The discovery of a "lost world" of ancient organisms that lived in Earth's waterways at least 1.6 billion years ago could change our understanding of our earliest ancestors, according to new research.
The study, led by researchers from the Australian National University (ANU) and published in the journal Nature this week, led to the discovery of microscopic, ancient creatures known as the 'Protosterol Biota' - part of a family of organisms called eukaryotes.
Eukaryotes have a complex cell structure that includes mitochondria, known as the "powerhouse" of the cell, and a nucleus that acts as the "control and information centre", the researchers said.
Fungi, plants, animals and single-celled organisms, such as amoebae, are examples of modern forms of eukaryotes that inhabit Earth today. Humans and all other nucleated creatures can trace their ancestral lineage back to the Last Eukaryotic Common Ancestor (LECA), which lived more than 1.2 billion years ago.
According to the researchers, these ancient creatures could have been the first predators on Earth, living at least one billion years before the emergence of any plant or animal.
The organisms were abundant in marine ecosystems across the world and probably shaped ecosystems for much of Earth's history, they added.
Commenting on the findings, Dr Benjamin Nettersheim, who is now based at the University of Bremen in Germany, said the remains of the Protosterol Biota appear to be "the oldest remnants of our own lineage".
"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," he said.
"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.
"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.
"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."
Professor Jochen Brocks from ANU, who made the discovery with Dr Nettersheim, said the Protosterol Biota were more complex than bacteria and presumably larger, although it's unknown what they looked like.
"We believe they may have been the first predators on Earth, hunting and devouring bacteria," Professor Brocks said.
The researchers believe the ancient creatures thrived from about 1.6 billion years ago up 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 unknown.
To make the discovery, 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 Australia's Northern Territory. The molecules featured a "primordial chemical structure" that hinted at the existence of early creatures that evolved before LECA and had since gone extinct.
"Scientists had overlooked these molecules for four decades," Professor Brocks said.
"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."
Dr Nettersheim completed the analysis as part of his PhD at ANU before accepting a position at the University of Bremen. This work involved scientists from Australia, France, Germany and the United States.
