Thesis on evolution of man
TABLE 3. Definitions and descriptions for the grandmother hypothesis over time and authors. Most explanations for older women halting reproduction implicitly involve kin selection whether stated explicitly. Older women historically risked danger due to senescing physiological systems and mortality from predation in surviving to produce next offspring as well as increased demands for resources as their daughters matured to reproductive age to produce their own offspring. Foregoing future, risky reproduction to help rear kin suggests that non-reproductive adult human females historically should have been industrious in provisioning for grandchildren.
Older women in contemporary hunter gatherer societies, indeed, are among the most productive in foraging and sharing food, consistent with kin selection theory Hawkes et al. Whether inclusive fitness gains are sufficient to explain reproduction cessation remains contentious. The grandmother hypothesis explanation implies that the menopause phenotype is adaptive and predicts that older women historically overcame the decrease in gene-pool contributions inherent in the twofold reduction in genetic relatedness typically observed over generations.
Grandmothers in contemporary hunter-gatherer societies e. Researchers also have tested whether menopause, itself, is adaptive and found that, in premodern European e. No research has demonstrated genetic accounting consistent with inclusive fitness gains expected through kin selection to compensate for lost individual fitness.
The grandmother hypothesis on its own has as its most challenging theoretical problem addressing this steep selective gradient. This intriguing result implies that grandmothering evolved before reproductive senescence and extended longevity. Care from non-reproductive women and associated effects were an important factor in human evolution and continue to be in modern human societies. But whence grandmothers i. Explaining the origin of human grandmothers is challenging, especially given the twofold-increased selective gradient? Speculations that preference for younger women arose only after reproductive senescence had arisen Cohen et al.
We prefer involving the mate choice hypothesis in menopause-origin scenarios because it constitutes an explanation for how grandmothers originated, and we propose alternatively that genuine grandmothering evolved later, as a consequence from accumulated mutations and extended longevity, reinforcing their associated effects.
Genuine grandmothering, from this perspective, constitutes an elaboration on parental care specifically maternal , which had evolved earlier. Whether genuine grandmothering evolved prior to paternal care Geary, becomes an interesting topic for future research.
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Relationships between grandmothers and kin interestingly appear to have an effect on offspring survivorship: maternal grandmothers offer benefits to grandchildren whereas paternal grandmothers increase risk for child mortality Jamison et al. Effects associated with grandmothers, therefore, are non-universal. Contemporary human populations are characterized by mating behaviors favoring young adult females whether through intergenerational competition or male preference , long lifespans, and non-reproductive older females, but whether these characteristics arose, respectively, in scenarios associated with the mate choice, lifespan artifact, or grandmother hypothesis remains controversial.
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Contemporary human populations also are characterized by menopause, and the relationships among this characteristic and the other three characteristics as well as their associated hypotheses are contentious Figure 1. We provide herein, in a narrative framework, a theory for the origin and evolution of the human menopause phenotype.
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The theory involves as a sufficient, mechanistic, neutral genetic originating explanation, the mating behavior change described in the mate choice hypothesis. This is combined with a necessary, adaptive, initiating explanation, the lifespan increase described in the lifespan artifact hypothesis. These two explanations, together, account for the origin of reproductive senescence.
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They are accompanied by a contributory, exaptive, emerging explanation, a social interaction change in which older non-reproductive women exclusively started assisting in rearing grand-offspring rather than giving birth to and caring for additional offspring, described in the grandmother hypothesis; this accounts genetically for reproductive cessation among older adult females emerging in an evolving population, ultimately leading to the menopause phenotype that is observed in modern human societies.
Evolutionary branching diagram presented as a phylogenetic tree depicting relationships among gorillas, chimpanzees, and humans. Hypotheses cited and described in the text exist to explain each change. Relationships among menopause and the other three characteristics as well as their associated hypotheses are contentious. The characteristics are presented in the order in which they are described and theorized to have evolved in the text. The theory for the origin of human menopause is formulated by involving the mate choice hypothesis with the lifespan artifact hypothesis and grandmother hypothesis.
Human populations initially were characterized by a relatively short individual lifespan with diminished survival and fertility toward the end, as in contemporary chimpanzee populations. A change in mating behavior was established in which only young women mated with men. The change in mating behavior rendered as effectively neutral late-onset fertility-diminishing mutations in women. Mutant alleles accumulated in women. An increase in longevity was manifested for adaptive reasons such as decreased mortality or increased fitness resulting from prolonged, reproductively active adulthoods.
The increased longevity revealed reproductive senescence i.
Expressing this phenotype provided an opportunity for kin selection to operate exaptively, when older non-reproductive women i. A crucial detail in this narrative concerns relative timing for the hypothesized change in mating behavior and documented increase in lifespan. The change in mating behavior logically could have preceded, coincided with, or followed the increase in lifespan.
The latter scenario was adopted previously Morton et al. Emphasis was achieved by requiring the change in mating behavior to diminish fertility from a hypothetical, biologically unreal, maximally extended condition rather than a parsimonious, ancestral but mysteriously intrinsically determined decline in fertility at a predetermined age, which would have been uninteresting computationally.
Evidence exists that mating behavior involving young adult women is favored in our lineage, whether through male preference Symons, ; Casterline et al.
Researchers have documented that human populations evolved dramatic increases in lifespan in the Upper Paleolithic Hawkes et al. This documentation provides up to approximately years for female-specific late-onset fertility-diminishing mutations to have accumulated, allowing precedence or coincidence as plausible alternatives.
Non-reproductive older females subsequently could recoup evolutionarily fitness lost in unrealised potential matings e. The theory retains the salient point from the mate choice hypothesis: mutations that ultimately produce a menopause phenotype can arise effectively neutrally — non-adaptively — and accumulate over time. Developing the theory provides the opportunity to introduce biological considerations beyond those used in describing the mate choice hypothesis with associated computational model Morton et al.
Combining hypotheses is common in science, and human menopause phenotype science is no exception. Researchers often describe scenarios implicitly involving multiple hypotheses, whether intentionally. Difficulties can occur during pregnancy and childbirth in older women. Increased risk could be attributed to somatic aging rather than decreasing oocyte numbers Cohen, From an evolutionary perspective, greatest fitness would be achieved when younger women mated, as they would be more likely to produce healthy offspring.
Many among these undoubtedly could be included in a synthesis to formulate a theory for menopause. We chose to synthesize the trio combined herein as a minimal, parsimonious first approach. The fundamental benefit in this synthesis is that it provides an ultimate, neutral causal mechanism — mate choice — with an adaptation — extended lifespan — and an exaptation — grandmothers regaining fitness through kin selection. Narratives for the human menopause phenotype hitherto have been fraught with theoretical problems in at least one among these three components.
Other benefits derived from the synthesis and associated narrative is that they render more-plausible the processes associable with the three synthesized hypotheses. The synthesis and narrative provide bounds for the relative timing issue between the hypothesized change in mating behavior and documented increase in longevity associated with the mate choice hypothesis considered previously, in the section Change 1: Mate-Choice Hypothesis. The years available i. The synthesis and narrative also overcome the main theoretical problem with the lifespan artifact hypothesis.
As lifespan increased, the menopause phenotype encoded by female-specific late-onset fertility-diminishing mutations would have started to express, providing an explanation why only male fertility was extended concomitantly with longevity mentioned previously, in the section Change 2: Lifespan Artifact Hypothesis. The synthesis and narrative also overcome the main theoretical problem with the grandmother hypothesis.
As older non-reproductive women emerged, they could have increased or reclaimed inclusively fitness otherwise lost, by assisting in rearing kin, providing an explanation for how the twofold-increased selective gradient might have been alleviated at least mentioned previously, in the section Change 3: Grandmother Hypothesis.https://betwadenalra.cf
The initiating mechanism in the theory — reproduction involving only young adult females and adult males — could arise as an intrinsic consequence from genetic variation in preference and phenotype. Classic theory Fisher, , , ; reviewed in Andersson, and Prum, suggests that the case involving male preference for younger females may be established ultimately as an arbitrary sensu Prum, mate choice interaction.
Assortative mating initially establishes a genetic correlation in preference and phenotype, for instance, life histories in which males prefer mating with females close in age. An equilibrium is established as a compromise between the genetic correlation and natural selection Fisher, , , ; Lande, , which operates to favor phenotypes that optimize survival independently from phenotypes that might be preferred intersexually.
This equilibrium may be affected by a neutral shift, like genetic drift or, for instance, arbitrary preference for younger mates, perturbing populations from equilibrium. Such neutral mechanisms for preference and phenotype evolution via intersexual selection have been proposed as necessary null models for statistical analysis Prum, We effectively argue from such a null hypothesis perspective, initializing the theory with Lower Paleolithic human populations characterized by short lifespans and diminished late-age survival and fertility.
Those populations might have established genetic correlations in preference and phenotype and, in concert with natural selection, an equilibrium. Three changes might have perturbed those populations from equilibrium, with consequences: 1 a neutral mate choice shift, a behavior change in which only young women reproduced, rendered as effectively neutral late-onset female-specific fertility-diminishing mutations, which accumulated; 2 an adaptive lifespan increase revealed the reproductive senescence phenotype encoded in the accumulated late-onset fertility-diminishing mutation genotypes that had been unexpressed in the shorter lifespan; 3 an exaptive social interaction change in which older non-reproductive women exclusively assisting in rearing grandchildren rather than giving birth to and caring for their own children produced the first grandmothers.
The arbitrary mate choice shift might have involved preference for younger mates as described in the preceding paragraph; Symons, ; Casterline et al. The other possibility — younger females out-competing older females for access to mates Cant and Johnstone, — constitutes an interesting adaptive alternative hypothesis for statistical analysis in future studies.
The theory as we have described it herein is applied specifically to humans. Alternatively, a similar synthetic approach but differing in details e. Menopause in mammals, for instance, intriguingly has been associated with populations wherein female relatedness to group members increases with age Johnstone and Cant, A framework similar to the theory could applied to elucidate menopause in killer whales, with older females functioning more as ecological information repositories than as caregivers Brent et al.
The theory entails that the human menopause phenotype may be conceived as a trait with a sufficient, non-adaptive, genetic ultimate cause: mating involving only young women allowing late-onset fertility-diminishing alleles to accumulate cryptically. Increased lifespan necessarily allowed reproductive senescence to be expressed. Grandmothers may have emerged as a means to reclaim lost fitness through kin selection.
Menopause thus theoretically may be considered a trait that originated neutrally, evolved adaptively, and was co-opted exaptively Gould and Lewontin, ; Gould and Vrba, The theory practically could prompt research into 1 searching for and identifying real-world counterparts for the late-onset fertility-diminishing alleles in the computer simulations described in the section Menopause , genes which then would become candidates for exerting deleterious effects on other life history traits through pleiotropy; 2 testing whether genuine grandmothering evolved prior to paternal care as mentioned in the section Change 3: Grandmother Hypothesis ; and 3 investigating whether younger females out-competing older females for access to mates could lead to a menopause phenotype as described in the section Prospectus.
Predictions formulated on the basis of the theory also could prompt empirical research. If only young adult males and adult females were to mate in a population under selection for increased longevity e. This possibility could be tested by investigating whether menopausal male fruit flies could be evolved in a laboratory setting. All authors contributed to the research described herein and the description, itself.
MT reviewed and summarized data on menopause, mating behavior, life expectancy, and grandmother hypothesis descriptions. RS conceived the mate-choice hypothesis. JS formulated the synthetic theory. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors thank D. Morton, H. Naveira, and N. Johnson for helping to gestate ideas.
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