Granny’s Dear Will Live a Looong Life!
It so seems that same words were expressed in totally strange languages ─ and just with mimics before the language was invented ─ millions of years ago. Let’s not be unfair to the fathers’ moms. Although in geneticists’ views the bride’s mother is more successful in passing the genes to new generations, it has now been confirmed that grandmothers, no matter from which lineage, have played the decisive role in the evolution of menopause in humans, extension of their lifespans and, consequently, the growth of the brain size. And the neutrality of the judge ending years of controversy on the subject is unquestionable: Mathematics.
According to the “grandmother hypothesis”, first proposed in 1997 by Kristen Hawkes, professor of anthropology at the University of Utah, because the help grandmothers provide to the feeding and upkeep of grandchildren at primitive societies reduces the energy cost of these chores for mothers, they can have more children. The infants, although weaned earlier, thus spend more years under care as children and adolescents before reaching adulthood.
Hawkes bases her hypothesis on her observations during the years she spent living together with the Hazda tribe of hunter-gatherers in Tanzania in 1980s.
For the grandmothers to support their grandchildren, first there has to be grandmothers around. Menopause enters the picture here. End of fertility with diysfunctioning of the ovaries is generally seen between ages 45-50 in humans. Although fertility ends at about the same time in bonobos and chimpanzees, the two species with closest affinity to humans from which we have branched off from a common ancestor 4-to-6 million years ago, in these and other ape species (gorillas and urangutans) lifespan either ends with the end of fertility or goes on for a few more years. In the wild, those who can live until the onset of menopause are very few due to external factors like diseases, starvation and violence .
In humans, however, post-menopausal years can approach half of the average lifespan. The reason, first and foremost, is the presence of grandmothers. Because of the ability of a few hominid grandmothers to live beyond the menopause and shoulder the upkeep of infants to support their daughters millions of years ago, the cost of childbearing to mothers were reduced; so they were able to bear more children. Thus the genes responsible for menopause were passed on to later generations through natural selection.
The vital role of grandmothers appeared two million years ago as the forests receded and large swathes of Africa turned into arid savannahs. In ape societies, when the infants are weaned (at the age 4.8 in chimpanzees, 2 in gorillas, 6 in orangutans,) they begin to find their own food and feed themselves. Therefore apes remain in the forests where the young can fend for themselves. But the humans, for whom the average age of weaning is 2.8, were faced with a difficult choice. Either, they , too, would have to stay in the forest where the infants could find their own food, or they would have to keep feeding the infants even after weaning. This, however, places an obstacle for reproduction. Because when a mother is busy caring for a child, it is difficult to have another.
It is here that grandmothers enter the equation. They take onto themselves such tasks as digging out tubers from the hard soil of the arid savannah or cracking open the hard shells of seeds (which the weaned young of apes and hominids are incapable of) and so help them survive.
According to Hawkes, those who remained near the food sources the just weaned young could gather (forests) were our “ape cousins”. And those who began exploiting food resources outside the reach of small infants opened the door to grandmother care and eventually evolved into humans.
In her new work ,called the “Second Grandmother Hypothesis” within the community of anthropologists to distinguish it from her 1998 study, Hawkes searches the answer for the question of what happens when the effect of grandmothers is added after starting out with a lifestyle like that of the apes. The computer simulations were employed to determine the average lifespan after attaining adulthood. Today, the chimpanzees, reaching adulthood at the age of 13, can live another 15-16 years on average in the wild. Humans, on the other hand, after reaching adulthood at 19, can look forward to 60 more years in developed countries (thanks largely to clean water, hygiene and health services in latter centuries.)
In determining the parameters of the grandmother effect in the new study, researchers made conservative assumptions that a woman couldn’t provide grandmother care before the age of 45 and after 75; could not take care of a child under the age of 2; and could care for only one child of her daughter or another woman. Furthermore, based on earlier research, the possibility of every newborn child carrying a mutatated gene that would extend or shorten his or her lifespan was assumed to be 5 percent.
The simulation, designed by mathematical biologist Peter Kim and anthropologist started with only 1 percent of women reaching an age allowing them to become grandmothers and look after the children. It was seen that the results obtained in the timespan from 24.000 to 60.000 years were in good agreement with rates observed at the hunter-gatherer societies of our day: 43 percent of adult women are grandmothers. In the same period, the lpost-adulthood life doubled from 25 years to 49.
Another finding of the new grandmother hypothesis was an extended childhood permitted by grandmother care, which, in turn, led to the evolution of larger brains. Dissenters argue, however, that large brain could have evolved earlier, and more time it needs to accumulate adequate information forced the extension of the childhood period.
REFERENCES
- 1. “Grandmas made humans live longer”, Utah Üniversitesi, 24 October 2012
- 2. “Sacrifice on the Serengeti – A guest Post by Eric M. Johnson”
- 3. http://blogs.scientificamerican.com/primate-diaries/2011/10/13/sacrifice-on-the-serengeti/
- 4. “Evolution of Menopause”, http://en.citizendium.org/wiki/evolution_of_menopause