Once upon a time, 570 million years ago, some organisms in what is now southern China died in mysterious circumstances. As they lay in state, the cells decayed and formed salt replicas of themselves through a process called phosphatization. When these so-called Doushantuo fossils were unearthed in 1997, their discoverers believed the dead cells were early animal embryos, which would make them the oldest animals ever found. Other scientists believed they were not animals, but were actually giant, sulfur-metabolizing bacteria called Thiomargarita. No one is sure yet, but one group of researchers now argues that if you look at dead Thiomargarita cells, they don't look anything like the Doushantuo fossils, suggesting the relics aren't bacteria.
The Doushantuo fossils are unique because they are so well preserved. If the fossils are indeed animal embryos, they provide "a real insight into both [a developmental stage] that doesn't make it into the geological record and the earliest stages of animal evolution," says paleobiologist Patrick Orr of University College Dublin, who was not involved in the research. The problem is, "you can't compare a fossil with the living version."
So palaeontologist Philip Donoghue and colleagues at the University of Bristol in the United Kingdom decided to investigate how dead Thiomargarita and dead embryos fall apart. Doing this in a controlled fashion is difficult, Donoghue says. " Lots of people do experiments involving sticking something in a bucket and watching it rot." The actual process is very complex, involving bacteria, fungi, and certain minerals present in the environment. And when the group tried to perform their experiments with Thiomargarita, "we had lots of trouble trying to kill this thing," he says. Although they tried toxic chemicals, drugs, and extreme conditions like heat, "it resisted all our attempts." In the end, they gave up and went to look for pre-killed Thiomargarita, finding examples in various stages of death and decay in sediment samples.
The researchers captured 3-D images of dead Thiomargarita, expired sea urchin embryos, and some of the original Doushantuo fossils using x-ray light from a synchrotron. The images of the partially dead and totally dead bacteria looked nothing like the famous fossils, the researchers report today in the Proceedings of the Royal Society B. For one thing, Thiomargarita cells became empty spheres as they decayed, whereas the fossils showed evidence of a lot of internal structure.
Donoghue believes the Doushantuo fossils were likely preserved through templating, a process through which bacteria that live in seawater cover the surfaces of a larger dead organism with a layer called a biofilm. When the bacteria in the biofilm die, their cells become phosphatized and take on the shape of the larger organism. So when researchers look at the fossil, they're actually looking at the remnants of a biofilm coat, not the original organism.
When the researchers tried to replicate the process, it worked fine with the sea urchin embryos, but when Donoghue's group tried to form a biofilm on Thiomargarita, the process didn't work as well. Rather than leaving templates of themselves, the Thiomargarita cells collapsed. It's therefore unlikely, the authors conclude, that the Doushantuo fossils are Thiobacteria.
"I like this kind of experimental work," says Frank Corsetti, a geobiologist at the University of Southern California in Los Angeles who led the original study suggesting the Doushantuo fossils might be Thiomargarita. But he says that just because biofilm bacteria can phosphatize embryos but not Thiomargarita, that doesn't mean the fossils formed through this process. Still, he says, "it's a good first step."
Donoghue says his group is continuing to work on the famous fossils, hoping to figure out more about what happened to the organisms after they died. The process of decay, he says, is "underappreciated."