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Wonder why you live longer than a chimp? Thank your MOTHER IN LAW

And your mum, other grandmas. Grandads not mentioned

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Grandmothers are the secret behind humans' living such long lives compared to our near relatives the apes, a computer simulation has revealed.

“Grandmothering was the initial step toward making us who we are,” says Kristen Hawkes, anthropology prof at Utah uni.

Hawkes and colleagues of hers have long sought to advance their theory of grandmothering, which goes roughly like this. In the remote past, the common ancestral mothers of humans and great apes couldn't generally move on to have another child until their most recent nipper was no longer breast-feeding (breast feeding acts as a contraceptive, in order to prevent a mother being overburdened with children she can't support).

This could take quite a long time, much longer than it now does, as the infants need to be quite old before they can feed themselves and generally in an ape-man tribe nobody else was going to get hold of suitable food - for instance by digging up suitable root veg etc - and feed it to them.

Back then, female apes would generally die once they were past childbearing age: but naturally a few genetically different individuals might live on for longer. According to Hawkes and her colleagues, these older ape ladies in many cases would get hold of suitable infant food and pitch in to help feed their descendants. The nippers would thus be weaned earlier, allowing their mothers to have another child sooner, so spreading the grandma's unusual long-life genes faster and conferring an evolutionary advantage.

That, according to the theory, is why human females can live so long past their reproductive lifespan: because it helps to spread their genes.

Various computer simulations have sought to confirm or disprove this idea. One recent study cast doubt on the idea that mothers-in-law and such types can actually be a help to the human race rather than a hindrance, but now Hawkes and colleagues of hers have struck back. A statement highlighting the research explains:

The researchers were conservative, making the grandmother effect “weak” by assuming that a woman couldn’t be a grandmother until age 45 or after age 75, that she couldn’t care for a child until age 2, and that she could care only for one child and that it could be any child, not just her daughter’s child.

Based on earlier research, the simulation assumed that any newborn had a 5 percent chance of a gene mutation that could lead to either a shorter or a longer lifespan.

The simulation begins with only 1 percent of women living to grandmother age and able to care for grandchildren, but by the end of the 24,000 to 60,000 simulated years, the results are similar to those seen in human hunter-gatherer populations: about 43 percent of adult women are grandmothers.

The new study found that from adulthood, additional years of life doubled from 25 years to 49 years over the simulated 24,000 to 60,000 years.

The full paper has just been published in the journal Proceedings of the Royal Society B.

Quite how male lifespan is affected by one's mother and mother-in-law being around longer is unclear. It doesn't seem to have been considered very important by the study authors, and to be fair may only have become an issue in comparatively recent times. ®

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