ABSTRACT...The analysis of 28 ancient genomes revealed that N-B482 was widespread in Eurasia during the Neolithic, Bronze, and Iron Ages, spanning from Baikal to Hungary and from the Arctic to Uzbekistan, but it is now considered a relict. The number of its modern carriers is vanishingly small: the analysis of our samples from North Eurasia’s indigenous populations (n ≈ 25,000) detects N-B482 presence only in North Altaians (Kumandins, Chelkans, Tubalars), Mongolians, and Kalmyks. The primary cause of extinction for N-B482 lineages is genetic drift. The Galton-Watson theory of branching processes suggests a high probability of extinction for lineages with uniparental inheritance.1. Introduction...It is well known that the number of family names passed down through male descendants grows smaller over time. In nations with a long tradition of family names, such as Han or Korean, there are relatively few modern family names. This phenomenon was first studied in the 19th century in France (Bienaymé) and the UK (Galton, Watson) in connection with the mass extinction of aristocratic family names. Genetics considers a family name similar to a genetic marker correlated most distinctly with the Y-chromosome of men [8]. The extinction of family names and Y-chromosome variants is observed not only within individual lineages but also within entire Y-chromosome haplogroups. Even the earliest studies of human Y-chromosome phylogeny reported a high rate of extinction of Y-chromosome lineages [9].4. Discussion...However, the main factor that reduces diversity among Y-chromosome lineages in small populations is genetic drift. The systematic extinction of individual haplogroup N-B482 lineages is a result of random genetic drift. The bottleneck effect is a form of genetic drift characterized by a dramatic reduction in the population’s size further followed by its growth. In the past, the bottleneck effect could have affected either an entire population (through natural disasters, famine, epidemics, etc.) or primarily its male members (through wars, sex imbalance, etc.) [60,61]. The extinction of some Y-chromosome lineages may have been caused by economic factors that affected population growth rates, such as unequal access to resources. If such factors had a lasting effect, they may have caused the extinction of those Y-chromosome lineages that were less competitive [62]. However, the key factor in eliminating Y-chromosome lineages which exerts its effect even when the analyzed populations are initially equal in size is a continuous universal extinction of male lineages due to the stochastic character of female and male births with a probability close to 0.5. This process was first described by Galton and Watson (see Appendix A).Appendix AExtinction of Genealogies in the Theory of Galton-Watson Branching ProcessesThe number of modern descendants of those men who lived in the past can vary significantly depending on a multitude of factors. An attempt to explain the extinction of British aristocratic family names by a stochastic branching process was made by Galton and Watson in the 19th century. The Galton-Watson branching process makes allowance for a variance in the number of male offspring, including the absence thereof, produced by different men. Although the model cannot cover the entire diversity of causes underlying the extinction of male lineages, its simplicity illustrates the universal, overarching character of stochastic elimination of paternal lineages. The Galton-Watson theory of branching processes is outlined in [63,64]....The growth rate of a Neolithic population was about 0.1% a year [66], i.e., A = 1.03, and the average generation time was 30 years. According to the branching process theory, 97% of the Neolithic Y-chromosome lineages (1/1.03 = 0.97) became extinct at this growth rate. But even in a rapidly growing population that doubles with every generation (A = 2), 50% of Y-chromosome lineages become eliminated over time. Therefore, the higher the average number of sons per man, the lower is the extinction probability for the Y-chromosome lineage. At the modern growth rate of the world’s population, which is 0.9% a year (А = 1.3), the extinction probability for its Y-chromosome lineages is 77%.The universal law of extinction of paternal lineages has its consequences. For example, old branches of the Y-chromosome tree usually split into only two new branches. If the number of male offspring follows a geometric distribution in the Galton-Watson branching process, then the probability of survival of three branches is very low and equals (1 − 1/А) of the probability of survival for two lineages. Another consequence of the extinction law is that the overwhelming majority of ancient men whose aDNA we study today are not direct ancestors of modern men.
