Around 1000 to 800 million years ago, the common ancestors of all of today's eukaryotes emerged - single-celled organisms whose chemical and structural cell structure was to become the basic blueprint of all living beings with a cell nucleus. But what was before that? This is now revealed by fossil molecules that come from a lost habitat of primeval eukaryote precursors. These "protosterols" discovered by scientists in rocks between 0.8 and 1.64 billion years old suggest that there was a whole group of nucleated protozoa that were widespread in the oceans of the primeval earth, which was still low in oxygen. It was only when the environmental conditions changed, when it became colder and richer in oxygen, that this “protosterol environment” was supplanted by the ancestors of today's eukaryotes and died out.
All living multicellular and eukaryotic protozoa such as green algae or ciliates descend from a common ancestor. This last common ancestor of all eukaryotes - called LECA - developed the molecular and cellular structures that still characterize eukaryotic cells today, including the cell nucleus, cell organelles and also important molecules of cell metabolism such as steroids. "Almost all eukaryotes produce steroids, such as cholesterol, which is produced by humans and most other animals," explains first author Benjamin Nettersheim from the University of Bremen. "These lipid molecules are an integral part of eukaryotic cell membranes, where they perform a variety of physiological functions." Preserved relics of these steroids and their derivatives can be found in rocks dating back 800 to 1200 million years. Biologists therefore assume that the last common ancestor of today's eukaryotes lived around this time.
Steroid Preforms as Biochemical Fossils
But how this ancestor of today's eukaryotes arose and from which ancestors was previously unclear - fossil evidence of possible ancestors of this LECA was previously missing. “Scientists have long searched for fossil evidence of such ancestral eukaryotes, but their physical relics are extremely rare. Instead, the oceans of the primeval earth seemed to have been more of a kind of bacterial soup," says Nettersheim. In contrast, however, DNA comparisons suggested that the roots of eukaryotes must go back much further than 800 to 1200 million years. In their search for possible traces of these primeval protozoa, Nettersheim and his colleagues have now chosen a new chemical approach: Almost 30 years ago, the biochemist and Nobel Prize winner Konrad Bloch postulated that there could have been organisms before the advanced eukaryotes that had not yet possessed the complete metabolic pathway for the production of the typical eukaryotic steroids. They therefore only produced primordial sterols - precursors of the modern steroid molecules, which today only occur as intermediates in the cellular synthesis chain.
Theoretically, so the assumption of Nettersheim and his team, relics of such primeval protosteroids could have been preserved in ancient rock formations. "We used a combination of techniques to first convert various modern steroids into their fossil equivalent - otherwise we wouldn't have known what to look for," explains co-first author Jochen Brocks from the Australian National University. Then the team began to examine samples from up to 1.7 billion year old rock formations in different countries for these molecules - with success: "The oldest collection of these biomarkers came from the 1.64 billion year old Barney Creek Formation in Australia, it contained almost 100 different protosterol derivatives,” the scientists report. These proto- and primordial sterols were also found in other rocks from the period up to around 800 million years ago. "Once we knew our target, we discovered that dozens of other rocks collected from billions-year-old water bodies around the world were studded with similar fossil molecules," says Brocks.
Lost lifeworld from stem group eukaryotes
The scientists believe these results suggest that before the advent of modern-style eukaryotes, there must have been an entire lost habitat of more ancestral stem-group eukaryotes. "The new finds of these biomarkers reveal a protosterol environment that was widespread and very common in the mid-Proterozoic," write Nettersheim and his colleagues. During this time, 1600 to 800 million years ago, these primordial eukaryotes dominated the open water habitats and formed a large, widely ramified group of organisms that were probably perfectly adapted to the then oxygen-poor primeval earth. "These protosterol biota were in the oceans and lakes of the time—their relics were visible all along," says Brocks. "But scientists didn't know what to look for - until now." These stem group eukaryotes were so successful at the time that they initially largely displaced the ancestors of modern eukaryotes.
However, around 1000 to 800 million years ago – in the geological period of the tonium – this changed. During this time, the earth's atmosphere became increasingly oxygenated, the supply of nutrients to the oceans increased, and the climate became cooler and more changeable. This change also ushered in a far-reaching ecological change: the ancestors of today's eukaryotes, previously disadvantaged because of their more complex, less efficient synthesis of "modern" steroids, now had an advantage. Because unlike the protosterols, their steroids were more robust and better protected the cells from dehydration, cold shocks and osmotic stress. "The early representatives of the advanced eukaryotes could have been something like the extremophiles of their time," the researchers explain. When the environment changed in tonium, these "modern" single-celled organisms were able to displace their protosterol competitors, and the stem group eukaryotes died out a little later. "This transformation in tonium may have been one of the most profound ecological changes in the evolution of complex life," write Nettersheim and his colleagues. "The fossil proto- and primordial sterols are thus witnesses to a lost world of these stem group eukaryotes of the Mesozoic."
Source: Jochen Brocks (Australian National University, Canberra) et al., Nature, doi:10.1038/s41586-023-06170-w