Life on Earth appeared more than 3.5 billion years ago - it is difficult to pinpoint the moment more precisely, if only because it is not easy to draw the line between “almost alive” and “truly alive”. However, we can say for sure that this magical moment stretched out for many, long millions of years. Still, it was a real miracle.
To appreciate this miracle at its true worth, you need to get acquainted with a number of modern theories describing different options and stages of the birth of life. From a brisk but lifeless set of simple organic compounds to protoorganisms that have known death and entered an endless race of biological variability. After all, aren't these two terms - mutability and death - giving rise to the entire sum of life?..
1. Panspermia
The hypothesis about bringing life to Earth from other cosmic bodies has a lot of authoritative defenders. This position was held by the great German scientist Hermann Helmholtz and the Swedish chemist Svante Arrhenius, the Russian thinker Vladimir Vernadsky and the British lord physicist Kelvin. However, science is a realm of facts, and after the discovery of cosmic radiation and its destructive effect on all living things, panspermia seemed to die.
But the deeper scientists plunge into the issue, the more nuances emerge. So, now - including putting on numerous experiments on spacecraft - we take much more seriously the ability of living organisms to tolerate radiation and cold, lack of water and other "delights" of being in outer space. The finds of all kinds of organic compounds on asteroids and comets, in distant gas and dust clusters and protoplanetary clouds are numerous and beyond doubt. But claims about the discovery in them of traces of something suspiciously resembling microbes remain unproven.
It is easy to see that for all its fascination, the theory of panspermia only transfers the question of the origin of life to another place and another time. Whatever brought the first organisms to Earth - whether a random meteorite or a cunning plan of highly developed aliens, they had to be born somewhere and somehow. Let not here and much further in the past - but life had to grow out of lifeless matter. The question "How?" remains.
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1. Unscientific: Spontaneous generation
The spontaneous origin of highly developed living matter from non-living matter - like the birth of fly larvae in rotting meat - can be associated with Aristotle, who generalized the thoughts of many predecessors and formed a holistic doctrine of spontaneous generation. Like other elements of Aristotle's philosophy, spontaneous generation was the dominant doctrine in Medieval Europe and enjoyed some support until the experiments of Louis Pasteur, who conclusively showed that even fly larvae needed parent flies to appear. Do not confuse spontaneous generation with modern theories of the abiogenic origin of life: the difference between them is fundamental.
2. Primary broth
This concept is closely related to the classic experiments that had managed to acquire the status of the 1950s by Stanley Miller and Harold Urey. In the laboratory, scientists modeled the conditions that could exist near the surface of the young Earth - a mixture of methane, carbon monoxide and molecular hydrogen, numerous electrical discharges, ultraviolet light - and soon more than 10% of the carbon from methane was converted into the form of various organic molecules. More than 20 amino acids, sugars, lipids, and nucleic acid precursors were obtained in the Miller-Urey experiments.
Modern variations of these classic experiments use much more sophisticated setups that better match the conditions of the early Earth. They simulate the impact of volcanoes with their emissions of hydrogen sulfide and sulfur dioxide, the presence of nitrogen, etc. So scientists manage to get a huge and varied amount of organic matter - potential building blocks of potential life. The main problem of these experiments remains the racemate: isomers of optically active molecules (such as amino acids) are formed in a mixture in equal amounts, while all life known to us (with a few and strange exceptions) includes only L-isomers.
However, we will return to this problem later. It should also be added here that recently - in 2015 - Cambridge professor John Sutherland and his team showed the possibility of forming all the basic "molecules of life", components of DNA, RNA and proteins from a very simple set of initial components. The main characters of this mixture are hydrogen cyanide and hydrogen sulfide, which are not so rare in space. To them it remains to add some minerals and metals that are present in sufficient quantities on Earth, such as phosphates, copper and iron salts. Scientists have built a detailed reaction scheme that could well create a rich "primordial soup" so that polymers appear in it and full-fledged chemical evolution comes into play.
The hypothesis of the abiogenic origin of life from "organic broth", which was tested by the experiments of Miller and Urey, was put forward in 1924 by the Soviet biochemist Alexander Oparin. And although in the "dark years" of the heyday of Lysenkoism, the scientist took the side of opponents of scientific genetics, his merits are great. In recognition of the role of an academician, his name bears the main award presented by the International Scientific Society for the Study of the Origin of Life (ISSOL) - the Oparin Medal. The prize is awarded every six years, and at various times has been awarded to both Stanley Miller and the great chromosome researcher, Nobel laureate Jack Shostak. In recognition of Harold Urey's enormous contribution, ISSOL awards the Urey Medal in between the Oparin Medal (also every six years). The result is a unique, real evolutionary award - with a changeable name.
3. Chemical evolution
The theory tries to describe the transformation of relatively simple organic substances into rather complex chemical systems, the precursors of life itself, under the influence of external factors, the mechanisms of selection and self-organization. The basic concept of this approach is "water-carbon chauvinism", representing these two components (water and carbon - NS) as absolutely necessary and key for the emergence and development of life, whether on Earth or somewhere beyond. And the main problem remains the conditions under which “water-carbon chauvinism” can develop into very sophisticated chemical complexes capable, first of all, of self-replication.
According to one of the hypotheses, the primary organization of molecules could occur in the micropores of clay minerals, which played a structural role. Scottish chemist Alexander Graham Cairns-Smith put forward this idea a few years ago. Complex biomolecules could settle and polymerize on their inner surface, as on a matrix: Israeli scientists have shown that such conditions make it possible to grow sufficiently long protein chains. Here, the required amounts of metal salts could accumulate, which play an important role as catalysts for chemical reactions. Clay walls could function as cell membranes, dividing the "inner" space, in which more and more complex chemical reactions take place, and separating it from the outer chaos.
The surfaces of crystalline minerals could serve as "matrices" for the growth of polymer molecules: the spatial structure of their crystal lattice is capable of selecting only optical isomers of one type - for example, L-amino acids - solving the problem that we talked about above. Energy for the primary "metabolism" could be supplied by inorganic reactions, such as the reduction of the mineral pyrite (FeS2) with hydrogen (to iron sulfide and hydrogen sulfide). In this case, neither lightning nor ultraviolet radiation is required for the appearance of complex biomolecules, as in the Miller-Urey experiments. This means that we can get rid of the harmful aspects of their action.
Young Earth was not protected from harmful - and even deadly - components of solar radiation. Even modern, evolutionarily tested organisms would be unable to withstand this harsh ultraviolet radiation - despite the fact that the Sun itself was much younger and did not give enough heat to the planet. From this arose the hypothesis that in the era when the miracle of the origin of life was happening, the whole Earth could be covered with a thick layer of ice - hundreds of meters; and that's for the best. Hiding under this ice sheet, life could feel completely safe from ultraviolet radiation and from frequent meteoric strikes that threatened to destroy it in the bud. The relatively cool environment could also stabilize the structure of the first macromolecules.
4. Black smokers
Indeed, ultraviolet radiation on the young Earth, whose atmosphere did not yet contain oxygen and did not have such a wonderful thing as the ozone layer, should have been deadly for any nascent life. This gave rise to the assumption that the fragile ancestors of living organisms were forced to exist somewhere, hiding from the continuous stream of sterilizing all and all rays. For example, deep under water - of course, where there are enough minerals, mixing, heat and energy for chemical reactions. And such places were found.
Towards the end of the twentieth century, it became clear that the ocean floor could in no way be a haven for medieval monsters: the conditions here are too harsh, the temperature is low, there is no radiation, and rare organic matter can only settle from the surface. In fact, these are vast semi-deserts - with some notable exceptions: right there, deep under water, near the outlets of geothermal springs, life is literally in full swing. Black water saturated with sulphides is hot, actively mixed and contains a lot of minerals.
Black ocean smokers are very rich and distinctive ecosystems: the bacteria feeding on them use the iron-sulfur reactions, which we have already discussed. They are the basis for a fully blooming life, including a host of unique worms and shrimp. Perhaps they were the basis for the origin of life on the planet: at least theoretically, such systems carry everything necessary for this.
2. Unscientific: Spirits, gods, ancestors
Any cosmological myths about the origin of the world are always crowned with anthropogonic ones - about the origin of man. And in these fantasies one can only envy the imagination of the ancient authors: on the question of what, how and why the cosmos arose, where and how life - and people - appeared, the versions sounded very different and almost always beautiful. Plants, fish and animals were caught from the seabed by a huge raven, people crawled out of the body of the ancestor Pangu as worms, molded from clay and ashes, were born from marriages of gods and monsters. All this is surprisingly poetic, but of course it has nothing to do with science.
5. The world of RNA
In accordance with the principles of dialectical materialism, life is a "unity and struggle" of two principles: changing and inherited information, on the one hand, and biochemical, structural functions, on the other. One is impossible without the other - and the question of where life began, with information and nucleic acids or with functions and proteins, remains one of the most difficult. And one of the well-known solutions to this paradoxical problem is the RNA world hypothesis, which appeared in the late 1960s and finally took shape in the late 1980s.
RNA - macromolecules, in storage and transmission of information are not as efficient as DNA, and in the performance of enzymatic functions - not as impressive as proteins. But RNA molecules are capable of both, and until now they serve as a transmission link in the information exchange of the cell, and catalyze a number of reactions in it. Proteins are incapable of replicating without DNA information, and DNA is incapable of this without protein "skills." RNA, on the other hand, can be completely autonomous: it is able to catalyze its own "reproduction" - and this is enough for a start.
Studies within the framework of the RNA world hypothesis have shown that these macromolecules are capable of full-fledged chemical evolution. Take, for example, an illustrative example demonstrated by California biophysicists led by Lesley Orgel: if ethidium bromide is added to a solution of RNA capable of self-replication, which serves as a poison for this system, blocking RNA synthesis, then little by little, with a change in generations of macromolecules, in the mixture RNAs appear that are resistant even to very high concentrations of the toxin. Something like this, evolving, the first RNA molecules could find a way to synthesize the first tools-proteins, and then - in combination with them - "discover" for themselves the double helix of DNA, the ideal carrier of hereditary information.
3. Unscientific: Immutability
No more scientific than the stories about the first ancestors can be called the views bearing the loud name of the Theory of a Stationary State. According to her supporters, no life has ever arisen at all - just as the Earth was not born, nor did the cosmos appear: they simply were always, always and will remain. All this is no more justified than the Pangu worms: in order to take such a "theory" seriously, one will have to forget about the countless findings of paleontology, geology and astronomy. And in fact, to abandon the entire grandiose building of modern science - but then, perhaps, it is worth giving up everything that is due to its inhabitants, including computers and painless dental treatment.
6. Protocells
However, simple replication is not enough for "normal life": any life is, first of all, a spatially isolated area of the environment that separates metabolic processes, facilitates the course of some reactions and allows excluding others. In other words, life is a cell bounded by a semipermeable membrane composed of lipids. And "protocells" should have appeared already at the earliest stages of the existence of life on Earth - the first hypothesis about their origin was expressed by Alexander Oparin, who is well known to us. In his opinion, droplets of hydrophobic lipids resembling yellow droplets of oil floating in water could serve as "protomembranes".
In general, the scientist's ideas are accepted by modern science, and Jack Shostak, who received the Oparin Medal for his work, was also involved in this topic. Together with Katarzyna Adamala, he managed to create a kind of “protocell” model, the analogue of the membrane of which did not consist of modern lipids, but of even simpler organic molecules, fatty acids, which could well have accumulated in the places of origin of the first protoorganisms. Shostak and Adamala even managed to "revive" their structures by adding magnesium ions (stimulating the work of RNA polymerases) and citric acid (stabilizing the structure of fatty membranes) to the medium.
As a result, they ended up with a completely simple, but somewhat living system; in any case, it was a normal protocell that contained a membrane-protected environment for RNA reproduction. From this moment you can close the last chapter of the prehistory of life - and begin the first chapters of its history. However, this is a completely different topic, so we will talk about only one, but extremely important concept associated with the first steps of the evolution of life and the emergence of a huge variety of organisms.
4. Unscientific: Eternal Return
A "corporate" representation of Indian philosophy, in Western philosophy associated with the works of Immanuel Kant, Friedrich Nietzsche and Mircea Eliade. A poetic picture of the eternal wandering of every living soul through an infinite number of worlds and their inhabitants, its transformation into an insignificant insect, then into an exalted poet, or even into a being unknown to us, a demon or a god. Despite the lack of ideas of reincarnation, Nietzsche is really close to this idea: eternity is eternal, which means that any event in it can - and should be repeated again. And each creature endlessly revolves on this carousel of universal return, so that only the head is spinning, and the very problem of primary origin disappears somewhere in a kaleidoscope of countless repetitions.
7. Endosymbiosis
Take a look at yourself in the mirror, peer into the eyes: the creature with whom you are looking at each other is a complex hybrid that has arisen in time immemorial. Back in the late 19th century, the German-English naturalist Andreas Schimper noticed that chloroplasts - the organelles of plant cells responsible for photosynthesis - replicate separately from the cell itself. Soon there was a hypothesis that chloroplasts are symbionts, cells of photosynthetic bacteria, once swallowed by the host - and left to live here forever.
Of course, we do not have chloroplasts, otherwise we could feed on sunlight, as some pseudo-religious sects suggest. However, in the 1920s, the endosymbiosis hypothesis was expanded to include mitochondria, organelles that consume oxygen and supply energy to all of our cells. To date, this hypothesis has acquired the status of a full-fledged, repeatedly proven theory - suffice it to say that mitochondria and plastids have their own genome, more or less cell division mechanisms, and their own protein synthesis systems.
In nature, other endosymbionts have also been found that do not have billions of years of joint evolution behind them and are at a less deep level of integration in the cell. For example, some amoebas do not have their own mitochondria, but there are bacteria included inside and performing their role. There are hypotheses about the endosymbiotic origin of other organelles - including flagella and cilia, and even the cell nucleus: according to some researchers, all of us eukaryotes are the result of an unprecedented fusion between bacteria and archaea. These versions have not yet found strict confirmation, but one thing is clear: as soon as it emerged, life began to absorb its neighbors - and interact with them, giving birth to new life.
5. Unscientific: Creationism
The very concept of creationism arose in the 19th century, when this word began to be called the supporters of various versions of the appearance of the world and life, proposed by the authors of the Torah, the Bible and other holy books of monotheistic religions. However, in essence, creationists did not offer anything new in comparison with these books, over and over again trying to refute the rigorous and thorough findings of science - and in fact, over and over again, losing one position after another. Unfortunately, the ideas of modern pseudoscientists-creationists are much easier to understand: it takes a lot of effort to understand the theories of real science.
Sergey Vasiliev
