The answer is Alexander Markov, Doctor of Biological Sciences, paleontologist, leading researcher at the Paleontological Institute of the Russian Academy of Sciences, head of the Department of Biological Evolution of the Biological Faculty of Moscow State University.
By and large, we cannot say anything concrete about life on other planets, because we know only one form of life - earthly. We know that it has a single origin, although, in principle, life on Earth could have arisen many times, and living beings living on the planet could have come from different "roots". But we do not observe this on Earth - in particular, this is clear from the fact that all living beings known to us have the same genetic code. Therefore, we can say little about which of the properties of earthly life are common to life in general, and which are unique. You can only build not very well-founded hypotheses.
Alexander Markov
Quite confidently, we can probably say that the property of any life is the ability for Darwinian evolution. At the moment, we have only two models for creating complex and diversely arranged objects, such as living organisms. The first is intelligent design, when some intelligent being creates them. The second is Darwinian evolution. We do not yet know any third way and cannot even imagine. Therefore, if we find life somewhere in the Universe, we can be sure that this is life capable of Darwinian evolution, or artificial life.
The latter kind of life may not be capable of Darwinian evolution. Moreover, even if we find something similar to life and can prove that this life does not evolve, then we should assume that it is most likely created artificially and purposefully. If life arose naturally, then it will certainly evolve according to Darwin. This means that it will necessarily have four properties: the ability to reproduce - the ability to create its own copies; variability - that is, this copying should not be absolutely accurate, there should be slight deviations from the original; heredity - at least some of the changes that occur during copying must be inherited by next generations; and something elsethat at least some hereditary differences should affect the efficiency of reproduction. This fourth point is also called "natural selection".
Symmetad from the novel by Stanislav Lem “ Solaris ”
As for the chemical foundations of other life, there are some scientific developments on this topic. Many have tried to speculate and even experiment on whether some other chemical basis of life is possible, not like ours. Is the presence of water as the main solvent necessary, is carbon really necessary, etc. The more or less satisfactory results of such studies are such that the elements from which our earthly life is created is what is easiest to build life from in general. Attempts to replace, say, carbon with silicon, and oxygen with sulfur or fluorine, lead to very great difficulties. Although here, of course, we can always assume that we simply do not know something and, perhaps, someday we will find life based not on water, but, for example, on hydrogen fluoride, as in one of Efremov's stories.
At the same time, some other polymer is quite possible as a heredity substance. The main thing is that it can reproduce. Our DNA and RNA are convenient because thanks to the principle of complementarity (mutual interaction of biopolymer molecules or their fragments, which ensures the formation of bonds between spatially complementary fragments of molecules or their structural fragments due to supramolecular interactions - NS), these molecules are very well suited for copying. But proteins and carbohydrates, for example, cannot multiply like that. That is, life must be based on some kind of polymer that is easy to copy. Already today, other similar polymers have been artificially created, which always use the principle of complementarity, that is, there are always complementary nucleotides there. Not necessarily our A = T, G = C,there may be some other pairs, but complementarity between them is required. You can read about alternative biochemistry in the book by biologist Mikhail Nikitin “The Origin of Life. From nebula to cell”.
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Mimoid from the novel by Stanislav Lem “ Solaris ” simulates a helicopter
In connection with the theme of extraterrestrial life, someone may recall the existence of such ancient and unlike all other living organisms, like viruses. But they have no cells, no metabolism, they are not able to reproduce without the help of another living cell, so there is no need to talk about their separate existence - at least in their earthly and modern versions. They could exist independently, if you supply them with what they lack for independent life, but in this case it will, in fact, no longer be a virus, but something like a cell.
Another thing is that viruses are probably older than modern cells. Therefore, in the early stages of the origin of life, when there were no cells yet, there was probably a community of multiplying, replicating (replication is the process of synthesis of a daughter DNA (or RNA) molecule on the template of the parent DNA or RNA molecule, NS) molecules. And at that time it was impossible to draw a clear line between cellular life forms and viruses - not a single molecule was then self-sufficient, they cooperated and jointly somehow multiplied each other. Some of this mess of cooperating molecules later united into strong alliances, surrounded by a shell and became cells, and some became independent molecules, but which needed the help of others - that is, viruses.
Can you imagine evolution taking a different path, forming a non-cellular life form? I guess, yes. Something not subdivided into cells, something like a plasmodium - a huge, thick spread cell thousands of kilometers in size, in which there are many sets of chromosomes and genomes that synthesize some kind of proteins around them. Something approaching the sentient ocean from the Solaris novel. All this can be imagined. But in this case, selection will still take place, only within the organism itself - at the level of individual droplets, fragments of the "ocean", at the level of sets of its chromosomes and genomes.
Why am I thinking this way? Because just the other day I wrote a note on a new article that came out in September in the journal Nature Communications. A completely unimaginable bacterium with a fundamentally new type of genetic architecture was discovered. It is a giant bacterium - more than a tenth of a millimeter - with calcite granules. It turned out that in one cell of such a bacterium there is not one genome, as it should be, but many, and all of them are very different from each other. These differences are comparable to the level of differences between different types of bacteria. That is, it is like a whole multi-species community within one shell. And the authors of the study believe that in this bacterium, the selection takes place at the level of parts of one cell, that is, the genomes inside it somehow multiply, somehow help each other and partly compete with each other. So it is probably possible to imagine something similar and even more grandiose.
Alexander Markov