In the cosmic hierarchy, the Earth and the star around which it revolves, so to speak, are still in infancy. The Earth was formed from the substance left after the birth of the Sun 4.6 billion years ago, while the age of the Universe as a whole is considered to be 11-16 billion years. As during the formation of all planets, the initial stage of our planet's existence was so turbulent that it is almost impossible to imagine.
And even after the globe took its shape, its surface was molten for another 600 million years, overheating was caused by heat coming from the inside, from the earth's core, and by asteroid bombardment from outside, which raised the temperature of the evaporating oceans to the boiling point. During this period, which some of the geologists call Hed, hell truly reigned on our planet.
After the constant bombardment by asteroids ceased, and the remaining asteroids were in certain orbits and could hardly harm the Earth, carbon, nitrogen, hydrogen and oxygen in various combinations "formed amino acids and other basic building material of living matter." As Nobel laureate Christian de Duve wrote in his book Life-giving Dust, published in 1995, "the products of these chemical processes, deposited by atmospheric precipitation, comets and meteorites, gradually formed the first organic matter on the lifeless surface of our recently condensed planet."
This carbon-rich film has been affected by both processes taking place in the Earth itself and by the falling bodies of space on its surface; the effect of ultraviolet radiation was many times stronger than at present, because now we are protected by the earth's atmosphere. All of these materials were eventually deposited in the seas, and, as the eminent scientist JB Haldane wrote in his famous 1929 article, "the primordial oceans had the consistency of hot, diluted broth."
The main by-product of these processes was something viscous, brownish, called "gummy", "sticky" and in other words, awakening memories of childhood. Those who oppose Charles Darwin's conclusion that man is a relative of chimpanzees and orangutans, in fact, put a person before this last insult - we came from some kind of slime!
So, we have a primary "broth" in which a lot of something sticky is mixed everywhere. How could life on Earth arise from this raw material? This is where the real mystery begins. It is generally accepted that the decisive role was played by RNA - ribonucleic acid, a close relative of DNA, which determines the genetic code of humans and other living things. And yet, there are still numerous disputes about how, when and where life actually originated. Let's look briefly at some of the issues fueling these discussions.
For a long time, biologists and chemists believed that life on Earth should have arisen no earlier than a billion years after the planet cooled and the intense bombardment of it by asteroids stopped, and this happened about 3.8 billion years ago. Hence it follows that life on Earth has existed for no more than 2.8 billion years. But geological evidence, and even organic fossils, increasingly suggests that bacteria already existed long before that.
The Greenlandic Isua formation, composed of the oldest rocks of our planet, whose age is determined at 3.2 billion years, contains carbon - the main building material of all known forms of life, and in proportions characteristic of bacterial photosynthesis. Many biologists come to the conclusion that even at such an early period bacteria must have existed, and if so, then even earlier there were more primitive organisms than bacteria.
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Relatively recently, a geologist from the University of Western Australia, Bigir Rasmussen, discovered in the Pilbara craton in northwestern Australia, fossil remains of filamentous microorganisms aged 3.5 billion years, as well as "possible" fossil remains that date back to 3.235 billion years ago, in erupted volcanic deposits in western Australia. Because of such finds, a serious problem arises: the origins of life are postponed to 200 thousand years after the end of the Hed period, which many biologists consider a rather short time for the necessary chemical processes to take place.
Rasmussen's more recent find, reported in June 1999 in Nature, raises another dilemma. Because biomolecules necessary for living matter, such as proteins and nucleic acids, are very fragile and survive better at lower temperatures, many chemists have long been convinced that life on Earth should have arisen at low temperatures, maybe even negative … And yet Rasmussen dug up his microscopic filaments in the material originally located near the volcano's vent, where the temperature was extremely high.
In fact, the most ancient organisms that continue to exist today are bacteria that live in preserved volcanic vents or in springs with water temperatures up to 110 ° C. The existence of these ancient bacteria in the vents of volcanoes provides strong evidence in favor of the assumption of high-temperature conditions for the origin of life on Earth, supported by other scientists.
One of the adherents of the view of the origin of life on Earth in cold conditions is Stanley Miller, who instantly became famous in 1953 after conducting a series of experiments at the University of Chicago. He was then a graduate student and studied with Nobel Prize-winning chemist Harold Urey, who won the Nobel Prize for discovering heavy hydrogen called deuterium. According to Yuri, the planet's atmosphere originally consisted of a mixture of molecules of hydrogen, methane, ammonia, water vapor and was especially rich in hydrogen. (Note that oxygen was present only in the composition of water vapor. It was only after the emergence of life in the atmosphere that oxygen began to appear as a result of the release of carbon dioxide during photosynthesis, which eventually led to the development of more complex biological forms.)
Miller prepared a mixture of the elements Yuri had indicated in a sealed vessel and for several days exposed it to electrical discharges that simulated lightning. To his surprise, a pinkish glow appeared in the glass jar, and analysis of the results obtained revealed the presence of two amino acids (a component of all proteins), as well as other organic substances, which were believed to be formed only by living cells. This experiment, which his leader reluctantly approved, not only made Miller famous, but also led to the emergence of a new field of science - abiotic chemistry, the main task of which was to obtain biological substances in conditions that are believed to have existed on Earth before the emergence of life.
The word "consider" is crucial here. Assumptions about the composition of the earth's atmosphere before life developed on our planet change all the time. And although many experiments were carried out after Miller's work in 1953, they did not lead to results that could be associated with the concept of "life", despite the formation of various kinds of organic molecules in them. As de Duve notes in Life-giving Dust, such experiments are often conducted “under more contrived conditions than are necessary for a truly abiotic process.
Among all these experiments, Miller's original experiment remains classic. It was practically the only one conceived solely for the purpose of replicating plausible prebiological conditions without the intention of obtaining a specific end product. In other words, it is always quite easy to organize an experiment in such a way as to most likely get the desired result, but the experimental conditions will be too suitable.
At least in such experiments, it was not possible to reproduce life even in its most elementary form - in the form of a separate cell without a nucleus. As Nicholas Wade wrote in his June 2000 New York Times article on Rasmussen's latest discovery, "The most intense attempts by chemists to create molecules in the laboratory typical of living matter have only shown that it is a devilishly difficult task."
Thus, the main problems are concentrated on two main lines of research in order to establish how life on Earth originated. The moment of the origin of life is pushed even further into the past, so that there is, apparently, too little time left for the chemical processes necessary for the origin of life to take place. And these chemical reactions themselves, as before, remain just as mysterious.
Despite colossal technical advances and a huge amount of accumulated genetic data, Stanley Miller's 1953 experiment remains virtually the only convincing result of such research. Nevertheless, the discovery itself raised doubts - many of the scientists now believe that the balance of the elements he used based on the work of his leader G. Juri was wrong. When the ratio of the components changes, the amino acids obtained by Miller are not formed.
Due to new difficulties, the whole picture of the evolution of life has become more obscure. Once it seemed that it could be clearly traced by phylogenetic (genealogical) trees reflecting the evolutionary history of an organism from its very roots. Phylogenetic trees were first built in the 19th century in accordance with Charles Darwin's theory in order to clearly demonstrate the evolutionary history of individual groups of animals. The first branched tree was built by the German evolutionary biologist Ernst Haeckel (who also proposed the term "ecology").
The discovery of DNA made it possible to create such phylogenetic trees not only for animals and plants, but also for their genetic material, which made it possible to understand much deeper the processes underlying the concept of "life". To obtain genealogical trees, researchers conduct a comparative analysis of the sequences of the molecular building blocks of nucleic acids (nucleotides) or amino acids in proteins. The results are compared for different organisms.
Based on the mechanisms of branching of evolution and mutations, using this technique, it is possible to determine the distances between two branches on the phylogenetic tree, that is, to find out to what extent two species have moved away from their common ancestor and from each other. (In addition, this method has helped scientists find the age of the ancient organisms that still exist today in super-hot volcanic vents.) The task of performing a comparative analysis of sequences is perhaps easiest to understand if we draw an analogy with a word game where one is asked a long word with the aim of forming as many short words as possible from its constituent letters.
In the late 1970s, Carl Wose of the University of Illinois applied sequence comparative analysis to the RNA molecules found in all living things, resulting in a more complex phylogenetic tree than anticipated. The three main branches of the tree corresponded to the three fundamental kingdoms of living organisms: prokaryotes, archaea and eukaryotes. Prokaryotes are microorganisms such as bacteria.
Wose's proposed new subdivision - the archaea - includes a second group of bacteria found in very hot places on Earth, such as hot springs. Eukaryotes are organisms consisting of large cells that have a formed nucleus; this includes all multicellular organisms - plants and animals, including humans.
But since the early 1980s, when more genomes have been decoded across all three kingdoms, the picture has become more uncertain. Trees based on genes other than Wase's original protein model turned out to be completely different. In addition, genes are rearranged in surprising, even unexpected ways. These variations make it extremely difficult to trace such genes back to common ancestors and, even more unpleasant, suggest that the primary gene - the founder of life - itself had a rather complex structure, more complex than the “original” gene should have.
The only plausible solution to this problem is to assume that instead of growing all the time upward to form vertical branches in the early stages of the evolution of life, the tree gave off side branches, and some genes were transferred horizontally. This idea is reinforced by the fact that even today, bacteria can transmit some genes horizontally, including, unfortunately, those that make bacteria resistant to antibiotics. This conclusion means that the tree of life, instead of having a beautiful straight trunk, turns into something resembling a painting by Jackson Pollock. This is discouraging to say the least.
But Karl Wose was not embarrassed. He hypothesized that a single-celled organism, which for a long time was considered the original form of life, may have been a kind of colony, consisting of several types of cells, capable of quite easily exchanging genetic information horizontally. Some scientists are confused by this perceived lightness. It means that the mechanism of replication (reproduction) of genes, which is observed in DNA and is a fairly precise mechanism, developed in cells only at a later time. The colony eventually had to rise to a higher stage of development, when each organism took on its own form. But when did this happen?
So how did life on earth come about?
Nowadays, experts attribute completely different dates to the moment when slender DNA trees began to form vertical branches - in the range from only a billion years ago and almost to the previously assumed 4 billion years. As in the situation with the theory of the Big Bang in the origin of the Universe, thanks to new discoveries and methods of measurements as our knowledge expands, theories of the origin of life on Earth are not simplified, but more complicated. For this reason, other explanations for the emergence of life, long dismissed as fantastic, have retained some supporters.
Could life have been brought to Earth from the surrounding space? Of course, asteroids, meteorites and comets contain the elements that form the building blocks of living matter, and it is generally accepted that life on Earth arose from a combination of such materials - already existing on Earth and brought from space. But building material is one thing, and life itself is quite another. Some prominent scientists are of the opinion that the primary life was brought to our planet from space already fully formed, that is, not just constituent parts, but the organisms themselves. Back in 1821, Sals-Guyonde Montlivol suggested that the moon was the source of life on our planet.
This idea was revived in relation to Mars in 1890, when the American astronomer Percival Lovell (who predicted the existence of the planet Pluto and calculated its orbit) said that the channels visible on the surface of the red planet could only be built by intelligent beings. William Thomson (Lord Kelvin), who developed the perfect temperature scale, at the end of the 19th century, suggested that life was brought to our planet by meteorites.
No one was more obsessed with such ideas as the Swedish chemist Svante Arrhenius, who won the 1903 Nobel Prize for his founding work in electrochemistry. According to his theory of panspermia, bacterial spores scattered in the cold world space are able to travel long distances in a state of suspended animation and are ready to wake up if they meet a hospitable planet on their way. He was not familiar with the problem of deadly cosmic radiation.
Fred Hoyle promoted some version of the panspermia hypothesis in connection with his theory of a stationary universe, which is described in Ch. 1. Hoyle went so far as to assert that epidemics such as the 1918 Spanish flu pandemic were caused by germs from space, and that the human nose had evolved to prevent spaceborne pathogens from entering the body.
Francis Crick (who received the Nobel Prize in Medicine in 1962 with James Watson and Maurice Wilkins for the discovery of the DNA double helix) and the founder of prebiological chemistry, Leslie Orgel, went even further, supporting the idea that life was "seeded" on Earth by representatives of the highly developed extraterrestrial civilization. They called this hypothesis "directed panspermia."
UFO devotees are, of course, happy to have the Nobel laureate Scream among their supporters, and science fiction writers are always ready to jump at these kinds of ideas. Lovell's Martian Canals inspired HG Wells to some extent in the famous War of the Worlds, published in 1898. While many respected scientists openly protest against the idea of panspermia, either directly or indirectly, some are more cautious.
Christian de Duve wrote: “With such famous supporters, the panspermia hypothesis can hardly be rejected without detailed analysis,” despite the fact that, in his opinion, such theories have no convincing evidence. This conclusion was made in 1995, but the next year, the whole world went around the headlines with a statement made by NASA.
The NASA report related to one of the rocks discovered in 1984 in Antarctica. The samples were fragments of a meteorite called SNCs (pronounced "snix") - an abbreviation for the names of the places where the first three such fragments were found, Shergotty - Nakhla - Chassigny. At a press conference dedicated to this event, a sample of the rock lay on a blue velvet pillow, and the head of NASA Dan Goldin addressed those present with the words: "Not today or tomorrow we will know if only life exists on Earth," which turned out to be a great way attract the attention of journalists.
Then NASA scientists talked about what was definitely known about these rocks. Studies have shown that they formed on Mars about 4.5 billion years ago. For half a billion years, the rock was under the surface of Mars, but after cracks appeared on the surface of Mars as a result of meteoric impacts, it was exposed to water. New events took place with this rock about 16 million years ago, when a space object, perhaps an asteroid, fell on Mars, as a result of which a fragment of the Martian crust was thrown into the surrounding space.
After traveling in space for millions of years, this fragment fell into Antarctica just 16,000 years ago. Back in 1957, science fiction writer James Blish released the novel Cold Year, which focused on the rock found in the Arctic and turned out to be the remnant of a planet destroyed by the Martians during the war of two worlds, which made the hero exclaim: “The history of the universe in a cube ice! The events at the NASA conference were less dramatic, although newspapers did their best to hype the story.
The rock, discovered by NASA, contained carbonates similar to those that form on our planet with the participation of bacteria. Also found were fine-grained iron sulfides and other minerals that resemble the waste products of bacteria. In addition, using a scanning electron microscope, tiny structures were identified that could be fossil remains of Martian bacteria - they were submerged so deep that they could not form on Earth.
Not wanting to be embarrassed, NASA officials had a scientist on hand who said that these structures were too small to be bacteria, and that carbonates seemed to have formed at very high temperatures incompatible with life. However, his skeptical remarks in no way could prevent the appearance of giant screaming headlines in newspapers: "Life on Mars!"
The subsequent discussion of this issue by scientists took place on the basis of scientific terminology that can scare off any journalist. The problem could be solved if one of those tiny fossilized awns could be opened. If we find a cell wall, or better yet, a fragment of a cell, we would get an answer.
Unfortunately, there is no developed methodology for such research. When the answer is still received, even if it is positive, many scientists will probably say that this only proves that life on Mars, like on Earth, existed in the form of bacteria. This will not be evidence that life originated on Mars and was brought to our planet (or vice versa), and will not confirm the theory of panspermia. But now it can no longer be said that there are no grounds at all to assume such possibilities.
J. Malone