It would not be a surprise to anyone that men in the UK become fathers, on average, roughly three years later than women. Indeed, anthropologists have observed such discrepancies in parental age, or “generation times”, in over 99 per cent of contemporary cultures. But has this always been the case?
A study published in Science Advances in January provides the first evidence that these sex differences have been a feature across all 250,000 years of human history. Improving the precision of these sorts of estimates helps evolutionary geneticists to better date major population events, such as mass migrations or interbreeding events between early humans and extinct hominins such as the Neanderthals.
The researchers at Indiana University first studied a large genetic dataset of Icelandic families, which had data on parental age. This allowed them to model how parental age and sex relates to the types of new DNA mutations that arise between generations. The model was then applied to a dataset of genomes from across the world, allowing the researchers to estimate both when in human history a mutation was likely to have arisen, and also the likely age and sex of the parents who preceded it.
The results found that the average generation time for humans across history was 26.9 years, with more than seven years difference between the typical age at conception for men (30.7 years) compared to women (23.2 years).
Whilst reaching puberty at basically the same age as women, men can typically reproduce for an additional 20 years, which likely explains some of the difference. However, the researchers suggest that social factors could also play a role, such as expectations to accrue assets or status before starting a family.
Some have raised valid concerns that the model does not sufficiently account for factors other than parental age affecting the pattern of mutations, such as the impact of environmental exposures. However, the work takes us one step closer to understanding the biology, demography and social structures of our ancient ancestors.
This piece is from the New Humanist summer 2023 edition. Subscribe here.
