At least 1.6 billion years ago, microscopic organisms lived in waterways whose molecular fossils have been found in rocks from that time and could be the earliest traces of our lineage.
The discovery of this microscopic ‘lost world’, published today in the journal Nature, could change understanding of our early ancestors.
All eukaryotic cell organisms (those in animals, plants, and fungi, with their DNA enclosed in a membrane) have a common ancestor known as LECA, which lived 1.2 billion years ago, but must have been preceded by other forms of eukaryotes.
A group of Australian, French and German researchers has discovered abundant amounts of sterols in rocks from the middle of the Proterozoic period (2.5 billion to 542 million years ago), primitive compounds hitherto unnoticed as fossil evidence of early life.
These vital signs indicate the existence of a whole group of organisms, hitherto unknown, that dominated complex life on Earth and that would have lived at least a billion years before the appearance of any animal or plant.
The existence of these microscopic organisms that are part of eukaryotic organisms was discovered by a team led by Jochen Brooks of the Australian National University (AUN), which are generically termed “protosterol organisms”.
“Molecular remains of protosterol organisms discovered in 1.6-billion-year-old rocks appear to be the oldest remains of our lineage: they lived even before LECA,” says co-signator Benjamin Nittersheim of the University of Bremen, Germany). .
These creatures, which thrived from about 1.6 billion years ago to about 800 million years ago, were abundant in marine ecosystems around the world and likely shaped ecosystems for most of Earth’s history.
Although it is not known what they were like, Brooks noted in a statement from his university that they would have been more complex and likely larger than bacteria: “We think they may have been the first predators on Earth, hunting and devouring bacteria.”
They differed from complex eukaryotes as we know them (animals, plants and algae) in their cellular structure and possibly in their metabolism, which were adapted to a world with much less oxygen in the atmosphere than it does today, according to the center. Earth Science Center (GFZ).
It is also unknown when exactly these ancient organisms became extinct, although it does point to the Tunnic period, when more advanced eukaryotes, such as fungi and algae, began to flourish.
“Just as the dinosaurs had to die out in order for our mammalian ancestors to become large and plentiful, protosterol organisms probably had to go extinct a billion years ago to make way for modern eukaryotes,” Brooks said.
To make the discovery, the researchers studied fossil lipid particles, known as protosteroids, found inside a 1.6 billion-year-old sedimentary rock from the mid-Proterozoic that formed on the ocean floor near Australia.
The molecules have a primitive chemical structure that suggests complex primitive organisms that evolved before LECA and have since become extinct. Without these biomarkers “we would never have known that protosterol organisms existed—scientists have overlooked these molecules for four decades because they don’t fit into typical molecular research pictures,” according to Nettersheim.
But once they knew what they were looking for, they found that dozens of other rocks, mined from billion-year-old streams around the world, “also secrete similar fossil particles.”
The 1958 Nobel Prize in Medicine, Konrad Bloch, predicted in 1994 the existence of this type of primitive molecule, so these findings prove him correct.
However, it was considered that these primitive intermediate particles would never be found because they could not survive in the geological record, although this research now indicates that this is not the case and suggests that the remains of protosteroids may persist in rocks for more than billion years.
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