Meet your oldest eukaryotic ancestor found in 1.6-billion-year-old rock

A “lost world” of organisms that lived in Earth’s ancient waterways about 1.6 billion years ago has been found fossilised in rock from the ocean near Australia’s Northern Territory.

The find could change scientist’s understanding of our earliest ancestors.

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Animals, including humans, as well as plants, fungi, and single-celled organisms such as amoebae are all made up of eukaryotic cells. Every one of these organisms can trace its ancestral lineage back to the Last Eukaryotic Common Ancestor (LECA) which would have lived more than 1.2 billion years ago.

Eukaryotes have a complex cell structure that includes mitochondria and a nucleus.

Mitochondria act as the “powerhouse of the cell”, producing energy, and the nucleus is the “control and information centre.”

Evidence for early eukaryotes has been elusive until now.

Researchers from the Australian National University (ANU) have described “Protosterol Biota” known from fossilised fat molecules in 1.6-billion-year-old rock that possess a chemical signature that indicates they came from a now extinct ancient eukaryote.

Their research is described in a paper published in Nature.

“Scientists have long searched for fossilised evidence of these early eukaryotes, but their physical remains are extremely scarce. Earth’s ancient oceans rather appeared to be largely a bacterial broth,” says co-author Dr Benjamin Nettersheim, who completed his PhD at ANU and is now based at the University of Bremen in Germany.

“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.”

The new finding represents the oldest remnant of our lineage, and the origins of all modern eukaryotic forms. But, given the prevalence of eukaryotic biota today (and for the last 600 million years), why are we only finding early eukaryotes now?

“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?  

“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.”

“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,” Nettersheim explains.   

“Scientists had overlooked these molecules for four decades because they do not conform to typical molecular search images,” says first author and ANU professor Jochen Brocks.

“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.”

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First life on Earth emerged three to four billion years ago and would have been prokaryotic. These microbes lack a nucleus and other cell organelles. Prokaryotes make up the two other main domains of life on Earth apart from eukaryotes – Bacteria and Archaea.

The Protosterol Biota are from a period of Earth’s history known as the Tonian Transformation when more advanced organisms including algae and fungi started to flourish.

Brocks says that the Protosterol Biota were more complex than bacteria and were probably larger. “We believe they may have been the first predators on Earth, hunting and devouring bacteria.”

Brocks believes the Protosterol Biota thrived from about 1.6 billion to about 800 million years ago.