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.