Microscopic 570-million-year-old fossils from China may represent the earliest evidence for animal life on Earth, suggests a new study in the Proceedings of the Royal Society B.
Previous theories have said that the fossils represented giant bacteria.
"One of the proponents of the bacteria theory was a co-author of this paper (Jake Bailey of the University of Minnesota) and he now agrees that the fossils do not represent a giant sulfur bacteria," co-author Philip Donoghue, a professor of palaeobiology at the University of Bristol, told Discovery News.
Images previously taken by Shuhai Xiao, a professor of geobiology at Virginia Tech, reveal that many of the fossils from the Dousantuo Formation in South China look like mini baseballs and soccer balls.
With the bacteria hypothesis negated, that leaves a few possibilities as to what these unusual fossils represent. One, argued by Xiao and others, is that the fossils are of metazoan embryos. If so, they would present one of the oldest records of the animal evolutionary lineage.
Another theory is that the fossils are protists, which are unicellular organisms lacking a definite cellular arrangement. Protists include bacteria, algae, diatoms and fungi. Although not animals, early protists may have given rise to the world’s first animals and plants.
To test out the theories, Donoghue and his colleagues focused on the possibility that the sports equipment-looking fossils were bacteria. Living and decayed Thiomargarita, a modern bacteria, were compared with modern embryos.
The researchers used a big particle accelerator in Switzerland to study the fossils down to their most minute details -- just one quarter of a micron. The extreme up-close look revealed that the bacteria and the Doushantuo fossils are indeed very different.
Negation of the bacteria theory now strengthens the argument that the fossils, be they embryos or some kind of protist, sit at the base of the animal tree of life.
Donoghue doesn't think all animal life on Earth emerged from this particular site. The location was "just chance," in terms of preservation.
"It is the most awesome fossil deposit," he said. "Every single grain is a fossil, and the deposit is 8 meters (over 26 feet) thick."
Donoghue explained that there was a lot of dissolved phosphate in the ocean at this now-China location during the Ediacaran Period. The phosphate helped to preserve the fossils over the many millions of years.
He said at least two animals have already been identified at the site, but the finds are very controversial at present. One has been called Vernanimalcula, meaning "small spring animal,” referring to its appearance in the fossil record at the end of what is known as the “Snowball Earth” freeze period.
Jun-Yuan Chen of the Nanjing Institute of Geology and Paleontology at the Chinese Academy of Sciences and his colleagues believe Vernanimalcula is the first known bilateral animal, meaning the first with body symmetry. Donoghue and others, however, dispute that claim.
Unfortunately, the jury is still out on what exactly the baseball and soccer ball-shaped fossils represent.
Yet another rival for "world's oldest known animals" are fossils from the Flinders Ranges of South Australia. They may also be animals and date to nearly the same time period of around 570 million years ago.
Although China and Australia seem very disconnected today, that may not always have been the case.
"According to paleogeographic reconstructions, South China and South Australia were close to each other at the time, belonging to a supercontinent called Gondwana," Maoyan Zhu, a scientist at the Nanjing Institute, told Discovery News.
Findings concerning additional research on the Doushantuo fossils are expected to be released soon, however, so the mystery over what the fossils are may at last be resolved.