Steroids have been found in ancient rocks, indicating that the ancestors of eukaryotes lived around 1.6 to 0.8 billion years ago.
The discovered protosteroids thus suggest that ancestors of today's eukaryotes were more widespread much longer ago than previous biochemical evidence suggested. According to the authors, the organisms differed from eukaryotic organisms as we know them today in their cell structure and possibly also in their metabolism. This was adapted to a world that had far less oxygen in the atmosphere than today.
This "trunk" represents the common lineage of those organisms that were the ancestors of all branches of eukaryotes living today. Their representatives have long since died out, but further investigations into their nature could shed light on how complex life once evolved, according to the researchers.
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Bacteria, archaea and eukaryotes: Life on earth is traditionally divided into these three domains. Multicellular organisms such as plants, fungi and animals, which have more complex cells with a real cell nucleus, are categorised as eukaryotes. But when did the first eukaryotic organisms live on earth? The oldest microfossil finds come from rocks from Australia that are around 1.5 billion years old and were identified as the remains of algae by micropalaeontological investigations. However, biochemical analyses are far more difficult. So far, they have only been able to prove that ancestors of eukaryotes lived around 1000 to 800 million years ago.
An international team of researchers has searched for fossil biomarkers in rock samples up to 1.64 billion years old from Australia and other countries around the world and discovered primordial steroids. These so-called protosteroids point to a whole range of previously unknown eukaryotic organisms that lived in aquatic environments between 1.6 and 0.8 billion years ago, as the researchers show in their publication in "Nature".
"Almost all eukaryotes produce steroids, such as cholesterol, which is produced by humans and most other animals," explains one of the first authors of the study, Benjamin Nettersheim from the University of Bremen. "These lipid molecules are an integral part of eukaryotic cell membranes, where they fulfil a variety of physiological functions. By searching for fossilised steroids in ancient sediments, we can trace the development of increasingly complex life," says the researcher.
But how do you find such molecules in ancient rocks? "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 Jochen Brocks, Professor at the Australian National University, also first author of the publication. Researchers had overlooked these molecules for decades because they did not fit into the typical molecular search grid. "Once we knew our target, we discovered that dozens of other rocks from billion-year-old waters around the world were littered with similar fossil molecules," says Brocks.
The oldest samples with the biomarker come from the Barney Creek Formation in Australia and are 1.64 billion years old. Only fossilised molecules of ureukaryotes are found in the rock layers of the next 800 million years, before molecular signatures of modern eukaryotes first appear in the so-called Tonium period. According to Nettersheim, "the Tonium transformation proves to be one of the most profound ecological turning points in the history of our planet". During this period, the Earth's atmosphere was increasingly enriched with oxygen - a metabolic product of cyanobacteria and the first eukaryotic algae, which was toxic to many other organisms. In addition, global glaciations later occurred and the protosterol communities largely died out. The descendants were probably better able to survive heat and cold, as well as UV radiation, and displaced their original relatives.
The special thing about the discovery, however, is not only the molecular evidence of eukaryotes, which can be dated much earlier, says Christian Hallmann, another author of the study. "Since the last common ancestor of all modern eukaryotes, including humans, was probably capable of producing 'normal' modern sterols, there is a high probability that the eukaryotes responsible for these rare signatures belonged to the trunk of the evolutionary tree," says the researcher from the Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences.
