Ancient rocks house evidence of our earliest ancestors

Researchers from The Australian National University (ANU) have found evidence of microscopic creatures that lived in Earth’s waterways at least 1.6 billion years ago, which could change our understanding of our earliest ancestors. Their discovery has been detailed in the journal Nature.

Known as the Protosterol Biota and suspected to be the first ever predators on Earth, the newly discovered creatures are part of a family of organisms called eukaryotes. Modern forms of eukaryotes that inhabit Earth today include fungi, plants, animals and single-celled organisms such as amoebae, but Protosterol Biota were believed to live at least one billion years before any animal or plant emerged. Up until now, humans and all other nucleated creatures have traced their lineage back to the Last Eukaryotic Common Ancestor (LECA), which lived more than 1.2 billion years ago.

“Molecular remains of the Protosterol Biota detected in 1.6-billion-year-old rocks appear to be the oldest remnants of our own lineage — they lived even before LECA,” said Dr Benjamin Nettersheim, who completed his PhD at ANU and is now based at the University of Bremen.

“These ancient creatures were abundant in marine ecosystems across the world and probably shaped ecosystems for much of Earth’s history.”

The researchers studied fossil fat molecules found inside a 1.6-billion-year-old rock that had formed at the bottom of the ocean near what is now the Northern Territory. The molecules possessed a primordial chemical structure that hinted at the existence of early complex creatures that evolved before LECA and had since gone extinct.

“Without these molecules, we would never have known that the Protosterol Biota existed,” Nettersheim said. “Early oceans largely appeared to be a bacterial world, but our new discovery shows that this probably wasn’t the case.”

ANU’s Professor Jochen Brocks, who made the discovery with Nettersheim, added, “Scientists had overlooked these molecules for four decades because they do not conform to typical molecular search images.

“But once we knew what we were looking for, we discovered that dozens of other rocks, taken from billion-year-old waterways across the world, were also oozing with similar fossil molecules.”

According to Nettersheim, modern forms of eukaryotes are so powerful and dominant today that researchers thought they should have conquered the ancient oceans on Earth more than a billion years ago — but despite having long searched for fossilised evidence of early eukaryotes, their physical remains were extremely scarce.

“One of the greatest puzzles of early evolution scientists have been trying to answer is: why didn’t our highly capable eukaryotic ancestors come to dominate the world’s ancient waterways? Where were they hiding?” he continued.

“Our study flips this theory on its head. We show that the Protosterol Biota were hiding in plain sight and were in fact abundant in the world’s ancient oceans and lakes all along. Scientists just didn’t know how to look for them — until now.”

Brocks said the Protosterol Biota may have been the first predators on Earth, hunting and devouring bacteria, although it’s unknown what they looked like. He added that these creatures thrived from about 1.6 billion years ago up until 800 million years ago, coinciding with a period known as the Tonian Transformation when more advanced nucleated organisms, such as fungi and algae, started to flourish. But exactly when the Protosterol Biota went extinct is unknown.

“The Tonian Transformation is one of the most profound ecological turning points in our planet’s history,” Brocks said.

“Just as the dinosaurs had to go extinct so that our mammal ancestors could become large and abundant, perhaps the Protosterol Biota had to disappear a billion years earlier to make space for modern eukaryotes.”

Image caption: An AI-generated artist’s interpretation of what the Protosterol Biota may have looked like. Photo orchestrated in MidJourney by TA 2023.