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The evolution of menopause?

Instead of having more children, a grandmother may pass on her genes more successfully by helping her existing children and grandchildren. Such an advantage could have driven the evolution of menopause in humans.

Instead of having more children, a grandmother may pass on her genes more successfully by using her intelligence to directly or indirectly help her existing children and grandchildren. Such an advantage could have driven the evolution of menopause in humans, according to new research published in PLOS Computational Biology.

Women go through menopause long before the end of their expected lifespan, and researchers have long hypothesized that menopause and a long post-reproductive lifespan provide an evolutionary advantage. That is, each increases the chances a woman will pass on her genes. The precise nature of this works is still being debated.

To investigate the evolutionary advantage of menopause, Carla Aimé and colleagues at the Institute of Evolutionary Sciences of Montpellier (ISEM) in France, developed computer simulations of human populations using artificial neural networks. They recently tested which conditions were required for menopause to emerge in these artificial populations.
Specifically, the research team used the simulations to model the emergence and evolution of resource allocation and decision-making in the context of reproduction.

Menopause can be considered a resource allocation strategy in which reproduction is halted so that our biologic resources can be reallocated elsewhere than reproduction.

Researchers found that with the emergence of menopause, along with a long (sometimes 30 yeats) post-reproduction lifespan in simulated populations, requires intelligence in combination with caring for grandchildren. The importance of intelligence rather than physical strength lends support to a previously proposed hypothesis for the evolution of menopause known as the Embodied Capital Model.

"Embodied capital" is the organized somatic tissue such as muscles, immune system components, and brains, into functional senses which include: strength, skill, knowledge, and other abilities. Humans are specialists in using our brainbased capital. High levels of our knowledge and skills are used to discover the suite of high-quality, yet difficult-to-acquire foods hunter gatherers consume.

"Cognitive abilities allow accumulation of skills and experience over one's lifespan, thus providing an advantage for resource acquisition," Aimé says. "Stopping reproduction during aging allows allocating more of these surplus resources to assist offspring and grand-offspring, thus increasing children's fertility and grandchildren's survival."

Menopause, the permanent cessation of ovulation, occurs in humans well before the end of the expected lifespan, leading to an extensive post-reproductive period which remains a puzzle for evolutionary biologists. All human populations display this particularity; thus, it is difficult to empirically evaluate the conditions for its emergence. In this study, we used artificial neural networks to model the emergence and evolution of allocation decisions related to reproduction in simulated populations. When allocation decisions were allowed to freely evolve, both menopause and extensive post-reproductive life-span emerged under some ecological conditions. This result allowed us to test various hypotheses about the required conditions for the emergence of menopause and extensive post-reproductive life-span. Our findings did not support the Maternal Hypothesis (menopause has evolved to avoid the risk of dying in childbirth, which is higher in older women). In contrast, results supported a shared prediction from the Grandmother Hypothesis and the Embodied Capital Model. Indeed, we found that extensive post-reproductive lifespan allows resource reallocation to increase fertility of the children and survival of the grandchildren. Furthermore, neural capital development and the skill intensiveness of the foraging niche, rather than strength, played a major role in shaping the age profile of somatic and cognitive senescence in our simulated populations. This result supports the Embodied Capital Model rather than the Grand-Mother Hypothesis. Finally, in simulated populations where menopause had already evolved, we found that reduced post-reproductive lifespan lead to reduced children’s fertility and grandchildren’s survival. The results are discussed in the context of the evolutionary emergence of menopause and extensive post-reproductive life-span.

This work was funded by the French national agency for research (ANR, http://www.agence-nationale-recherche.fr/). Grant number: ANR-12-BSV7-0008. Project: HUMANWAY (led by MR). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.

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Jul 26, 2017   Fetal Timeline   Maternal Timeline   News   News Archive

Inuit grandmother and her grandchildren. Image credit: Pinterest

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