In a commentary published recently in the Proceedings of the Royal Society, the University of Adelaide's Jack da Silva says grandmothers may have helped to lower the risk of cancer, reports New Scientist's Michael Slezak. A study from the US last year found that women with either of the BRCA genes — which are linked to breast cancer — tend to have more children than women without the genes. In fact, da Silva says, the increase in fertility is so high that after four generations, the BRCA mutations should be ubiquitous. And yet, they're not. Why? Well, says Slezak, da Silva thinks "it all comes down to grandma."
Most people who develop breast cancer and die because of BRCA mutations do so after menopause, after they've had children, Slezak says. It doesn't limit the number of children they have, but it may limit the number of grandchildren they have, since they can no longer help their daughters care for offspring. Under da Silva's theory, "when humans were mostly hunter-gatherers, grandmother care was so important for the survival of youngsters that it may have cancelled out the fertility advantage of those with BRCA1 or BRCA2," Slezak adds. Though the majority of humans haven't lived in a hunter-gatherer society for many generations, da Silva says that the effect may have been strong enough in our ancestors to carry through to today.