Having studied the metabolism of primitive microbes, biologists groped for traces of a system that could be the precursor of living cells. We are talking about a complex self-sustaining cycle of biochemical reactions, which became the basis of the first living organisms.
The achievement is described in a scientific article published in the journal Proceedings of the Royal Society B: Biological Sciences.
Even the most primitive living cell is very complex. It is hard to believe that it could have arisen "ready-made" from a mixture of the right chemicals. Probably, cells were preceded by simpler systems, also capable of multiplying and evolving, and in this sense, living. Gradually becoming more complex and improving under the pressure of natural selection, they gave rise to the first cells.
What kind of systems could they be? Almost 50 years ago, renowned biologist and co-author of a new study, Stuart Kauffman, suggested that it was about autocatalytic cycles of chemical reactions.
Let us explain what we are talking about. When the same substances can enter into different chemical reactions, competition for the starting reagents begins between the reactions. Essentially, natural selection is turned on. The reaction that goes faster wins. And the fastest are reactions, the product of which is at the same time a catalyst that accelerates the course of the same reaction (such processes are called autocatalytic, that is, they spur themselves on).
But what happens when all the available reagents are used up? The reaction, of course, will stop, that is, it will "die". To prevent this from happening, it must be reversible: along with the direct process, where the initial substances are converted into a product, there must also be a reverse process, in which the product again "breaks down" into reagents.
Calculations and experiments show that autocatalytic chemical cycles are capable of "multiplying" and "evolving", that is, they really could be the precursors of life.
Diagram of a self-sustaining autocatalytic network of chemical reactions that could be the forerunner of life. The original components are highlighted in red.
Illustration by Joana C. Xavier.
But what kind of reaction cycles gave rise to the first living cells? New article co-authors Mike Steel of the University of Canterbury in New Zealand and Wim Hordijk of the Konrad Lorenz Institute for Evolution and Cognitive Research have put forward an intriguing hypothesis.
They found a whole class of chemical cycles remarkably similar to the actual metabolism of living cells. The authors called them reflexively autocatalytic food generated networks, or FAR.
FARs are interesting not only because they resemble real biochemical processes. They are also easy to start if the necessary reagents are available, it is worth pushing the system a little by adding an initial catalyst to it.
"This is what physicists call self-organization, a kind of holy grail in the study of the origins of life," explains Hordejk.
The authors of the new article set out to search for RAF in the metabolism of primitive microbes. They focused on bacteria and archaea that do not need oxygen, since free oxygen appeared on Earth billions of years after the first living organisms and, moreover, as a product of their activity.
They found several different RAFs in the metabolism of certain microbes. The most complex of the found systems includes 1335 reactions. Approximately the same number of processes determine the metabolism of a modern cell. However, researchers were primarily interested in the simplest cycles, which could be the predecessors of complex ones.
In microbes, which receive energy from the absorption of hydrogen and carbon dioxide (both were present in abundance on the early Earth), experts have found two simpler RAF systems, which, however, are capable of performing the basic functions of metabolism. The first is present in organisms that produce methane and contains 209 reactions. The second is characteristic of microbes that release acetate and consists of 394 reactions.
In this case, 172 reactions are common for both sets. They are found in both bacteria and archaea, and, apparently, go back to the last common ancestor of all living things.
Unlike almost all biochemical reactions, the isolated processes do not require large and complex protein molecules as catalysts. Much simpler substances are enough for them - cofactors and metal particles. Nevertheless, they synthesize amino acids (building blocks of proteins) and nitrogenous bases (one of the main components of DNA and RNA).
These biochemical cycles are probably older than genes. Perhaps this is the last biochemical relic of the precellular era. Interestingly, the microbes in which they were found live in geothermal springs.