HUMANS may owe their longevity to grandmothers, a new study argues.
This stems from a weakness that distinguishes humans from other primates: the
helplessness of our offspring, says Kristen Hawkes of the University of Utah in
Salt Lake City. 鈥淎 chimp child gets its own lunch,鈥� she says. 鈥淏ut even years
after weaning their kids, human mothers use up a lot of resources by sharing
蹿辞辞诲.鈥�
Hawkes鈥檚 team studied modern tribes, like the Hadza of Tanzania, that subsist
on wild foods. Earlier, they had found that postmenopausal women still gather
large amounts of food. Since these women no longer care for their own children,
the surplus often goes to grandchildren. This would have helped humans to
compete against species with self-sufficient young.
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Now the researchers claim this could explain why humans live so long. Other
primates live for up to about 50 years, just beyond their females鈥� menopause.
But if food contributed by a postmenopausal female promotes the survival of her
grandchildren鈥攁nd the transmission of her genes鈥攖here should be
selection for long life. Sure enough, humans can live for a century, and for a
woman, nearly 50 of these years are after the menopause.
The researchers also suggest that grandmothers drove evolution to create late
maturity and a small size at weaning (Proceedings of the National Academy of
Sciences, vol 95, p 1336). 鈥淭his challenges the idea that the nuclear family is
key to human evolution,鈥� says Sarah Hardy, a primatologist at the University of
California, Davis.