Categorized | Animals, Research

Computer Simulation Indicate Grandmothers Extend Humans Longevity and Could Increase Ape Lifespan

ApesThe “grandmother hypothesis” has received mathematical support from a recent study involving computer simulation. This renowned theory postulates that human developed longer lifespan as adults than apes due to grandmothers helping in the feeding of their grandchildren. The theory was first advanced in 1997 by university of Utah anthropologists Kristen Hawkes and James O’Connell, together with Nicholas Blurton Jones, an anthropologist at UCLA.

Kristen Hawkes, a prominent anthropology professor at Utah University conducted this recent study together with Peter Kim, a mathematical biologist and former postdoctoral researcher at the same university currently based in university of Sydney and James Coxworth, an anthropology doctoral student at the University of Utah. The study was financed by the Australia’s Research Council and National Science Foundation.

The computer simulation shows that animals with lifespan similar to Chimpanzees evolve to acquire human lifespan in less than 60,000 years with a little “grandmothering”. While human females live for a number of decades after child bearing, female chimpanzees hardly ever live past their child-bearing years, which are normally their 30’s and at times their 40’s. This simulation did not consider assumptions about the human brain size.

The Grandmother Theory

The grandmother theory postulates that through feeding their grandchildren after weaning, grandmothers ensure their daughters have the opportunity to bear more children within shorter time intervals. Consequently, the children grow to be weaned at younger ages but can first feed themselves at older ages. As a result the females end up having human like post-menopausal lifespan when they attain adulthood. Hawkes says that humans are who they are as a result of grandmothering. The new study’s senior author also adds to this saying that grandmothering accounts for the huge differences observed between modern apes and humans in terms of their life history events including the weaning age, age of attaining adulthood and their longevity.

While on a visit to Tanzania in east Africa, Hawkes observed that older women routinely spend their time collecting food for their grandchildren. The theory’s proponent conjectured that this offered grandmothers a window of opportunity to step in to food collecting tasks which the newly weaned children could not handle; offering the mother the time to focus on other duties, including producing other children.

However, possibilities exist that benefits grandmothers offer to their grandchildren may be a product of longer post-menopausal lifespan which evolved for other purposes. With this in mind the new research study, set out to establish whether grandmothering could lead to the evolution of Chimpanzees and other ape-like life histories into the long post-menopausal lifespan observed among humans.

Another recent simulation conducted by other researchers pointing out that few women live past their fertile ages, thus curtailing the “grandmothering effect”, was received with skepticism by Hawkes, spurring conduction of the new research. The new study further attended to the lack of any mathematical underpinning which characterized the theory when it was first advanced in 1997. Lack of any mathematical support was an apparent shortcoming which formed the theory’s major criticism, and part of the recent new study’s main objective.

Simulation of the Development of Adult Lifespan

In this study, the simulation calculated changes in the mean lifespan from the point adulthood starts. Adult longevity is different between humans and apes. Chimps become adults at the age of 13 years and averagely live an additional 15 or 16 years. In contrast, people living in the developed countries become adults around the age of 19 and can further live for another 60 or more years. The researching team took a conservative approach, weakening the grandmother effect by making several vital assumptions. It was assumed that the woman could not be a grandmother before the age of 45 years or after reaching 75 years, and that she could only care for a child of at least 2 years and beyond. The study also assumed the grandmother could care for only one child, belonging to anyone including her daughter’s.

The computer simulation begins with 1% only of women attaining grandmotherly age, who have the ability to take care of their grandchildren. By the end of the simulation, within a simulated time period of 24,000 years to 60,000 years, the population results are similar and comparable to those observed among human hunter and gatherer populations. In the study, additional years of life after adulthood doubled to 49 from 25 years over the same period.

Increase in adult longevity in this simulation involved evolution in the prehistoric time. However, increasing lifespan within the recent centuries have been largely attributed to several public health measures including access to clean water and development of efficient sewer systems.

Grandmothering versus Big Brains

Many anthropologists have argued that increase in brain size among human’s past ape-like ancestors was in fact the major cause leading to the evolution of lifespan dissimilar to those observed among the apes. However, the recent computer simulation indicates that even weaker grandmother effect could make simulated creatures develop to human longevity from chimp-like longevity. In this process, the simulation did not consider brain size together with hunting or pair bonding.

According to Hawkes, the move to longer adult lifespan, a result of grandmothering effect, underlies subsequent critical changes in human evolution such as the increase in brain size. The anthropology professor adds that it is what gave rise a wide range of social capacities which are the basis for development of distinct human traits such as pair bonding, learning of new skills, bigger brains and the tendency to cooperate with others.

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