that an animal origin, though not definite, is more likely than the new paper suggests. “But I think that’s okay,” Xiao said. “It’s hard to interpret things that are more than half a billion years old.”
While scientists debate if the fossils are embryos, the specimens face a threat worse than uncertain taxonomy: bulldozers.
Twenty years ago, when Xiao was a graduate student, the land around the Doushantuo Formation was green and beautiful. Xiao returned two years ago to collect more samples. “Now it’s very ugly,” he said, turned gray by trucks and quarry machinery. The site is rich not just in fossils but also in phosphate, used as raw material to produce fertilizer.
Xiao said that the mining operation had already destroyed the aboveground outcroppings. To find a continuous section of rock from which to collect samples, he relied instead on the walls of mining tunnels. But because the tunnel network risks collapse, the miners plan to fill the holes with loose rock, he said, also sealing away the fossils.
“They’re being quarried at a really massive rate,” Cunningham said. “If they’re all quarried out it would be very sad. The chance to find more fossils from this deposit would be lost.” Chinese paleontologists and others around the world have petitioned the Chinese government to curb the mining operation, though one researcher told Science magazine in April that a third of the fossil sites are already gone.
By using advanced techniques like synchrotron X-ray machines — akin to medical CT scanner, but meant for much smaller targets — scientists can wring more information from fossils already collected. But there remains one question best answered by a continued hunt: If these are indeed animal embryos, might adults exist in the deposit?
The answer, again, is ambiguous. Perhaps the eggs sank to the bottom of the ocean, where conditions favored their fossilization over the preservation of squishy, free-swimming adults. But scientists like Xiao are optimistic that there are new discoveries waiting in the rocks.
“Despite its importance, there have been relatively few studies of the Weng'an biota,” Bottjer said. “So, there is every reason to think that future studies will turn up additional evidence for the existence of animal life, including larvae and adult specimens.”