Evolution of sexual dimorphism in human lifespan (1)
Evolution of sexually dimorphic longevity in humans
David Gems, Aging (2014).
Translated by Evgenia Ryabtseva
Part 1. Evolutionary theoryWhy do humans live longer than other higher primates?
Why do women live longer than men? What is the significance of menopause? The answers to these questions could explain the mechanisms underlying the evolution of human aging. This article presents an evolutionary hypothesis that, based on the assumptions listed below, can provide answers to all three questions. The first assumption is that the evolution of increased human life expectancy could have been triggered by the prevalence of late reproduction of males in primitive communities practicing polygyny or polygamy. According to the second, the absence of a corresponding increase in the duration of the female reproductive period reflects the evolutionary limitation of the formation of oocytes in old age. And thirdly, it can be assumed that antagonistic pleiotropy (the influence of one gene on several phenotypic traits), affecting androgen-induced secondary sexual characteristics in men, increased reproductive success at a young age, while shortening life expectancy. The validity of the fact that sex differences in aging are due to androgens is confirmed by data on the outstanding longevity of eunuchs. At the same time, antiandrogenic therapy, currently used to treat prostatic hyperplasia, exacerbates a number of aspects of aging (for example, a tendency to cardiovascular diseases). One possible explanation for this is the assumption that low levels of androgen hormones throughout life slow down the aging process, whereas in old age the absence of these hormones has the opposite effect.
The nature of human aging has a number of pronounced features that have long attracted the attention of evolutionary biologists. For example, humans differ from higher primates in unusual longevity. The maximum life expectancy of orangutans and gorillas is 58.7 and 54 years, respectively, and our closest relatives bonobos and chimpanzees are 50 and 53.4 years, respectively [1]. At the same time, the maximum life expectancy of a person varies from 85 years for gathering tribes such as the Ache tribe (Paraguay) and the Kung Bushmen (South Africa) to 122 years in developed countries [1, 2]. Based on this, it can be assumed that the evolutionary breakthrough, consisting in an increase in life expectancy, occurred after the appearance of the last common predecessors of humans and chimpanzees/bonobos about 5-7 million years ago [1].
Another distinctive feature of human aging is sexual inequality. In parallel with the increase in life expectancy that accompanied the improvement of living conditions over the past century, there was an increase in the difference in longevity of representatives of different sexes [3, 4]. For example, in the UK, the average life expectancy of women and men born in 2012 is 82.4 and 78 years, respectively. The difference between these indicators is 4.4 years [5]. For the USA, the difference between the life expectancy of different sexes is 5 years (81 for women and 76 for men), for France – 6.4 years (84.7 and 78.3), for Russia – 13 years (73.1 and 60.1). This gender difference reflects the greater susceptibility of men to the development of a wide range of pathologies associated with aging, including cardiovascular diseases, type 2 diabetes mellitus, infectious diseases and sarcopenia (age-related decrease in muscle mass) [3]. The reasons for this "weakness" of the male sex are unclear both from the point of view of evolutionary prerequisites and from the point of view of somatic causes.
Another sexual difference of aging concerns the reproductive period. Women lose the ability to conceive a child at the age of about 50 with the onset of menopause, while men can maintain fertility until at least 70 years old. The significance of early loss of the ability to reproduce in women is the subject of active discussions, in particular, the discussion of the possibility that this phenomenon has an adaptive character and provides advantages in the course of evolution.
Apparently, aging itself is not an evolutionary adaptation. On the contrary, it is caused by the fact that the influence of genes on viability in old age practically does not contribute to survival. This is due to the fact that in the wild animals very rarely live to old age due to external causes of mortality, such as hunger, predators, diseases [6]. For example, a mutation whose detrimental effect on the health of mice manifests itself only after they reach the age of three will not be subject to selection, since in the wild mice rarely live to this age. Thus, the life expectancy formed by evolution reflects the intensity of mortality caused by external causes of representatives of the species in their usual habitat, as well as their ability to reproduce in the later stages of life.
The same gene can affect the phenotype in different ways at different stages of life. Such pleiotropy means that the same gene can theoretically be useful at an early age and harmful in old age (antagonistic pleiotropy). Since signs that appear at an early age are more important for survival, such genes can accumulate in a population, shortening the life expectancy of its representatives [7].
The evolutionary theory of aging includes a number of hypotheses that explain the appearance of the characteristic features of human aging. Apparently, human longevity was an evolutionary result of a decrease in mortality from external causes, which contributed to an increase in reproductive activity in the later stages of life. Probably, the development of intelligence and the use of language contributed to reducing the influence of such external causes of mortality as predation. The sexual dimorphism formed in the course of evolution may be the result of sexual differences in mortality from external causes and age-related features of reproductive activity [7]. For example, our male ancestors could, in comparison with women, be at a higher risk of mortality associated with the search for sexual partners and hunting. As for menopause, the hypothesis that it has an adaptive character is currently being actively considered [7-9]. According to the "theory of grandmothers", the help of postmenopausal women increases the reproductive success of their young relatives [10-12]. From this point of view, the genetic determinants of menopause persist in the population due to their impact on overall survival. That is, postmenopausal women in the later stages of life can contribute to the appearance of more offspring due to the care of their relatives' children than as a result of their own pregnancies.
Earlier reviews offered various ideas explaining the significance of menopause, for example [7-9], as well as the reasons for the sexual characteristics of human aging, for example [4, 13-15]. In this article, several earlier ideas concerning human evolution are summarized with observations about the longevity of eunuchs, as a result of which a plausible hypothesis about the causes of human longevity is formulated.
Continuation: The Patriarchal hypothesisPortal "Eternal youth" http://vechnayamolodost.ru