stem group eukaryotes on microbial mat

Artist’s imagination of an assemblage of primordial eukaryotic organisms of the ‘Protosterol Biota’ inhabiting a bacterial mat on the ocean floor. Based on molecular fossils, organisms of the Protosterol Biota lived in the oceans about 1.6 to 1.0 billion years ago and are our earliest known ancestors. Orchestrated in MidJourney by TA 2023 (CREDIT: Orchestrated in MidJourney by TA 2023)

CANBERRA, Australia — A “lost world” of organisms that lived in Earth’s waterways at least 1.6 billion years ago is shedding fresh light on the emergence of complex life. Researchers believe these ancient marine fossils represent the earliest known predators on the planet.

They are primitive forms of eukaryotes called “protosteroids” — specialized cells encased in membranes. Known as the “Protosterol Biota,” scientists discovered the microscopic creatures inside a rock at the bottom of the ocean near what is now the Northern Territory in Australia.

Modern eukaryotes include fungi, plants, animals, and even single-celled organisms such as amoebae. Humans can trace their ancestral lineage back to the Last Eukaryotic Common Ancestor (LECA) – which lived more than 1.2 billion years ago. The Protosterol Biota lived at least one billion years before any animal or plant emerged.

Artist’s imagination of an assemblage of primordial eukaryotic organisms
Artist’s imagination of an assemblage of primordial eukaryotic organisms of the ‘Protosterol Biota’ inhabiting a bacterial mat on the ocean floor. Based on molecular fossils, organisms of the Protosterol Biota lived in the oceans about 1.6 to 1.0 billion years ago and are our earliest known ancestors (credit: Orchestrated in MidJourney by TA 2023). (CREDIT: Orchestrated in MidJourney by TA 2023)

“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. These ancient creatures were abundant in marine ecosystems across the world and probably shaped ecosystems for much of Earth’s history,” says Dr. Benjamin Nettersheim, who completed his PhD at the Australian National University and is now based at the University of Bremen in Germany, in a media release.

“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,” Dr. Nettersheim continues.

“Scientists have long searched for fossilized evidence of these early eukaryotes, but their physical remains are extremely scarce. Earth’s ancient oceans rather appeared to be largely a bacterial broth.”

“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?”

“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.”

Artist’s imagination of two primordial eukaryotic organisms
Artist’s imagination of two primordial eukaryotic organisms of the ‘Protosterol Biota’ on the ocean floor. Based on molecular fossils, organisms of the Protosterol Biota lived in the oceans about 1.6 to 1.0 billion years ago and are our earliest known ancestors. Orchestrated in MidJourney by TA 2023. (CREDIT: Credit: Orchestrated in MidJourney by TA 2023)

The Protosterol Biota were certainly more complex than bacteria and presumably larger, although it’s unknown what they looked like, according to the ANU team.

“We believe they may have been the first predators on Earth, hunting and devouring bacteria,” adds co-lead study author Professor Jochen Brocks, who made the discovery with Dr. Nettersheim.

The creatures, described in the journal Nature, are believed to have thrived until about 800 million years ago. The end of this period in Earth’s evolutionary timeline is known as the “Tonian Transformation,” when more advanced nucleated organisms, such as fungi and algae, started to flourish. Exactly when the Protosterol Biota went extinct is still unclear.

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

The findings are based on an analysis of fossilized fat molecules. They 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. Early oceans largely appeared to be a bacterial world, but our new discovery shows that this probably wasn’t the case,” Dr. Nettersheim continues.

The discovery provides new insights into the emergence of life on Earth, showing complex multicellular eukaryotes populated the marine biosphere more than a billion years before the Cambrian Explosion 550 million years ago when most modern animal species emerged.

“Scientists had overlooked these molecules for four decades because they do not conform to typical molecular search images.” Prof. Brocks notes. “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.”

South West News Service writer Mark Waghorn contributed to this report.

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1 Comment

  1. Paul E. says:

    What did they feed off of if there weren’t any plants or animals, fish? What did the fish eat if there weren’t any other animals, plants? Remember- no plants?