A "lost world": scientists discover the trace of our ancestors who lived 1.64 billion years ago
All animals – including humans – but also fungi and plants, among others, share a common characteristic: the fact of having cells with a nucleus. This places us among living beings called "eukaryotes". Scientists have discovered a molecular trace of our distant ancestors, in rocks over a billion and a half years old.
What if steroids were the key to tracing our evolutionary history? This family of molecules may remind you of bodybuilding, or the doping of certain top athletes. Yet such uses relate only to one particular category, that of anabolics – to put it simply, synthetic versions of testosterone that build muscle.
In reality, steroids in the broad sense, or more exactly "sterols", include, for example, cholesterol and phytosterol which make up the membranes of eukaryotic cells (with a nucleus), vitamin D which strengthens our skeleton and our joints , or bile acids essential for digestion.
In Australian rocks that correspond to a marine ecosystem dated 1.64 billion years ago, scientists have discovered a new type of sterols, called "protosterols". Their presence testifies to the existence of a "complex life" at this time, they explain in an article published by the journal Nature on June 7 ( Brocks, JJ, Nettersheim, BJ, Adam, P. et al. 2023 ) .
Thus, the common ancestors of animals, fungi and plants, among others, already populated our planet more than one and a half billion years before our era. To find the very first forms of life – at the origin of eukaryotes but also of bacteria and archaea without a cell nucleus – however, we have to go back even further, some 3.8 billion years ago.
The idea of a Nobel laureate
Finding fossils over a billion years old is nevertheless a challenge, because the preservation of these ancestral forms of life up to the present day requires special conditions. This is why the team of Pr Jochen Brocks, researcher at the Australian National University (ANU), chose a different approach: that of going in search of molecular traces.
But it was still necessary to know what to look for precisely. “Almost all eukaryotes synthesize steroids, such as the cholesterol produced by humans and most other animals , explains Professor Benjamin Nettersheim of the University of Bremen, co-author of the study, in a press release. ..) By searching for fossilized steroids in ancient rocks, we can trace the evolution of increasingly complex life."
An idea that does not come out of nowhere, since it was first put forward in the 1990s by the German biochemist Konrad Bloch – laureate with Feodor Lynen of the 1964 Nobel Prize in Physiology or Medicine for their discoveries on the mechanisms and cholesterol regulation and fatty acid metabolism.
The authors of the study therefore began by comparing current steroids in order to deduce the chemical structure of potential fossil equivalents. Then they went through rocks with a fine-toothed comb – or rather, with a laser and a high-resolution mass spectrometer.
"Once we identified our target, we discovered that dozens of other rocks, taken from billion-year-old streams around the world, were teeming with similar fossil molecules," he said . Prof Brocks.
Oxygen and "Ice Ball Earth"
These first complex communities contributed, with certain bacteria (cyanobacteria), to emit large quantities of oxygen into the atmosphere, underline the authors of the study. Soon after, the Earth became almost completely covered in ice – a phenomenon called "Snowball Earth". Most of these organisms then disappeared… except for a few, probably favored through natural selection by their resistance to cold or UV rays. Our ancestors !
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