Evolution of sexual dimorphism in human lifespan (2)
Part 2. The Patriarchal hypothesis
The beginning of the article is here.
The increase in life expectancy was the result of evolution for the reason that it contributes to increased reproduction in the later stages of life. In the case of humans, longevity contributes to increased late reproduction in men, but not in women (at least not directly). In fact, the reproductive life expectancy of women does not differ much from the same period in other higher primates: on average, the ability to procreate in female chimpanzees and women remains up to 42 and 45 years, respectively [1]. Based on this, human longevity was the result of selection by the ability to late reproduction of the male half of our distant ancestors [16, 17].
This conclusion underlies the patriarchal hypothesis [18], according to which outstanding intellectual abilities and the ability to accumulate resources allowed successful older men to continue reproductive activity by acquiring additional young wives. The existence of such long-lived polygynous families (for example, consisting of one husband and several wives) would be supported by the accumulation of resources. In fact, the human species could well have descended from polygynous ancestors. Despite the widespread use of monogamy at the present time, in a number of communities, traditional polygyny is still practiced or even the dominant trend. Moreover, the polygyny of our ancestors is confirmed by the results of the analysis of the diversity of the Y chromosome [19].
Age-related reproduction models of men and women in polygynous communities, as a rule, demonstrate pronounced sexual differences. For example, in the Bimoba and Kusasi tribes living in Northern Ghana, female reproduction reaches a maximum level at 20 years old and begins to decline after 25. The reproductive activity of men, on the contrary, steadily increases from late adolescence to about 30 years [17, 20] (Fig. 1). An increase in reproductive age causes selection on the basis of longevity [7]. Moreover, the ability of men to reproduce until the age of 90 is a clear reflection of the concept of young wives.
Figure 1. Example of fertility distribution in a polygynous population (Northern Ghana):
the number of children born per year, depending on age and gender (according to [17]).
Marlowe et al. suggested that the absence of an increase in the reproductive period in women reflects an evolutionary limitation due to sexual differences in the mechanisms of gamete formation [18, 21]. Whereas in men, new spermatozoa are formed by the seminal glands throughout adulthood [22], in the ovaries of women, oocytes are formed only during embryogenesis. As we age, the number of oocytes decreases due to the activation of the process of programmed cell death. Perhaps this is a protective mechanism that ensures successful reproduction by preventing the formation of defective embryos. The rate of oocyte death in female chimpanzees and women is almost the same [23]. Thus, an increase in the female reproductive period requires a radical change in the functioning of the ovaries characteristic of primates. This approach implies that menopause, like aging itself, is maladaptive, and also replaces the "theory of grandmothers" with a less interesting explanation. Perhaps this reflects a recurring historical trend of poor treatment of older women as a group, for example, the fight against witches in medieval Europe, [24] as well as the cannibalistic use of older women by the Indians of Tierra del Fuego mentioned by Darwin during periods of famine [25]. However, if evolutionary constraints are the cause of menopause, this does not exclude the contribution of postmenopausal women to the overall survival of the population. For example, in a recent study conducted in the polygynous communities of Ghana, it was found that the presence of older women increases the number of children born to young women by 2.7% [17].
The probability that human longevity is an evolutionary result of reproduction of older men in polygynous families leads to several very interesting assumptions. Polygamy associated with reproductive longevity, including colonies with a single breeding individual, is a model found in the animal world. As an example, we can cite social insects, such as ants, whose "queens" can live for a very long time (for example, up to 28 years in black garden ants Lasius niger) [26]. Thus, an aging male reproducing individual can be protected by his large family from external causes of mortality, which, accordingly, can prolong its reproductive period [27]. Such a family structure could well play a critical role in the evolution of human longevity. According to the following assumption, the transition from polygamy to monogamy eliminated the most important guiding factor in the evolution of human longevity.
It is possible that ancient women contributed to increasing their reproductive success through behavior that maximizes the likelihood that their sons will have a long reproductive life. One method of achieving this goal is to choose men with a long reproductive period, that is, older men who already have children. Moreover, choosing an older man means choosing a man who potentially has fewer survival-reducing mutations [28]. The tendency of female individuals to interbreed with older male individuals is observed in various organisms, including fish (e.g., sea dogs) [29], birds (e.g., shaggy-legged mice) [30] and mammals not related to primates (e.g., African elephants) [31].
An intriguing speculative assumption is that similar sexual selection occurred in our ancestors. At the same time, the advantage was given to men who looked older than their years. This may explain the mystery of male baldness, which is a secondary sexual trait formed under the action of hormones of the seminal glands [32, 33]. It should be noted that even now women tend to choose men whose age exceeds their own. For example, according to the US census conducted in 1999, 33% of women married men 4 years or more older than them, while only 6.4% of women married men 4 years or more younger than them, which corresponds to a difference of 6.2 times [34].
Continuation: Why are men the short-lived sex?Portal "Eternal youth" http://vechnayamolodost.ru