How did sex come about? There is no answer yet - the science of the 21st century has to solve the problem. But there are hypotheses: according to one of them, this is a side effect of the biochemical mechanism for repairing damaged genes, which arose more than a billion years ago. Evolution has proven that sex is useful. In fact, we already knew about it. Science has given rational arguments
Yuri Panchul
biologist, USA
Sex arose already in the distant unicellular ancestors of man more than a billion years ago. Virtually all animals (with very few exceptions) can reproduce sexually. Even jellyfish have male and female genders. Those animals that are deprived of this happiness (for example, some species of lizards) undoubtedly had ancestors that reproduced sexually.
Why is sex necessary?
Sexual reproduction is so widespread in nature that, until the 19th century, people did not even wonder why animals and plants reproduce by copulation or other combination of germ cells. This question arose in the middle of the 19th century in connection with the spread of Charles Darwin's theory of evolution and natural selection. Indeed, according to this theory, in order for sexual reproduction to be preserved, it must help the survival of the species, and the “profit” must be greater than the costs of sexual reproduction. Otherwise, for the continuation of the species - including man - nature would have found something more effective.
The cost of sexual reproduction
Every action has its price. Reproduction through sex is also very costly. Flowering plants have to work hard to produce nectar to attract bees, which carry pollen from flower to flower, helping the plants to reproduce sexually. And Homo Sapiens men, for example, are forced to buy flashy sports cars to attract girls, girls – well, it’s known that “the devil wears Prada”…
In addition, only half of sexually reproducing individuals - females - give birth to children. If a mutant reproducing by budding appeared in a population of sexually reproducing creatures, then, all other things being equal (for example, the birth of an average of 2-3 children), the descendants of this mutant would easily displace the descendants of a pair of sexual creatures - after only 11 generations, the difference in the number of descendants would be more than 1000 times (2 eleven /2 = 1024). Why do we still reproduce sexually? The answer to this question became clear only after the development of genetics.
In terms of genetics
Charles Darwin believed that the existence of sex is beneficial for heterosis - the effect of increased vitality of crossbreeds of different breeds of animals, often observed in animal husbandry: it is known, say, that mestizos of different breeds of horses run faster than their "pure" relatives. But nature, of course, does not care about races - it is interested in the survival of the species. At the same time, Charles Darwin could not explain the nature of this effect, since in his time it was believed that the heredity of two parents mixes like blue and red paint, resulting in something in between - purple paint.
After the work of Gregor Mendel, August Weismann and other pioneers of genetics in the 19th century, an explanation for the usefulness of sex was found. According to genetics, each person carries two sets of genes: one received from the mother, the other from the father. Moreover, due to the shuffling of genes through a process called crossing over, a person receives a rather complex collection of genes from all his grandparents. However, genes sometimes get broken - defective. So, genetics has shown that a healthy gene received from the father can cover the negative effect of a defective gene received from the mother, and vice versa. Thus, Charles Darwin's observation about the increased vitality of crossbreeds of various animal breeds became clear.
However, the survivability of mestizos is not the main benefit of sexual reproduction. Much more important is the possibility of combining several positive mutations. This phenomenon is most evident in bacteria. While bacteria reproduce by simple division, in some species there is sex in the form of the exchange of genetic material through a special hose (pilus) with which bacteria are connected to each other, like airplanes during refueling in the air. It is in this way that bacteria exchange resistance genes with each other for each newly discovered antibiotic by doctors. Without sex, it would take thousands of years for bacteria to successively acquire all the mutations that protect them from several antibiotics. Because of sex, they can acquire these mutations independently, and then exchange "achievements".
Evolution puts on egoists
Explanations of the usefulness of sex for the survival of the species were considered sufficient until 1962, when the British zoologist VineEdwards began a dispute about the essence of Darwinism with the American biologist George Williams. Edwards believed that evolution selects species, and Williams was convinced (and was able to prove) that evolution selects selfish individuals. According to Williams, a hare in the forest does not compete with a wolf, but with another hare. If one hare runs a little faster than another hare, then it is the other hare that will be eaten by the wolf, thereby ensuring the evolutionary victory of the first hare.
In other words, any feature of an organism (including sex) will be fixed by evolution only if this feature helps a particular individual right now, and not his whole species in the abstract future. A large number of observations and mathematical models confirmed the correctness of the "individualist" Williams. This theory was further developed by Richard Dawkins, who generally stated that the true players of evolution are not species and not even individuals, but genes. The body is only a way to provide a future for the gene.
Politically Incorrect Sex Since the publication of Dawkins' book The Selfish Gene, many researchers have attempted to explain human sexual behavior as an optimal strategy for transferring these genes into the future, imposed by one's genes.
Matthew Ridley, for example, showed how humans evolved in small groups living in the African savannah. In such an environment, the man's genes told him to have one wife and the maximum number of mistresses. Wife - for the birth of children, to whom he would transfer his status in the group, and mistresses - for the widest possible distribution of his genetic material. And the genetically based strategy of women, according to Ridley, was different - to become the wife of a “reliable breadwinner” to feed the children, but at the time of ovulation, try to get genes from a “pirate” randomly running nearby with better “quality” genes than her husband.
When people began to build cities and large states, this genetically based strategy led to the formation of societies where the majority of women were usurped by big bosses (kings) and specialized don Juans. The rulers of China, the Inca Empire, Babylon had thousands of concubines - and a "beloved wife" to transfer status. In ancient Rome, slave owners did not hesitate to use their slaves for sexual purposes, leaving most male slaves without women.
According to Ridley, these trends led to a lot of violence and a low level of cooperation between men in the ancient world. In his book, Ridley suggested that the fall of the Roman Empire in the face of the Germanic barbarians was partly due to the fact that the Germanic tribes had a rigidly enforced monogamy, which helped men unite against a common enemy - Rome.
Why doesn't sex disappear?
Biologist George Williams explained it this way. Suppose there is a tree growing in the forest. It produces thousands of seeds that can fall in a variety of places - sunny or shaded, wet or dry. Therefore, it is important for a tree to produce different descendants - with more or less long roots, different trunk heights. The offspring will compete with each other, and the most fortunate one will survive. In other words, Williams believed that sex produces diversity, in which brothers compete with each other.
The opposite hypothesis was expressed by the American biologist Michael Giselin. He believed that, on the contrary, sex and the diversity associated with it allows the brothers to avoid competition with each other, as they begin to occupy slightly different ecological niches. As an analogy, let's assume that a blacksmith's family lives in a small village. If all five children of a blacksmith inherit his craft, then four of them will die of starvation, since the village only needs one blacksmith. But if they have different professions (weavers, roofers), then their total income will be higher, since they will not compete for one place under the sun.
The third hypothesis was put forward by the American biologist Lei Van Valen. It is called the Red Queen Hypothesis, after a character in Lewis Carroll's work. For the Red Queen, fast running was the only way for Carroll to stay in place. According to Van Valen, sex arose as a way for the individual to fight parasites. In the process of this struggle, no victory occurs - the individual and the parasites evolve simultaneously, remaining in the same place in relation to each other due to the simultaneous "arms race".
Finally, in the last two decades, the Russian-American biologist Alexei Kondrashov, who emigrated from Russia to the United States in 1991, made a great contribution to clarifying the question “why sex is needed”. According to Kondrashov's theory, sex is primarily needed not for the combination of useful genes, but for the combination and removal of harmful genes from the population. Each of us, a member of the Homo Sapiens species, carries dozens of new mutations that were not present in our parents. Most of these mutations have no effect, some (about 1%) are harmful, a negligible part is beneficial.
Sex, according to Kondrashov, allows you to collect combinations of harmful genes in a small subgroup of individuals, after which these “particularly damaged” die, “cleansing”, like a vacuum cleaner, their population from harmful mutations.
The beginning of time
The theories of Williams, Giselin, Van Valen and Kondrashov explain why sex retains its usefulness and is not replaced by, say, budding. Unfortunately, they don't explain how sex came about. There are many hypotheses. Unknown - even more. Finding the answer is the task of the science of the future.
