2023-06-07 16:59:28
A team of researchers from the Australian National University (ANU) and the University of Bremen in Germany have discovered that, around 1.6 billion years ago, the Earth’s oceans and lakes were populated, and probably dominated, by some organisms that could have been the first predators on our planet. The finding of these creatures, the oldest members of our own lineage and known as the ‘protosterol biota’, is published this Wednesday in ‘Nature’.
These microscopic creatures are part of a family of single-celled organisms called eukaryotes. Unlike prokaryotes, eukaryotes have a complex cell structure that includes a membrane, mitochondria, which are the ‘powerhouses’ of the cell, and a well-differentiated nucleus that acts as a control and information center.
Currently, the eukaryotes that inhabit the Earth include all fungi, plants, animals and a large number of single-celled organisms such as amoebas. Humans and all other ‘nucleated’ creatures can trace their ancestral lineage back to the Last Eukaryotic Common Ancestor (LECA). LECA, not to be confused with LUCA (Last Ultimate Common Ancestor, the last common ancestor, the first form of life to exist on Earth) lived more than 1.2 billion years ago.
These ancient creatures were very abundant in marine ecosystems around the world and likely shaped those ecosystems for most of Earth’s history, the researchers explain. In fact, the protosterol biota lived at least a billion years before the appearance of any animal or plant.
“The molecular remains of the protosterol biota detected in 1.6 billion-year-old rocks,” says Benjamin Nettersheim of the University of Bremen and co-author of the study, “seem to be the oldest remains of our own lineage: they lived even before LECA. Modern forms of eukaryotes are so powerful and dominant today that they surely conquered and dominated Earth’s ancient oceans more than a billion years ago.”
In search of fossil evidence
Scientists have long searched for fossil evidence of these early eukaryotes, but their physical remains are extremely rare. Earth’s ancient oceans were largely akin to a vast bacterial broth. However, one of the biggest puzzles of early evolution, still unanswered by scientists, is why our eukaryotic ancestors failed to master the world’s ancient waterways. Or at least they left no trace on them.
“Our study -says Nettersheim- turns this theory on its head. We showed that protosterol biota was hiding in plain sight but was 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.”
more complex than bacteria
Jochen Brocks, of the Australian National University and co-lead author of the paper with Nettersheim, believes that the protosterol biota was surely more complex than bacteria and presumably larger, although exactly what they looked like is unknown. “We believe -says the scientist- that they may have been the first predators on Earth, hunting and devouring bacteria.”
According to the article, these extraordinary creatures thrived for more than 800 million years, from around 1,600 to around 800 million years ago. The end of this period in Earth’s evolutionary timeline is known as the ‘Tonic Transformation’, when more advanced nucleated organisms such as fungi and algae began to flourish. But exactly when the protosterol biota went extinct is unknown.
“The Tonic Transformation,” says Brocks, “is one of the most profound ecological tipping points in the entire history of our planet. Just as the dinosaurs had to die out so our mammalian ancestors could become large and plentiful, so too the protosterol biota had to die out a billion years earlier to make room for modern eukaryotes.”
fat molecules
To make their discovery, the researchers studied fossil fat molecules found inside a 1.6 billion-year-old rock that had formed on the ocean floor near what is now Australia’s Northern Territory. The molecules possessed a primordial chemical structure that hinted at the existence of early complex creatures that evolved before LECA.
“Without these molecules,” Nettersheim says, “we would never have known that the protosterol biota existed. The first oceans appeared to be largely a bacterial world, but our new discovery shows that this was probably not the case.”
In Brock’s words, “For four decades, scientists had overlooked these molecules because they didn’t fit 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 around the world, also ooze similar fossil molecules.”
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