Sunday, January 22, 2012

Grandmothers, Menopause, and the Survival of Our Species

Sarah Blaffer Hrdy, in her book Mothers and Others: The Evolutionary Origins of Mutual Understanding, provides this chart which indicates the paths nature took in arriving at our species, Homo sapiens. 

Evolutionary anthropologists tell us that the human-vectored branch broke off from the chimpanzee line about 6 million years ago, thus the timescale in the chart. There were many false starts—more than are included here—and most species variations failed. Homo sapiens eventually propagated from the African branch of Homo erectus that originated about 2 million years ago. This branch managed to exist for well over a million years and eventually spawned, about 200,000 years ago, the even larger-brained Homo sapiens that was capable not only of surviving, but of thriving. Our species is thought to have emerged from Africa 50-100,000 years ago as it spread across the earth.

Hrdy provides an answer to the question of why this line survived while so many others did not. She suggests that the answer is not to be found in looking at known human history and trying to project human attributes back in time, nor is it to be found in studying the other great ape species. While those exercises can be useful, the most edifying pursuit is the evaluation of the human populations that most closely resemble those from eons ago in Africa: hunter-gatherer societies.

The Africa of long ago would have been a difficult environment in which to survive. Food would have been scarce and periods of feast and famine would have been the norm. Our ancient ancestors could have only survived as small bands that developed the tendencies to cooperate and share. Evolutionary pressures would then have favored such attributes. These intentions could be expressed audibly and with hand signals, but this would also be the beginnings of the capability to express and interpret subtle changes in facial expression as means of determining emotion and intent.

Hrdy suggests the most critical development was that of cooperative breeding. This term refers to the ability of the mother to have assistance from others (alloparents) in caring for infants. This meant that the mother could spend more time gathering food while assuming others would protect and nurture her helpless infant. This arrangement also meant that the mother could breed more often, rather than having to wait until her child could be trusted to survive her absence. These are important differences between humans and the other great apes.

For cooperative breeding to be enhanced by natural selection there must be feedback to improve survival probability. The most direct evolutionary pressure would be derived from the survival of the infant itself. The infants who would live to maturity would tend to be those who could elicit nurturing responses from others. Hrdy refers to human babies as "sensory traps." In many species there has developed an innate interest in the newborns. This can involve hormonal responses driven just by the nearness to an infant. This is important for defining potential caretakers, but the infant must also play an active role. It is necessary for the infant to make itself an object worthy of the nurturing response of others. Hrdy tells us that it would have been critical for infants to learn how to read the intent and emotions of others besides its mother if it were to encourage others to care for it. We have all been face to face with a human infant and have spent long periods of time staring into its face trying to figure out what was going on behind those eyes. Meanwhile the infant is staring back at our face trying to figure out what it can learn from looking at us. Learning to smile and imitate the facial expressions of others has always been an effective way to generate affectionate responses from a potential care giver. In earlier times it might have been necessary for survival. The need to interpret the intent of others would continue to grow in importance as the child matured.

Studies of the existing hunter-gatherer societies support Hrdy’s picture. Many exhibit gift-giving tendencies that are designed to promote as many social ties as possible can be viewed as a remnant of the ancient need for cooperation and sharing. Such societies are very flexible in terms of "family" organization, with a matrilocal orientation being the most efficient because it provided related females as potential care givers. Mothers would begin to breed as soon as they became capable. There is no innate response that tells them how to care for an infant. That is a response that must be learned. Where better to learn it than from female relatives?

The recent realization by anthropologists that hunter-gatherer women quite frequently gave birth in the vicinity of their own mothers, provided they were still living, has led to greater interest in the role of grandmothers. Human females are almost unique in the animal kingdom in that they live long lives after they can no longer bear children. Anthropologists have discovered that in all sorts of societies infant survival is always greatly enhanced if the mother’s parent, the child’s grandmother, is available for nurturing.

A "grandmother hypothesis" has emerged. Grandmothers enhance the survival of their daughters’ children, therefore it is valuable to have grandmothers who live long enough to be around to contribute. This selects long-lived females as an evolutionary outcome. It also suggests that it is important that these grandmothers not be burdened by children of their own if they are to be available to assist others, selecting the characteristic of relatively early menopause in these long-lived women as an additional attribute.

Our species survived where so many others faded away. Perhaps grandmothers were the difference makers. Perhaps mature women around the world, as they pass through periods of hormonal assault, can take some comfort in the notion that they are replaying a role that was critical to the survival of the species.

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