An international team has discovered a new type of molecular fossil in sedimentary rocks from the mid-Proterozoic - which spans from 2.5 billion to 542 million years ago. Protosteroids, a type of lipids found in abundance in those rocks, indicate that eukaryotes were a dominant life form in aquatic environments between 1.6 billion and 800 million years ago, the authors explain in Nature. The finding would confirm the theory of Nobel laureate Konrad Bloch, who predicted the existence of these primordial molecules.
Artist's rendering of an assemblage of primordial eukaryotic organisms that inhabited a bacterial mantle on the ocean floor. Credit: MidJourney - TA 2023.
Reconstructing the history of life on our planet is an exciting and extremely difficult task. The authors of this paper have exploited a new type of fossil biomarker to reconstruct the origin of eukaryotic organisms. These are relatives of cholesterol and other sterols that have remained in rocks, sediments and pockets of oil for billions of years.
We are eukaryotes –as are terrestrial plants, fungi and a great diversity of single-celled organisms. We all share the property of having certain sterols, such as cholesterol, in our cell membranes. The rest of living organisms (bacteria and archaea) are prokaryotes and have a different type of sterols called hopanopolyols. By looking for one or the other in the formations of the past we can reconstruct when they appeared and how they changed.
As we do not have a time machine, we have to find out what happened from the traces that successive living beings have left behind. These traces are of two kinds. The first is the fossil record. Under certain conditions, the bodies of organisms are buried and preserved in sediments. The older the sediment, the fewer fossils are well preserved. In other words, the further back in time we want to go, the less information we can gather. The oldest eukaryotic fossils have been found in formations around 1.65 billion years ago, but in addition to their scarcity and low diversity, there was the added difficulty of not finding steranes, the fossilised residues of eukaryotic sterols.
The second type of trace is molecular. The history of our ancestors is recorded in our DNA. The mutations that have accumulated over time show us who our ancestors were, as many of our mutations had to be in them. By comparing the DNA sequences of two living beings, we can estimate how long ago our common ancestor lived. For example, the most recent common ancestor between chimpanzees and us lived between six and eight million years ago. Again, the further back we want to go in our lineage, the more uncertain the estimate becomes. In the case of the emergence of eukaryotes, estimates gave a date of between 1,200 and 1,850 Ma. If the ancestor of modern eukaryotes lived at that time, it would be logical to think that there must have been primitive eukaryotes much earlier.
The authors have developed a method for detecting fossil sterols. Instead of looking for modern ones, they have reconstructed what the precursor molecules - the protosteroids - would have looked like, and have managed to find them in formations as old as 1.6 billion years. This shows that these rare fossils of early forms were indeed eukaryotic, reconciling the two types of evidence mentioned above.
These precursor steroids are actually the intermediates in the synthesis pathways of modern ones. Over the course of evolution, more and more modern eukaryotes added steps in that pathway until they achieved the highly efficient sterols that we eukaryotes have today. This fact, that successive intermediates would have been the only sterols in primitive eukaryotes, was predicted as early as 1994 by Konrad Bloch [Nobel Prize in Physiology or Medicine in 1964], the discoverer of the cholesterol synthesis pathway. Thus, this work not only reconciles two different lines of evidence, but also confirms a theory.
The main difficulties of this type of work are also twofold. On the one hand, extraordinary efforts have to be made to avoid contamination of the samples with modern sterols. In the methods section, the authors give details of their protocol, which seems to be very cautious. And the second type of difficulty is knowing which compounds to look for. The novelty of the study is that they were able to look for the right sterols, those that were most likely to have existed millions of years ago. All in all, a beautiful study that solves an enigma and helps us to understand the past of life on our planet.
Sterols are a type of molecule found in the membranes of various living beings, especially eukaryotes. These molecules are quite stable, so they can be found in the sediments of the fossil record and are therefore known as 'biomarkers' or 'molecular fossils'. Obviously, molecules are going to be affected by diagenesis (the processes that form sedimentary rocks). For example, due to diagenesis, cholesterol is modified to cholestane. If cholestane is present in a sediment, that indicates that modern eukaryotes were present when it formed. Knowing that, you can make a correlation between the type of biomarker found in a geological sample and the type of organisms that might have existed at that time. But there are a couple of problems. The first is that the more time passes, the more the biomarkers degrade and alter, so the correlation becomes less and less accurate. The other is that, even if we know the original molecule, if we don't know how it is modified, we cannot identify the biomarker that is produced.
Knowing how these molecular alterations and molecules are produced can allow us to identify new biomarkers. That is precisely what the authors of this study have done. The researchers have found a new type of eukaryotic biomarker, proto-steroids, which until now was not considered as such. These proto-steroids come from molecules such as lanosterol and cycloartenol, which are intermediary molecules of the sterol pathway in today's eukaryotes.
I think it is a good quality and rigorous study. They have collected samples from different geological periods and analysed them with the most appropriate technology available to determine their composition. In addition, they have been very careful to avoid contamination during the handling of the samples. The results show that proto-steroids are present in rock samples ranging in age from 1.64 billion to 800 million years. Until now, no eukaryotic sterols had been found in samples of these ages. This would indicate that the most abundant eukaryotes at that time were different from today's eukaryotes in terms of sterol synthesis and that substances such as lanosterol and cycloartenol were not intermediates, but end products with a biological function.
Modern eukaryotes capable of producing molecules such as cholesterol and the like appeared about 1 billion years ago, during the so-called Tonic period. They became the dominant eukaryotes thanks mainly to the proliferation of red algae (rhodophytes) 800 million years ago, coinciding with the increase of oxygen concentration in the atmosphere. From that time onwards, 'molecular fossils' such as cholestane and the like began to be detected.
As this is a pioneering work, both its methodology and its results will have to be replicated and confirmed by other groups. But the results obtained fit quite well with the results of other work being published recently on the origin and evolution of the eukaryotic cell.