Rocks hundreds of meters below the Australian outback have provided clues to a lost world of primitive microbes that once inhabited the world’s oceans and may have eventually given rise to modern plants and animals.
Analysis of fat molecules isolated from rocks suggests they were created by a previously undiscovered ancient population of organisms called eukaryotes, a group of living creatures whose cells typically contain a nucleus and other internal compartments. The molecules are 1.6 billion years old and suggest that eukaryotes were abundant and widespread much earlier than earlier biochemical evidence suggested.
“The previous story was that eukaryotes were extremely rare until about 800 million years ago,” says Phoebe Cohen, a paleobiologist at Williams College in Williamstown, Mass., who was not involved in the research. “Paleontologists really bristled at that because it’s not what we’ve seen in the fossil record.” The findings help bridge the gap between the two types of evidence, she said.
Chemical fingerprint
Most modern eukaryotes rely on fat-like compounds called sterols, such as cholesterol, to build cell membranes and perform other cellular functions. Because sterols are found throughout the eukaryotic lineage, they are thought to have been present in the last common ancestor of all modern eukaryotes. For this reason, paleontologists have used the compounds as a biomarker for the presence of eukaryotes in ancient rocks.
But look further back in time than 800 million years ago and the sterol trail dries up. Scientists have been unable to find traces of the compounds in older rocks, despite the existence of fossils of red and green algae—both eukaryotes—dating back about a billion years.
This absence has led to speculation that eukaryotes were not abundant enough to leave a detectable trace of sterols 800 million years ago.
Another possibility, however, was that the researchers were looking for the wrong molecules. Benjamin Nettersheim, a geobiologist at the University of Bremen in Germany, Jochen Brocks, a paleobiogeochemist at the Australian National University in Canberra, and their colleagues decided to focus on the short-lived molecules that make up modern eukaryotes in sterol synthesis. Such modern intermediates may have been the final product of prehistoric eukaryotes.
Wild ocean
The team combed rocks from around the world and found widespread traces of these “protosterols” evidence that the eukaryotes that produced them were abundant in aquatic environments between 800 million and 1.6 billion years ago.
This contradicts previous thinking, says Nettersheim. One possibility is that a eukaryote that produces more modern sterols gained a selective advantage between a billion and 800 million years ago, eventually crowding out its protosterol-producing counterparts.
The work could show why scientists haven’t been able to find biochemical clues to confirm the fossil record, says Laura Katz, a biologist who studies microbial eukaryotes at Smith College in Northampton, Massachusetts. “We were just looking for the wrong thing.
But Andrew Roger, who studies comparative genomics and eukaryote evolution at Dalhousie University in Halifax, Canada, notes that billion-year-old fossilized red and green algae look remarkably similar to living algae and likely produced modern sterols. This would suggest that modern sterols – not just their precursors should also be present in rocks that are more than 800 million years old. “The finding raises as many questions as it answers,” he says.
And while there are reasons to suspect that protosterols were created by eukaryotes, scientists have not yet been able to rule out the possibility that they were made by ancient bacteria, says Susannah Porter, a paleontologist who focuses on the early evolution of eukaryotes at the university. California, Santa Barbara.
But the team’s approach using hypotheses about the evolution of biosynthetic pathways to guide the search for ancient life could reveal more about early life, he adds. “It’s thinking about the biomarker record from an evolutionary perspective,” says Porter. “And I think it’s needed.
Reference: https://www.nature.com/articles/d41586-023-01847-8
