THE ORIGIN OF LIFE
Some 3800 million years ago, some chemicals formed bonds, becoming a complex compound capable of self-reproduction. This was the beginning of life on Earth.
The beginning of life on Earth and how it came into being are still a mystery. But since Charles Darwin first described the evolution of animals and plants, scientists have accepted the idea that all life forms are subject to a continuous process of development.
With each new generation, the advantages increase, the disadvantages are removed and new possibilities are explored. An ancestral species can give birth to several new species, and then disappear; or it can survive in its original form, perfectly adapted to its own niche in the system, while its descendants thrive in other niches.
The result is a complicated network of relationships that links all organisms found today on Earth to a group of ancestral species that have already disappeared. The ancient remains of many of these extinct organisms are preserved in the form of fossils.
The fossils are found in sedimentary rocks, whose age can be determined by using advanced radioactive dating techniques. This allowed scientists to form an approximate picture of life on Earth at every stage of its history – approximate because only a small part of animals and plants were preserved.
However, one thing is clear: judging from the fossils found in rocks, the network of relationships between modern and extinct organisms is tree-shaped, with new branches that have formed over time. Many branches collapsed and died – for example, dinosaurs – but, while some branches disappeared, others sprouted and bloomed. If we could start from any branch to its origin, we would eventually reach a single trunk, the ancestor of all organisms that have ever lived: the origin of life.
Unfortunately, determining the origin of life is not so simple. According to current estimates, the earth is about 4,500 million years old, but it seems that the oldest rocks in which visible fossils appear are less than 590 million years old, and were deposited at the beginning of the so-called Cambrian.
The fossils from these Cambrian rocks represent a wide range of life forms, such as worms and mollusks, which had obviously evolved from their primitive origins – in other words, they were halfway on the evolutionary scale. Their origins during the great pre-Cambrian era are hidden in the fact that the precambrian rocks do not appear to contain fossils.
The main reason for this is simple. The soft-body organisms do not fossilize well, because after they die, they generally break down completely before the sediments deposited around them can be petrified. It seems that most of the organisms that lived in the Precambrian – which accounts for 80% of Earth’s history – were too inconsistent to leave well-defined traces.
But that does not mean that they have left no trace. In the early 1950s, two researchers began to study in detail a 2000-million-year-old rock formation beneath the shores of Lake Superior, known as silica of cremains (because it was more a source of “cremains” for muskets). The rocks contained strange white rings, each about one meter in diameter. They didn’t seem to have anything organic, but despite this, the researchers decided to examine small fragments of the rings under a high-power microscope.
What they added were undoubtedly signs of life: the remnants of organisms that resembled algae and microscopic monocellular bacteria that are still alive today. Miraculously, these fragile organisms had been saturated with silicon, which resembles glass and has solidified, forming silica, and preserving them as insects are kept in amber. The white rings in the rocks proved to be the eroded remnants of their colonies: spherical, stony formations, known as stromatolites, which resemble coral colonies on tropical reefs.
The discovery of the specimens from the silica of cream was a revelation. In all corners of the world, scientists have begun to re-examine old rocks that they once considered fossil-free. Their work has been rewarded with amazing results: the oldest life forms discovered in Western Australia have appeared around 3500 million years ago. Meanwhile, the oldest sun-rock in the world: – the Amitsoq gnats in southwestern Greenland, 3800 million years old – have been under research, but so far no precise results.
The fact that these prehistoric life forms resembled algae and current bacteria was no surprise to biologists. Because monocellular organisms are so simple, it is relatively simple to find out how they work at the most elementary level. For example, instead of studying how raw chemicals are transformed into the foundation stones of life: proteins, lipids and carbohydrates.
These studies are particularly important in the pursuit of life because precisely such a transformation – from inert chemicals into living tissue – must have been the one that started the whole process.
A bacterium is a simple cell that prepares its own food: a gelatinous, liquid-filled wrap that absorbs simple chemicals, consisting of hydrogen, oxygen, carbon and nitrogen, and transforms them into more complex organic chemicals, such as proteins. which forms its body and the carbohydrates (carbohydrates) that give it energy.
All of these processes are controlled by an organic chemical called deoxyribonucleic acid, known as DNA shortening. This chemical is the one that provides the assembly instructions for the other complex chemicals. DNA still has an important property: it is capable of self-reproduction.
Each DNA molecule is constructed in the form of a spiral staircase. The sides are made up of chains of atoms, and at certain intervals connection bridges are created, such as steps. The whole structure can be separated, each “step” separating in the middle. When the two sides separate, the truncated “steps” attract other chemicals that form bonds, and which restore the sides of the stairs – thus, from a single staircase, two new stairs are formed.
This seemingly simple trick is the essence of life. It allows a single-celled organism to grow and self-reproduce, dividing itself right in the middle in a replica of the chemical processes that take place in its afterlife.
In more complex life forms, these multiplying cells cooperate to form multicellular structures, each structure representing only a part of an extremely complicated organism. The whole process is controlled by the genetic code carried by the DNA molecule, whose structure differs from one species to another, and even from one individual to another.
All other processes of life – eating, drinking, excretion, absolutely all are mechanisms that have been formed to serve the DNA and support its activities.
DNA is a very complex substance, and the more sophisticated the form of life, the more complex the DNA in it. The DNA in a bacterial cell is as implausible as it is, but it is a complicated construct consisting of thousands of atoms arranged in groups called nucleotides – compounds of carbohydrates, phosphates and bases.
Each nucleotide is a complex structure, and the same is true for other organic molecules, such as proteins and carbohydrates. For example, proteins are made up of amino acid chains – of which there are 20 different types – sorted into specific segments. A simple chain can contain 100 links, while others can contain thousands of links. The entire arrangement is determined by the genetic code in the body’s DNA.
The most elemental bacterial cell contains proteins, carbohydrates and DNA (and other similar nucleic acids). That should be the case, if it is to work. Because such cells are the most primitive forms of life discovered to date and the simplest known today – we must conclude that they were made up of non-living structures, which synthesized these essential components of life before they could be used. of them.
Although no one knows what the world was like about 3800 million years ago, in the 1920s researchers Oparin and Haldane hypothesized that the atmosphere at that time was almost completely devoid of oxygen, instead it was rich in ammonia, water, monoxide. carbon, meta, hydrogen and a variety of other substances. Oparin and Halden also claimed that the surface of the Earth was largely covered with hot water, which was continually boiling because of the incandescent molten rocks just beneath the thin scar of the ocean floor.
They claimed that this combination of gas and hot water formed a rich chemical-unsubstantial “soup” that had just the right ingredients for the synthesis of life. The key reaction could be triggered by volcanic activity, intense ultraviolet radiation through the thin atmosphere or electrical energy from lightning. The theory was tested in 1953 by American researcher Stanley Miller.
Using two jugs and some glass tubes, Miller built a model of the prehistoric world.
When Miller heated the mixture from the bottom pitcher, it boiled, turned to gas, passed into the chamber with sparks and finally condensed, then drained into the bottom pitcher.
For one week, the mixture was continuously heated, treated with sparks and condensed, after which it was siphoned for analysis.
The results were satisfactory. The heated and sparked mixture contained three amino acids – compounds that bind to form proteins. the idea was taken up by other researchers, who have done similar experiments and obtained even more amino acids, even simple nucleotides: the foundation stones of DNA.
Such experiments seem convincing. Therefore it is reasonable to assume that all proteins – and many other organic chemicals – could be synthesized over several thousands of years. Even DNA itself, with its thousands of precisely arranged atoms, could be successfully synthesized. And once it happened, it could self-reproduce, produce its own proteins and other complex organic substances, and become a fully functioning and self-reproducing life cheese, resembling a bacterial cell.
This must have happened, but the mathematical probability that a complex substance, such as protein or even DNA, will be generated by accidentally associating certain chemicals in a primitive ocean is infinite.
This sense of what may have happened can be compared to the speculation regarding the monkey and the typewriter: if the creature is given enough paper and a few years to beat it, it might reproduce some intelligible words; but the chances of a monkey accidentally producing a masterpiece are virtually nil. An amino acid can be compared to a word, but DNA would undoubtedly be masterpiece.
These unlikely chances are now largely accepted among scientists, and the search for a mechanism to determine amino acids, such as those created in Miller’s lab, to associate and form proteins without the DNA assembly instructions provided has not been completed.
If such a mechanism is to be found, it means that we are on the right track in understanding the origins of DNA, and ultimately the origin of life itself.