Paleontologists have detected abundant protosteroids — traces of ancient life forms — in 1.6-billion-year-old sedimentary rocks that had formed at the bottom of the ocean near what is now Australia’s Northern Territory.
Eukaryotic life appears to have flourished surprisingly late in the history of Earth. This view is based on the low diversity of diagnostic eukaryotic fossils in marine sediments of mid-Proterozoic age (around 1.6 billion to 800 million years ago) and an absence of steranes, the molecular fossils of eukaryotic membrane sterols. This scarcity of eukaryotic remains is difficult to reconcile with molecular clocks that suggest that LECA had already emerged between around 1.2 and more than 1.8 billion years ago. LECA, in turn, must have been preceded by stem-group eukaryotic forms by several hundred million years. Brocks et al. report the discovery of abundant protosteroids in sedimentary rocks of mid-Proterozoic age. Image credit: Arek Socha.
“All living eukaryotes evolved from the Last Eukaryotic Common Ancestor (LECA) that lived between around 1.2 billion and more than 1.8 billion years ago,” said University of Bremen paleontologist Benjamin Nettersheim and colleagues.
“LECA and all its descendants form the crown of the eukaryotic tree including algae, plants, fungi, animals and all extant, unicellular protists.”
“Yet, the domain Eukarya has a much deeper prehistory. The ancestral line leading towards LECA, and all its extinct side-branches, are stem-group Eukarya.”
“The genome and cell structure of living descendants provide only limited insights into the evolution of LECA’s ancestors, and almost nothing is known about their abundance, ecology and habitats.”
“To study the hundreds of millions of years of hidden eukaryote evolution and ecology, we have to search for fossil and chemical remains directly in the geological record.”
In their research, the scientists detected chemical traces of the so-called Protosterol Biota in 1.6-billion-year-old rocks of the Barney Creek Formation in northern Australia.
“These ancient creatures were abundant in marine ecosystems across the world and probably shaped ecosystems for much of Earth’s history,” Dr. Nettersheim said.
“The Protosterol Biota were certainly more complex than bacteria and presumably larger, although it’s unknown what they looked like,” added Australian National University’s Professor Jochen Brocks.
“We believe they may have been the first predators on Earth, hunting and devouring bacteria.”
According to the team, these 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. But exactly when the Protosterol Biota went extinct is unknown.
“The Tonian Transformation is one of the most profound ecological turning points in our planet’s history,” Professor Brocks said.
“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 team’s findings appear in the journal Nature.
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J.J. Brocks et al. Lost world of complex life and the late rise of the eukaryotic crown. Nature, published online June 7, 2023; doi: 10.1038/s41586-023-06170-w
