What if modern eukaryotes made modern protosterols? A paleontologist’s guide to the discovery of modern intermediates
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 water environments between 800 million and 1.6 billion years ago.
Most modern eukaryotes rely on fat-like compounds called sterols, such as cholesterol, to build cell membranes and carry out other cellular functions. Because sterols are found throughout the eukaryotic family tree, they are thought to have been present in the last common ancestor of all modern eukaryotes. The compounds are being used by the palaeontologists to get an idea of the presence of eukaryotes.
But Andrew Roger, who studies comparative genomics and the evolution of eukaryotes at Dalhousie University in Halifax, Canada, notes that fossilized red and green algae dating back one billion years look remarkably similar to living algae, and probably made modern sterols. That 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.
There is a chance that researchers are looking for the wrong molecule. Benjamin Nettersheim, a geobiologist at the University of Bremen in Germany, Jochen Brocks, a palaeobiogeochemist at the Australian National University in Canberra, and their colleagues decided to focus on short-lived molecules that modern eukaryotes make while synthesizing sterols. The end product might have been the modern intermediates.
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, she adds. “It’s thinking about the record of biomarkers from an evolutionary perspective,” Porter says. I think that is necessary.
While there are reasons to suspect that the Protosterols were made by humans, the researchers have not been able to determine if they were made by ancientbacteria, as a possibility, according to Susannah Porter, a paleontologist.
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