Grandmothers 'helped humans evolve'

GRANDMOTHERS might have played a key role in human evolution, helping us develop bigger brains and longer lives, researchers say.

The "grandmother hypothesis" says there are evolutionary benefits from having older, non-reproductive females caring for offspring. US and Australian scientists tested the theory by running computer simulations of many generations of chimpanzees.

In the real world, female chimps rarely live past their child-bearing years, usually dying in their 30s or 40s.

But when grandmothering was added to the virtual world of the computer simulation, it made a huge difference to evolving chimp lifespan. After 24,000 to 60,000 years of grandmothers caring for grandchildren, chimpanzees who reached adulthood lived another 49 years - as do human hunter gatherers.

Before the addition of grandmothers, the simulated animals lived only another 25 years after reaching adulthood, just like chimps do in the wild.

According to the hypothesis, when grandmothers help feed their grandchildren after weaning, their daughters can produce more children at shorter intervals.

The children become younger at weaning but older when they start feeding themselves and reach adulthood.

Because of the benefits of grandmothers helping with child rearing, females end up living long after they lose the ability to reproduce.

Natural selection favours longevity genes which are passed to offspring, thereby increasing both female and male lifespan.

In their computer model, the scientists deliberately made the grandmother effect "weak" by assuming that a female could not be a grandmother until at least the age of 45, that she could not care for a child until age two, and that she could care for only one child.

They found that even a weak grandmother effect bestowed human-scale longevity on chimpanzees.

The researchers, whose findings are reported in the journal Proceedings of the Royal Society B, believe the implications go beyond lifespan and underlie many key changes in human evolution including larger brain size.

Lead scientist Professor Kristen Hawkes, from the University of Utah, said: "Grandmothering was the initial step toward making us who we are."

She added: "If you are a chimpanzee, gorilla or orang-utan baby, your mum is thinking about nothing but you. But if you are a human baby, your mum has other kids she is worrying about, and that means now there is selection on you - which was not on any other apes - to much more actively engage her: 'Mum! Pay attention to me!'

"Grandmothering gave us the kind of upbringing that made us more dependent on each other socially and prone to engage each other's attention."

The grandmother effect gave rise to "a whole array of social capacities" that laid the foundations for the evolution of human traits such as pair bonding, bigger brains, learning new skills, and co-operative living, she said.

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