Geneticists are puzzled by the gender composition of the ancestors of Europeans and Asians. Either far more men than women came out of Africa, or the colonialists practiced matriarchy and polyandry, or men lived much longer than their girlfriends. Authors who have received an unusual result do not fully believe a single explanation
Nowadays, scientists no longer doubt that the human race appeared in Africa. From here, Homo sapiens settled throughout the world, occupying continent after continent and sometimes meeting with their distant brethren, who left Africa hundreds of thousands of years earlier. This happened in Europe, where people of the modern type drove out the Neanderthals about 30 thousand years ago. Perhaps something similar happened in Asia - for example, in Indonesia, where the "hobbits" of the Flores island lived only 20 thousand years ago, when almost all of the Earth, except perhaps America, was inhabited by Homo sapiens.
For centuries, historians and anthropologists have tried to reconstruct the sequence of the settlement of our planet by races and peoples. In recent years, geneticists have been offering them help. By comparing the chromosomes of representatives of different peoples, scientists can now restore family ties between them and try to calculate (based on estimates of the rate of accumulation of mutations in the genome) when there was a division between peoples, whose descendants are now known as, say, the Swedes and the Chinese.
Drift on the X chromosome faster
Genetic drift is a purely random phenomenon and obeys the laws of probability theory. It can be calculated that genetic drift in autosome DNA should be 25% slower than on the X chromosome.
During the formation of gametes - eggs and sperm - paired chromosomes diverge into different gametes in a random way, and the mother may well pass the 15th chromosome to the child from his grandfather, and the 22nd from his grandmother. Moreover, even one chromosome can carry genes from two ancestors due to the crossing-over phenomenon, when paired chromosomes exchange parts similar at the ends during cell division. However, not every egg cell, and very, very few of the spermatozoa will initiate a new life, passing genes by inheritance. Therefore, the factor of chance can radically change the frequency of occurrence of genes among descendants in comparison with ancestors.
The fewer individual chromosomes are involved in this process, the more the final population is subject to the "random factor", and the genetic composition of small populations changes much faster due to drift than in large ones. But we have two types of chromosomes - sex chromosomes X and Y, the last of which, in the amount of 1 piece, is available only in men, and autosomes, which are equally divided in men and women. In a population where men and women are equally divided, there are only 75 X chromosomes for every 100 autosomes of each type.
Based on this analysis, scientists believe that the majority (and maybe 100%) of those living outside Africa now are descendants of a group of people who left the continent about 60 thousand years ago (plus / minus 20 thousand years). How long this exodus lasted is more difficult to determine, but it is clear that very few people started the many billions of non-Africans. To be convinced of this, it is enough to look at genetic diversity - the abundance of different variants of genes in a given population. Among blacks it is several times more than calculated for all other races and peoples combined.
If you look closely at the genetic makeup of races and peoples, you can find much more subtle effects. For example, try to figure out how many of the “colonialists” who set off on a long journey from Africa were men, and how many women.
And it turns out that there were several times more men
However, this is just the simplest interpretation of the data.
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To assess the ratio of males to females in the founder population of the non-African population, scientists tried to estimate the number of female X chromosomes in relation to autosomes - the rest, paired, non-sex chromosomes. Since men have not two X chromosomes, but only one, in total any man and woman have only 3 X chromosomes, while, for example, the first or 15 chromosomes have 4 pieces. Likewise, in any population with equal sex composition - where there are equal numbers of males and females - there will be 33% (4/3 times) more autosomes than X chromosomes.
But here's how to find out how many chromosomes were in people who died 60 thousand years ago?
We need to look at the DNA of their descendants, and moreover, not one, but a large number. And compare how the gene pool as a whole has changed over time, namely, how the relative frequencies of various variants of the DNA sequence (gene alleles) have changed in the population. In this case, it is not so much about the emergence of a new mutation (the emergence of a new allele), but about how much the proportion of owners of a particular allele in the studied group of people changed.
The reason for these changes is the so-called genetic drift.
This is a purely probabilistic phenomenon, and its rate is inversely proportional to the number of chromosomes in the population.
Therefore, if there were equal numbers of men and women among the colonizers who left Africa, the gene pool associated with the X chromosome would change 33% faster than the autosomal gene pool.
American geneticists, led by Alon Kanan and David Reich of the recently formed Broad Institute of Harvard and MIT, have tried to measure these speeds. To do this, they compared three populations - "West Africans", "East Asians" and "North Europeans". Data on the relative frequencies of thousands of different DNA variations for them were collected in the framework of the international project "HapMap". West Africa is represented in it by the Yoruba people of Niger, East Asia - by half Japanese and Han Chinese, "Northern Europe" - by Americans of the corresponding origin.
It is assumed that the first are the descendants of people who remained in Africa, and the second and third are the descendants of the very "colonizers" who left the black continent. Comparing variations in hundreds of thousands of DNA positions, one can statistically separate the variations - to find out which arose when all people became Africans, which ones - during the "exodus", and which ones - after that, when the departed were divided into Europeans and Asians.
As the comparison showed, the X chromosomes of the "colonizers" changed not by 33%, but 60-70% faster than autosomes.
All other things being equal, this means that there were 3 or even 5 times more men among them than women!
Corresponding work is accepted for publication in Nature Genetics.
That there were more men on the long voyage is hardly shocking to anthropologists. By studying the hunter-gatherer communities that have survived to this day, these scientists have long concluded that it is men who are the driving force behind long-distance migration, while women are responsible for small displacements. However, the scale of the discrepancy - 3-5 times - and the fact that immigrants from Africa did not go to neighboring villages, but "into the void", where they had no one to continue their family, made Kanan and Reich think about alternative explanations.
For example, the size of the female population could be incorrectly imprinted in the genes if men, for some reason, left offspring with only a small proportion of their companions.
This behavior is practiced in many canines and even some monkeys, where the dominant female does not allow other members of the pack to mate with numerous males. But did people behave the same way? It is unlikely, the authors point out, since the same anthropological studies indicate the spread of polygamy rather than matriarchy and polyandry among hunting tribes.
Perhaps men lived longer? After all, genetic drift determines not time, but the number of changed generations. If women are replaced more often than men, then it seems to us that their drift speed is higher. Again, Keinan and Reich and colleagues believe that this explanation also contradicts anthropological observations.
Finally, is it possible that we are counting the drift speed in vain? Maybe the gene pool was not changing due to drift, but under the influence of natural selection? After all, external conditions changed dramatically with the exit from Africa. Maybe some X-linked genes, from which it was neither warm nor cold in Africa, suddenly turned out to be critical for survival in the Middle East? Again, unlikely, the authors write. First, it is not clear why only on the X chromosome. Secondly, the scientists failed to find any differences in the rate of change in the gene pool between the coding and non-coding regions of chromosomes.
Ultimately, accelerated genetic drift along the X chromosome remains a mystery, scientists admit.
By the way, during further resettlement - during the colonization of Europe and Asia - there were no drift anomalies, and the work of Keinan and Reich shows the ratio of the number of X chromosomes to autosomes at 3/4 (within the margin of error). How was the African exodus so different? There is no answer to this question yet, but it is possible that all of us, with the exception of Africans, are children of a very small number of women and a much larger number of men.
