The proverbial primordial soup from which our earliest, multi-cellular ancestors emerged was presumably seething with many much simpler, single-celled organisms. Finding the first indications of evolution into more advanced, embryonic development has proved difficult, however, both because of the organisms' small size and soft structures.
A famous collection of minute 570-million-year-old fossils, from the Doushantuo Formation in southern China, has long been thought to hold some of the earliest evidence of multicellular animal development: primitive embryos. But a new high-tech analysis, published online Thursday in Science, suggests that the fossils might instead represent spores of a single-celled, asexual organism.
The globular fossils show various stages of division. "We've been convinced for so long that these fossils represented the embryos of the earliest animals," Philip Donoghue, a professor of paleobiology at the University of Bristol and paper co-author, said in a prepared statement. If his team's new theory holds, "our colleagues are not going to like the result," because it would mean that "much of what has been written about the fossils for the last 10 years is flat wrong," he said.
The team used synchrotron-based x-ray tomographic microscopy, which uses accelerated particles, to peer into about 450 of the miniscule fossils and digitally reconstruct their interiors. "It allowed us to make a perfect computer model of the fossil that we could cut up in any way that we wanted, but without damaging the fossil," John Cunningham, also of Bristol and a paper co-author, said in a prepared statement. "We would never have been able to study the fossils otherwise."
The condition of the Doushantuo fossils is exceptional. In some of the samples, "even their nuclei have been preserved," Therese Huldtgren, of the Swedish Museum of Natural history and co-author, said in a prepared statement. Given the high quality of the ancient organisms from the Ediacaran Period, however, scientists were perplexed as to why only embryonic stage individuals would have been discovered, and not any adults or juveniles. The pattern of cellular division and fullness of the nuclei captured in the rock "is incompatible with the embryonic development." Instead, the researchers argue, the cells are undergoing straight up asexual reproduction.
Not everyone is convinced that the ancient specks are actually just spores. "In terms of progressivist storytelling, this all seems a little too good to be true," Nicholas Butterfield of the University of Cambridge's Earth Sciences department wrote in an accompanying essay in the same issue of Science. "Interpretation at this level is inevitably impressionistic," he noted. "But to my eye there is still a case for identifying the Doushantuo fossils as embryos, albeit algal rather than animal."