Genetics reveals parental age at conception over the past 250,000 years

The sharp increase in parental age, particularly in the industrialized world, has been one of the most noticeable demographic changes of the last century, reflecting our greatly increased life expectancy. Still, the shift isn’t quite as unprecedented as people might have thought, new evidence suggests. During the Ice Age, parents, especially fathers, were much older than during most of the agricultural period.

Unless a mother died in childbirth, fossils do not reveal the age of the parents. Consequently, this appeared to be an aspect of prehistory that anthropologists could only guess at by extrapolating from surviving hunter-gatherer societies.

Professor Matthew Hahn of Indiana University has changed that, announcing in Science Advances that the age of our ancestral parents at the time of conception is encoded into our DNA. Each child has 25-75 new (de novo) mutations in their DNA, along with many that appeared earlier in their ancestry. Most are harmless, or at least only very mildly harmful, and provide an overview of where we came from. Hahn and co-authors noted what kinds of mutations change as the parents age.

“Through our research on modern humans, we noticed that we could predict the age at which people had children from the types of DNA mutations they passed on to their children,” Hahn said in a statement. “We then applied this model to our human ancestors to determine the age at which our ancestors procreated.”

“These mutations from the past accumulate with each generation and exist in humans today,” added first author Dr Richard Wang. The authors were even able to trace which mutations came from which parent thousands of generations back.

On average, Hahn, Wang and co-authors found, conception has occurred at 26.9 years during the species’ existence: 30.7 years for the father and 23.2 years for the mother. However, this figure hides a lot of variation, as the chart below shows.

Estimates of the ages of fathers (blue) and mothers (red) at conception for human ancestors going back 10,000 generations (top) and the average age difference between males and females (bottom)

Calculations of the ages of fathers (blue) and mothers (red) at conception for human ancestors from 10,000 generations back (top) and the average age difference between males and females (bottom). Image credit: Wang et al./Science Advances

The results confirm that fathers have been older on average than mothers at all times, but the size of the gap has changed quite significantly between eras. The latest shift is easy to explain: When people expected to die in their 40s or 50s from diseases that have now become rare, there was a strong incentive to start having children early. Longer life expectancy makes waiting less risky, effective contraception makes it less of a sacrifice, and increased educational opportunities for women make it economically beneficial.

What is not as obvious are the reasons for the shift to earlier parenthood that accompanied the spread of agriculture, nor the apparent increase in paternal age around 38,000 years ago, before the last glacial maximum. Interestingly, until relatively recently, generation times were more than six years shorter for Asian and European populations than for African counterparts. Larger selections can enable more detailed divisions by region.

Although mutations in DNA sequences have been used to estimate parental age before, the methods used only allowed averaging over thousands of generations and did not differentiate by parental gender. Using the distribution of 25 million de novo mutations identified by the 1000 Genomes Project to estimate when they arose, Hahn and Wang claim their method is far more precise.

The findings are a side quest to the team’s primary theme; gain a more precise understanding of the number of mutations parents pass on to children. It was only when the authors compared the numbers they got with those of other mammals that they noticed patterns based on age and wondered if these could be used. Their conclusions fit well enough with what we know from written records to give the authors confidence in their technique, while also revealing information we could not obtain otherwise.

The paper is open in Science Advances.

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