Why women evolved to live so far beyond their reproductive years is a mystery long debated. Now there is new evidence backing the “grandma hypothesis” – that they stick around to invest in their grandchildren, safeguarding the genes they share.
Leslie Knapp and her team at the University of Cambridge reasoned that if the hypothesis is true, how much grandma invests in her grandchildren should depend on the proportion of genes they share. So they came up with a way to test this.
Due to the way X chromosomes are transmitted, grandmothers share an equal amount of DNA with their daughter’s sons and daughters, but a smaller proportion of DNA with their son’s son than with their son’s daughter. So the team reasoned that paternal grandmas might invest less in these grandsons than these granddaughters, which in turn might have a measurable effect on life expectancy.
Knapp’s team examined historical records from seven countries, including England, Japan and Ethiopia, which ranged from the 17th century to today. “We wanted to test whether the effect was independent of culture,” says Knapp.
For kids who grew up in the same village or lived in the same home as their grandma, they noted if she was paternal or maternal and when the children died.
Sure enough, the researchers found that in all seven countries, males died earlier if they had grown up with their paternal, rather than their maternal, grandmother. This was not true for the girls.
Grandmas might differ in levels of investment in their grandchildren through food provision or teaching, says Knapp.
The genes on the X chromosome only make up about 8 per cent of our genes – and so aren’t the only ones that grandmas have an interest in protecting. However, Knapp points out that “the X carries genes known to be involved in fertility and intelligence, which are important for future reproductive success”.
Before all you grandsons out there start firing dirty looks at your dad’s mother, the findings don’t mean that granny thinks any less of you. There’s no evidence that paternal grandmas have a conscious preference for their sons’ daughters, says Knapp.
Cheryl Jamison, a sociologist at Indiana University in Bloomington, calls the research “fascinating”. She also emphasises that the genes on the X chromosome are just some of the many factors that drive grandmothers’ behaviour towards their grandchildren. Culture and environment, as well as genes found on other chromosomes, must also play a role.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2009.1660