The last common ancestor of all living organisms (LUCA) lived about 4.5 billion years ago, that is, it is practically the same age as the planet. Biologists came to such an unexpected conclusion, applying dating by the "molecular clock" method … The result is presented in a scientific article published in the journal Nature Ecology & Evolution by a group led by Davide Pisani from the University of Bristol.
The traditional source of information about the evolution of life is, of course, fossils. But the Earth is a changing planet. Erosion, continental movement, volcanic activity and other geological processes are gradually erasing traces of the past. Information about the Archean Eon (4.0-2.5 billion years ago) is extremely fragmentary. In addition, sometimes it is completely difficult to understand whether the found fossils are of biological origin.
However, the fossil record is not the only source of information about the development of life. The evolutionary tree of living organisms can be built based not only on their anatomy, but also by interpreting the DNA code. The similarities and differences in the genome indicate how closely related certain groups of living beings are, and make it possible to establish who descended from whom. This is done in much the same way as with the analysis of fossils. Only if paleontologists have at their disposal a limited number of external traits that they can compare with each other, then genetics are in a better position: each of the many thousands of genes can in fact be considered a separate trait.
Genetic methods make it possible to produce and date evolutionary events. It is based on the molecular clock method. Its essence is simple in general terms: it is believed that mutations accumulate at the same average rate (although in some cases it is worth considering significant amendments to this rule). Therefore, by the number of mutations that distinguish two evolutionary branches, one can judge how long ago they diverged.
On the whole, the results obtained by two methods (genomic and paleontological) are in good agreement with each other. This proves once again that science in general terms correctly represents the history of life on Earth. In those cases where the fossil record is poor (as, for example, in the Archean era), genomic methods can compensate for the lack of knowledge.
Pisani's group re-calculated the rate of mutation and built an evolutionary tree of life, relying primarily on genomic techniques, but also using dated fossils. The results are impressive.
Promotional video:
Recall that the oldest fossil traces of life are 3.95 billion years old. However, this does not mean that the results of the authors of the new work contradict the paleontological data.
The fact is that almost all fossils of any epoch are buried in sedimentary rocks. The oldest traces of life, respectively, actually refer to the oldest surviving sedimentary rocks on Earth. That is, life could have existed much earlier, but there were no geological conditions for its traces to reach our time.
Several other key evolutionary events have been re-dated as well. So, bacteria and archaea, according to the calculations of the authors, separated no earlier than 3.4 billion years ago. Eukaryotes (which include, in particular, all multicellular) appeared about 1.84 billion years ago. The last common ancestor of all mitochondria (recall that these cellular organelles, according to generally accepted ideas, were previously separate organisms) lived 2.053-1.21 billion years ago.
The most sensational results, of course, relate to the lifetime of LUCA. They mean that life is actually the same age as Earth, because the formation of the planet ended about 4.5 billion years ago. Soon after that, Theia struck, after which the planet did not have a solid surface at all, representing a continuous ocean of fire-breathing lava.
Thus, the formation of living things from primary organic matter took tens, at least two or three hundred million years. If the results of Pisani and colleagues are confirmed, experts will have to answer the question of how the process of the birth of life could be completed so quickly.
However, the probability of an error cannot be ruled out yet. We have already mentioned that there are important subtleties in the molecular clock method. So, scientists have proved that in the history of life there have been eras when mutations occurred much more often than on average. In addition, some parts of the genome are more susceptible to change than others. Further, some organisms have a mutation control system: once in unfavorable conditions, they purposefully accelerate mutagenesis "in the hope" of obtaining a new property that allows them to adapt to a new environment. This is how, for example, bacteria develop antibiotic resistance.
Of course, biologists, using the molecular clock method, take into account all these nuances. But no one can guarantee that there is still no factor distorting the dating results. Experimenters nowadays make one surprise after another, and the molecular foundations of life are continually being more complex than we imagined.
Anatoly Glyantsev
