Foreword by Boris Stern
We have long planned to hold a discussion related to the eternal question of the place of man in the Universe. This, of course, is about extraterrestrial life and planets from other stars. At the moment, slightly less than 6 thousand exoplanets are known, of which more than two thousand are awaiting independent confirmation. But for statistical research, you can use all 6 thousand.
There are very few planets among them that are supposedly suitable for life. This is natural, because they are the hardest to find: a very powerful selection effect works against earth-like planets. They are too light to be caught by the radial velocity of a star, and their year is too long for their transits to be reliably unearthed from the Kepler space telescope 1. The exception is the planets in the habitable zone of red dwarfs, which are open under our very noses, it is much easier to find them. There are a lot of such planets, but, alas, red dwarfs are very inconvenient for life next to them. However, the extrapolation of Kepler's data for the "hot lands" of stars like the Sun gives a very optimistic result: at least 15% of these stars have planets in their habitable zone. This estimate was obtained independently by many authors,and over time it becomes more and more optimistic: 20% and even a quarter of the suns have lands. This means that the closest class G or K star to us with Earth in orbit in the habitable range is within 15 light years. There are few such stars, and candidates are already emerging, for example Tau Ceti. And there are many such planets within a radius of, say, 30 light years.
Observation methods are gradually progressing. New nearby Earth-like planets will be discovered using the improved HARPS instrument. In the next decade, we will learn a thing or two about the atmospheres of some Earth-like planets using instruments such as the giant Extremely Large Telescope (ELT) and the James Webb Space Telescope. And it is possible that oxygen will appear in the absorption spectrum of the atmosphere of some transit planet (passing through the disk of a star). If the star is not overly active and old enough, oxygen can only be biogenic. This is how extraterrestrial life can be detected.
Is it real? If life arises in any corner as soon as conditions arise for it - why not? But is it? The argument is often made that life on Earth appeared very quickly, which means that this is the case - a few hundred million years are enough for it to appear in some kind of soup. But there is also a counterargument - a suitable "soup" can only exist on a young planet - life arises either quickly or never.
And, of course, there is the opposite point of view: life is a rare phenomenon based on a completely incredible coincidence. The most detailed point of view on this matter, professional and with quantitative estimates, was expressed by Evgeny Kunin. Life is based on copying long molecules, originally they were RNA molecules. Copying is done by a certain device called a "replicase" (these lines were written by a physicist, therefore the terminology from the point of view of a biologist is somewhat awkward). Replicase will not come from anywhere if it is not programmed in the same RNA being copied.
According to Kunin, in order for the self-reproduction of RNA to start, and with it evolution, “at a minimum, the spontaneous appearance of the next one is necessary.
- Two rRNAs with a total size of at least 1000 nucleotides.
- Approximately 10 primitive adapters of 30 nucleotides each, for a total of about 300 nucleotides.
- At least one RNA encoding replicase is approximately 500 nucleotides in size (bottom score). In the accepted model, n = 1800, and as a result, E <10 - 1081 ".
In the given fragment, we mean a four-letter encoding, the number of possible combinations is 41800 = 101081, if only a few of them start the evolution process, then the probability of the required assembly per one "attempt" of spontaneous assembly is ~ 10-1081.
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There is no contradiction in the fact that the result is before our eyes, there is no: according to the theory of inflation, the Universe is huge, tens of orders of magnitude larger than its visible part, and if we understand the universe as a closed space, then there are universes with the same vacuum as ours, a gigantic set … The smallest probability is realized somewhere, giving rise to a surprised beholder.
These two extremes mean a lot in terms of our place in the universe. In any case, we are alone. But if life exists dozens of light years away from us, this is technological loneliness overcome by development and millennial patience. If Kunin's assessment is correct, this is a fundamental loneliness that cannot be overcome by anything. Then we and earthly life are a unique phenomenon in the causally connected volume of the Universe. The only and most valuable. This is important for the future strategy of humanity. In the first case, the basis of the strategy is search. In the second case - sowing (there is even such a term "directed panspermia"), which also includes the search for a suitable soil.
All of this deserves a discussion. Are there loopholes through Kunin's argument? Are there any mechanisms that can be discerned bypassing the "irreducible complexity" of the RNA replicator? Is it really that unforgivable? Etc.
We asked several biologists for their opinions.
Alexander Markov, Ph. D. biol. sciences, led. scientific. sotr. Paleontological Institute RAS, head. Department of Biological Evolution, Biological Faculty, Moscow State University:
Evgeny Kunin's assessment, which implies that we are hopelessly alone in the universe, is based on one key assumption. Kunin believed that in order to start the process of RNA replication (and with it Darwinian evolution; it is logical to consider this moment as the moment of the origin of life), it was necessary that purely by chance - as a result of a random combination of polymerizing (for example, on mineral matrices) ribonucleotides - a ribozyme with RNA polymerase activity appeared, that is, a long RNA molecule that has a completely definite (and not just any) nucleotide sequence and, due to this, is able to effectively catalyze RNA replication.
If there is no other way, another "entrance" into the world of the living from the world of inanimate matter, then Kunin is right, and we should give up the hope of finding any life in the Universe except earthly. It can be assumed that it all started not with a single highly efficient polymerase, but, for example, with a certain community of small, ineffective polymerases and ligases (ribozymes that are able to stitch short RNA molecules into longer ones): perhaps this will make the assessment a little more optimistic, but will not fundamentally change the situation. Because the first replicator was still very complex, and it should have appeared without the help of Darwinian evolution - in fact, by accident.
A viable alternative is non-enzymatic RNA replication (NR RNA): a process by which RNA molecules are replicated without the aid of complex ribozymes or protein enzymes. Such a process exists, it is catalyzed by Mg2 + ions, but it goes too slowly and inaccurately - at least under the conditions that the researchers had time to try.
However, there is a hope that nevertheless it will be possible to find some plausible conditions (which, in principle, could exist on some planets), when the NR RNA goes fast enough and accurately. Perhaps this requires some kind of relatively simple catalyst that can be synthesized in an abiogenic way. It is possible that simple abiogenic peptides with several negatively charged residues of aspartic acid that retain magnesium ions can act as such catalysts: protein RNA polymerases have similar active centers, and this possibility is now being explored.
The question of the possibility of an effective NR RNA is of fundamental importance for assessing the probability of the origin of life. If NR RNA is possible, then there may be quite a few living planets in the observable Universe. The fundamental differences between the two scenarios - with possible and impossible NR RNA - are shown in the table. If NR is possible, then Darwinian evolution could begin almost immediately after the appearance of the first short RNA molecules. The selective advantage should have been obtained by those RNA molecules that multiplied more efficiently by means of HP. These could be, for example, molecules with palindromic repeats, which could themselves serve as primers - "seeds" for replication; palindromes can fold into three-dimensional structures - "hairpins", which increases the likelihood of the appearance of catalytic properties in the RNA molecule. Anyway, afterAs Darwinian evolution began, the further development of life was determined not only by chance, but also by law.
Estimates of the probability (frequency) of the origin of life under these two scenarios should differ by a huge number of orders (although, of course, no one will give exact figures). It is also important to note that if life originated "according to Kunin", that is, due to the random assembly of an effective ribozyme-polymerase, then the principle of complementarity (specific pairing of nucleotides), on which the ability of RNA to reproduce and evolve, turns out to be a kind of "piano in the bushes”, Which had nothing to do with the fact that such a huge amount of RNA molecules had accumulated on the planets that an effective ribozyme with RNA polymerase activity accidentally appeared on one of the planets. If life arose "according to Shostak" (Nobel laureate Jack Shostak is now actively studying NR RNA and believes that this process is the key to the mystery of the origin of life),then complementarity was not a "piano in the bush", but worked from the very beginning. This makes the entire origin-of-life scenario a lot more compelling and logical. I would bet on Shostak.
Thus, now everything depends on the success of specialists in the field of prebiotic chemistry. If they find realistic conditions in which NR RNA is going well, then we have a chance to find life on other planets. And if not, then … we must look further.
Armen Mulkidzhanyan, Dr. biol. Sci., University of Osnabrück (Germany), led. scientific. sotr. MSU:
It is difficult to argue with the fact that life arose long ago and on the young Earth. The earth is composed of chondritic rocks, like meteorites. The heating of these rocks during the formation of the Earth caused the melting of the water brought with chondrites. The interaction of water with a heated, reduced rock should have led to the release of electrons, the formation of hydrogen and the reduction of carbon dioxide (CO2) to various organic compounds. Similar processes are still taking place in areas of geothermal activity, for example, in geothermal fields, but with low intensity. So the formation of organic matter in large quantities can be expected on the young planets of other stars. The likelihood that life can arise in this case can be estimated by considering the evolution of earthly life.
For the first two billion years, only microbes lived on Earth. It would have continued this way, but about 2.5 billion years ago, photosynthetic bacteria learned to use the energy of light to decompose water. Photosynthesis originally arose as a replacement for the damped geochemical processes of "dumping" excess electrons. In photosynthesis, the energy of light is used to oxidize various compounds, that is, to "take away" electrons from them, to photoactivate these electrons and ultimately reduce CO2 to organic compounds by them. The water decomposition system has arisen as a result of the gradual evolution of simpler photosynthetic enzymes preserved in some bacteria. There are several very plausible scenarios for how such enzymes, using light and chlorophyll, first oxidized hydrogen sulphide (and even now some people do it), then,as the hydrogen sulfide in the medium was exhausted, electrons were taken from ferrous iron ions, then from manganese ions. As a result, they somehow learned to decompose water. In this case, the electrons taken away from the water went to the synthesis of organic matter, and oxygen was released as a by-product. Oxygen is a very strong oxidizing agent. I had to defend myself against him. The emergence of multicellularity, warm-bloodedness and, ultimately, intelligence are all different stages of protection against oxidation by atmospheric oxygen.warm-bloodedness and, ultimately, reason - these are all different stages of protection against oxidation by atmospheric oxygen.warm-bloodedness and, ultimately, reason - these are all different stages of protection against oxidation by atmospheric oxygen.
Water decomposition occurs in a unique catalytic center containing a cluster of four manganese atoms and one calcium atom. In this reaction, which requires four quanta of light, two water molecules (2 H2O) decompose at once to form one oxygen molecule (O2). This requires the energy of four quanta of light. In response to the absorption of three light quanta, three electron vacancies ("holes") accumulate on the manganese atoms, and only when the fourth light quantum is absorbed, both water molecules are oxidized, the holes are filled with electrons and an oxygen molecule is formed. Although the structure of the manganese cluster has recently been determined with high precision, how this four-stroke device works is not fully understood. It is also unclear how and why in the catalytic center, where in primitive photosynthetic bacteria, apparently,manganese ions were oxidized, four of its atoms combined with a calcium atom into a cluster capable of decomposing water. The thermodynamics of chlorophyll participation in water oxidation is also mysterious. Theoretically, chlorophyll under illumination can oxidize hydrogen sulfide, iron, and manganese, but not water. However, it oxidizes. In general, it's like about a bumblebee: “According to the laws of aerodynamics, a bumblebee cannot fly, but he does not know about it and only flies for this reason.
It is very difficult to assess the likelihood of a water decomposition system. But this probability is very small, since in 4.5 billion years such a system emerged only once. There was no particular need for it, and without it, microbes would thrive on Earth, being included in geochemical cycles. Moreover, after the appearance of oxygen in the atmosphere, most of the microbial biosphere should have died or, more precisely, burned up - the interaction of organic matter with oxygen is combustion. Only microbes survived, having learned to breathe, that is, to quickly restore oxygen back to water directly on their outer shell, preventing it from inside, as well as the inhabitants of the few remaining oxygen-free ecological niches.
This story can serve as an example of a relatively recent (some 2.5 billion years ago) and relatively understandable event that led to a sharp increase in the complexity of living systems. It all started with gradual changes in photosynthetic enzymes. Then there was a one-time and very nontrivial evolutionary invention (manganese-calcium cluster), which might not have been. Subsequent tremendous changes were a reaction to the appearance of "toxic" oxygen in the atmosphere: Darwinian selection turned on at full power, I had to learn to breathe deeply and move my brains.
In total, we have a process that takes place in three stages: (1) gradual changes - (2) a one-time unlikely event - (3) further evolution, but at a different level or in different conditions. This scheme can be regarded as a molecular analogue of Severtsov's classical scheme of aromorphoses.
If you look at post-oxygen evolution, you can identify several more such unlikely one-time events that changed the course of evolution. This is the "assembly" of a complex eukaryotic cell, and the emergence of vascular plants, and various "breakthroughs" in the evolution of animals, which, in fact, Severtsov wrote about.
The emergence of life, which within the framework of the RNA world hypothesis is understood as the emergence of self-reproducing ensembles of RNA molecules (replicators), can also be represented as a three-stage process.
1) Preparatory stage: RNA-forming ribonucleotides are able to spontaneously "assemble" from simple molecules like cyanide or formamide under the influence of ultraviolet (UV) light. He was in abundance on the young Earth; there was no ultraviolet-absorbing ozone in the atmosphere yet, since there was no oxygen, see above. As Pouner and Saderland (University of Manchester) have shown, nucleotides in a special, “activated” cyclic form are “selected” in UV light; such nucleotides are able to spontaneously form RNA chains. Moreover, double, Watson-Crick RNA chains are significantly more resistant to UV radiation than single ones - this result was described by Evgeny Kunin in his very first published work back in 1980. That is, on the young Earth, due to the flow of "extra" electrons, a variety of organic molecules could be formed,but under the influence of hard solar radiation, it was primarily RNA-like molecules, preferably coiled into helical structures, that survived.
2) One-time, unlikely event: an ensemble of several RNA-like molecules began to copy itself (billions of years later, similar self-copying RNA ensembles were obtained by RNA selection under laboratory conditions).
3) Subsequent evolution: RNA replicators began to compete with each other for resources, evolve, unite into larger communities, etc.
The disadvantage of this hypothetical scheme is that neither the molecular details of the origin of RNA replicators, nor the natural factors that contribute to their selection are known. Hope is given by the fact that in the case of the next most important (and in turn) evolutionary event, namely the emergence of ribosomes, machines for protein synthesis, molecular details have been reconstructed. This was done by various methods in four laboratories; the results of the reconstructions are very similar. In short, the ancestor of modern very complex ribosomes was a construct of two RNA loops of 50-60 ribonucleotides each, capable of combining two amino acids with a peptide bond. The intermediate stages on the path from this two-loop structure to modern ribosomes were tracked in detail by Konstantin Bokov and Sergey Stadler (University of Montreal),Nobel laureate Ada Yonath and colleagues (Weizmann Institute), George Fox and colleagues (University of Houston) and Anton Petrov and colleagues (University of Georgia).
The ribosome, which at first had one catalytic RNA subunit, gradually grew in complexity and size, all this time synthesizing protein sequences from a random set of amino acids. Only at the last stages of its evolution did it merge with another RNA molecule, which became a small subunit of the ribosome, and the encoded protein synthesis began. Thus, the emergence of the genetic code is an unlikely evolutionary event separate from the emergence of ribosomal protein synthesis.
Most likely, further research will make it possible to reconstruct both the emergence of replicators and other unlikely events, for example, those associated with the emergence of the first cells, the exchange of genes between the first cells and viruses, etc.
Returning to the questions posed about probabilities: our detailed consideration shows that the evolution of earthly life is not one "absolutely incredible coincidence", but many successive extremely unlikely events.
Powerful generation of organic matter was most likely going on on other young planets. But this did not necessarily lead to the emergence of life. If the self-replicating RNA ensemble had not gathered on Earth, there would have been no life. The production of organic matter would gradually fade, and the Earth would become similar to Mars or Venus.
But even in the case of the emergence of life on other planets, this life could "get stuck" at any initial stage, and the probability of forever remaining at a primitive level of development was incomparably higher than the probability of climbing the next step and moving further.
Therefore, the likelihood of meeting wise aliens on another planet is immeasurably lower than the chance of getting into a simple but living slime there (and this is if you are very lucky). The likelihood that there is oxygen life somewhere is also immeasurably small: the decomposition of water to form oxygen is a very nontrivial four-electron reaction.
So building any strategy in the hope of finding an alien intelligence is just not very smart. The fact that there are (for now) intelligent beings on Earth is a very great success. Therefore, it makes much more sense to invest in the creation of "alternate airfields" for the already existing intelligent life in case nature fails or the carriers of the mind themselves fail. This means we need a spare Earth, or even better a few.
Evgeny Kunin, led. scientific. sotr. National Center for Biotechnology Information, Member of the US National Academy of Sciences:
I can confine myself to very brief remarks, since I fully agree with everything said by Alexander Markov … except, of course, the conclusions. Indeed, the limiting stage in the emergence of life is the spontaneous formation of a population of ribozyme-polymerase molecules with a sufficiently high rate and accuracy of self-copying. The probability of such an event is vanishingly small. To increase it significantly, a process is needed that creates the possibility of evolution without the participation of such ribozymes, in a much simpler system. The non-enzymatic replication discussed by Alexander is a good candidate for such a process. The only trouble is that based on everything I know from chemistry and thermodynamics, there is no chance of bringing these reactions to the level of sufficiently accurate replication of long molecules. Replication of very short oligonucleotides would be very interesting as a possible intermediate step, but will not significantly increase the likelihood. Thus, my conclusion remains the same: the emergence of life requires extremely unlikely events, and, therefore, we are alone in our Universe (the issue of multiple universes is not necessary to discuss here). Not only we are intelligent beings, but more broadly, living beings in general.
It is important to note the following: the extremely low probability of the occurrence of life does not mean in any way that it all happened by a miracle. On the contrary, they are all a series of normal chemical reactions, only including stages with a very low probability. Consequently, studying the mechanisms that somehow facilitate the emergence of life is not only meaningless, but extremely important and interesting. It just isn't visible (yet) that this could significantly increase the likelihood, but creating a scenario of events may well help.
Well, I will end with a quasi-philosophical, but, in my opinion, relevant consideration. The extremely low probability of the emergence of life violates the mediocrity principle: the events that have occurred on our planet are exceptional, even unique in the Universe. The principle of mediocrity in this case loses to the anthropic principle: no matter how incredible the emergence of life was a priori, UNDER the CONDITION of the existence of intelligent beings, and just cells, its probability is exactly equal to 1.
Mikhail Nikitin, researcher sotr. Department of Evolutionary Biochemistry, Research Institute of Physicochemical Biology. A. N. Belozersky Moscow State University:
It seems to me that life of a bacterial level of complexity is widespread in the Universe, but development to multicellular animals and potentially intelligent beings is much less likely.
Why do I think bacterial life is highly probable?
Kunin's reasoning is based on experiments on the artificial selection of ribozymes-replicases, which copy RNA molecules and can potentially copy themselves. All these ribozymes are on the order of 200 nucleotides in length, and the probability of obtaining them by random self-assembly is on the order of 4-200. However, these experiments did not take into account many important factors that could, firstly, ensure replication using shorter and simpler ribozymes, and secondly, before the start of any replication, direct self-assembly towards structured RNAs capable of working as ribozymes. Some of these factors have already been named by other authors: Shostak's non-enzymatic replication, selection for self-priming in Markov's "world of palindromes", selection for UV resistance, which directs RNA self-assembly towards hairpin structures proposed by Mulkidzhanyan). I will add to this list mineral substrates and "heat traps" (narrow pores with a temperature gradient) that make RNA copying very easy. Further, since we have a simple self-replicating genetic system, Darwinian evolution with a high probability will quickly create on its basis a bacterial cell or something similar - with a cell membrane that maintains a constant salt composition inside the cell.
Why do I believe that the evolution of life from simple cells to multicellular animals may be very unlikely? There are two considerations here, one more geological, the other purely biological. Let's start with the first one.
In paleontology, it has been reliably established that the evolution of organisms is very uneven. Crises and revolutions alternate with periods of stasis, sometimes very long. The longest period of stasis was called the "boring billion" and lasted most of the Proterozoic - from about 2 to 0.8 billion years ago. It was preceded by the appearance of oxygen in the atmosphere, the emergence of eukaryotic cells and the global Huron glaciation, and it ended with the largest Sturt glaciation in the history of the Earth, an increase in oxygen content to almost modern values and the appearance of multicellular animals. Evolution was also relatively slow in the Archean eon between 3.5 and 2.5 billion years ago compared to both the previous Catarchian eon (the time of the emergence of life and the late meteorite bombardment) and the subsequent oxygen revolution. The reasons for this unevenness are not fully understood. It seems to me personally convincing that the "oxygen revolution" (the massive spread of oxygen-producing cyanobacteria) was associated with the depletion of reserves of reduced (ferrous) iron in ocean water. As long as there was enough iron in the ocean, microbes thrived there using simpler and safer iron-oxidizing photosynthesis. It is not oxygen that is released in it, but compounds of iron oxide - magnetites and hematites, which were deposited on the seabed throughout the Archean. The supply of new iron to the sea (mainly from hydrothermal sources at the bottom) decreased as the geological activity of the planet subsided, and finally the resource crisis forced photosynthetic microbes to switch to a more complex “technology” of oxygen photosynthesis. Similarly,the cause of the “boring billion” could be the constant oxygen consumption for the oxidation of various minerals on land, which does not allow raising the oxygen content above 1–2%. In Proterozoic marine sediments, there are many traces of onshore oxidation of sulfide ores, due to which rivers carried sulfates, arsenic, antimony, copper, chromium, molybdenum, uranium and other elements into the ocean that were almost absent in the Archean ocean. The Late Proterozoic crisis with global glaciations, a rapid increase in oxygen content, and the emergence of multicellular animals may have been caused by the depletion of easily oxidized minerals on land.because of which the rivers carried sulfates, arsenic, antimony, copper, chromium, molybdenum, uranium and other elements into the ocean that were almost absent in the Archean ocean. The Late Proterozoic crisis with global glaciations, a rapid increase in oxygen content and the appearance of multicellular animals may have been caused by the depletion of easily oxidized minerals on land.due to which the rivers carried sulfates, arsenic, antimony, copper, chromium, molybdenum, uranium and other elements into the ocean that were almost absent in the Archean ocean. The Late Proterozoic crisis with global glaciations, a rapid increase in oxygen content, and the emergence of multicellular animals may have been caused by the depletion of easily oxidized minerals on land.
Thus, the timing of the onset of two key revolutions (oxygen photosynthesis and multicellular animals) was probably determined by the balance of biological (photosynthesis) and geological (release of ferrous iron and other oxidizable substances by hydrothermal vents and ground volcanoes) processes. It is quite possible that on other planets, these revolutions come much later. For example, a more massive planet (super-earth) will lose geological activity more slowly, release iron into the ocean longer, and could delay the oxygen revolution by billions of years. Planets in the habitable zone of red dwarfs will receive little visible light suitable for photosynthesis, and their biospheres also risk being stuck in an oxygen-free stage. The amount of water on the planet is also important. If the entire planet is covered by a deep ocean, then it will be deficient in phosphorus,coming mainly from land volcanoes, and if there is little water, then the ocean area available to photosynthetic microbes will also be small (before the appearance of multicellular plants, the productivity of terrestrial ecosystems was negligible compared to the seas). That is, there are plenty of reasons why the biosphere can get stuck in an oxygen-free microbial stage and not develop into animals. Time for development, by the way, is limited: the luminosity of stars grows with time, and the Earth in 1.5-2 billion years will begin to heat up irreversibly, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. Red dwarfs have a slower luminosity, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars.the area of the ocean available to photosynthetic microbes will also be small (before the advent of multicellular plants, the productivity of terrestrial ecosystems was negligible compared to the seas). That is, there are plenty of reasons why the biosphere can get stuck in an oxygen-free microbial stage and not develop into animals. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, the luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars.the area of the ocean available to photosynthetic microbes will also be small (before the advent of multicellular plants, the productivity of terrestrial ecosystems was negligible compared to the seas). That is, there are plenty of reasons why the biosphere can get stuck in an oxygen-free microbial stage and not develop into animals. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, the luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars.accessible to photosynthetic microbes (before the emergence of multicellular plants, the productivity of terrestrial ecosystems was negligible compared to the seas). That is, there are plenty of reasons why the biosphere can get stuck in an oxygen-free microbial stage and not develop into animals. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars.accessible to photosynthetic microbes (before the emergence of multicellular plants, the productivity of terrestrial ecosystems was negligible compared to the seas). That is, there are plenty of reasons why the biosphere can get stuck in an oxygen-free microbial stage and not develop into animals. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars. Time for development, by the way, is limited: the luminosity of stars increases with time, and the Earth in 1.5-2 billion years will irreversibly heat up, its oceans will evaporate, and the growing greenhouse effect will turn it into the second Venus. In red dwarfs, luminosity grows more slowly, but their planets can become uninhabitable due to the disappearance of the magnetic field and the subsequent loss of water into space, as happened on Mars.
The second consideration relates to the emergence of eukaryotes - cells with a nucleus. Eukaryotic cells are much larger and more complex than those of bacteria and archaea and appeared later, most likely during the "oxygen revolution". The eukaryotic cell appeared as a chimera from an archaeal cell, a symbiotic bacteria that settled inside it and, possibly, a virus that infected them (or even more than one). The structure of the genome of eukaryotes unambiguously shows that their early evolution occurred not due to natural selection, but in many respects in spite of. In small populations, selection is not very efficient, and slightly deleterious traits can become entrenched due to gene drift and other purely random processes. This is detailed in the corresponding chapter of Kunin's Logic of Chance and suggeststhat the emergence of eukaryotes may be very unlikely even in a suitable environment (bacterial biosphere entering the oxygen revolution). At a minimum, cases of intracellular symbiosis between bacteria and archaea are practically unknown - although bacteria settle easily inside eukaryotic cells.
Summing up: I think that the combination of the described factors should lead to the fact that in our Galaxy there will be millions of planets with bacterial life and much less (possibly, only a few) - with life of a eukaryotic and multicellular level of complexity.
Boris Stern's postscript
A few words to end the discussion. It is quite possible that Evgeny Kunin greatly underestimated the likelihood of the origin of life under suitable conditions. And all the same, this assessment must be taken seriously. If he was mistaken by 900 orders of magnitude, it does not change anything: we are all the same alone within the horizon of the Universe, where there are only about 1020-1021 suitable planets. Even if the rest of the participants in the discussion are right and all sorts of tricks of Nature like non-enzymatic replication can make the origin of life more or less likely, it will be a very primitive life, in the overwhelming majority of cases, not able to jump to a higher level of development. Two panelists wrote about this in black and white. That's the whole Fermi paradox.
Hence, at least two important organizational conclusions follow. First: Developed life is the rarest and most valuable phenomenon in the universe. Therefore, see the last paragraph of Armen Mulkidzhanyan's note: humanity has a noble total goal - the spread of this phenomenon. We will talk separately about the possibilities and methods of achieving this goal.
The second organizational conclusion: the destruction of this life will be an irreparable loss of a galactic or even cosmological scale. This should be taken into account in their own assessment of "hawks" and politicians who are ready to resort to nuclear blackmail in order to inflate their own "greatness." The same applies to a civilization of unbridled consumption.
Authors: Boris Stern, Alexander Markov, Armen Mulkidzhanyan, Evgeny Kunin, Mikhail Nikitin