With a life expectancy of almost eighty years, man is one of the longest living mammals. From an evolutionary perspective, this is noteworthy: after they reach the age of fifty, people rarely conceive offspring. So why do we live so long? David van Bodegom, now a doctor attached to the Leyden Academy on Vitality and Ageing, researched this question in his thesis.
David van Bodegom studied the selection advantage of a long post-reproductive lifespan in a polygamous society in Ghana. As part of the research, he investigated the impact on the number of offspring and the survival of those offspring when men and women above fifty years of age were present in the household.
A woman above the age of fifty took care of 2.3% more offspring, but this had no impact on survival. Equally, men above fifty years of age had no impact on the survival probability. However, within their polygamous society, the men could conceive children up to a high age. No less than 18.4% of the children were conceived by men older than fifty years of age. This showed a clear selection advantage.
“The selection pressure of older men [...] can only have played a role in the evolution of our longevity if people lived in polygamous populations during their recent evolutionary past. An initial indication of this comes from anthropological studies, which show that the majority of the populations that we know were indeed polygamous”, said David van Bodegom. Evidence from DNA testing also endorses this finding.
However, this immediately raises another question: if men are responsible for the long post-reproductive lifespan, why do women in our society live longer?
“In the past, the differences in lifespan between men and women were different to now, as many more women died in childbirth than is the case nowadays. Very different mechanisms may underlie the current differences in lifespan; biological, which are related to pregnancy and childbirth, as well as more recent cultural and social differences.”