The sharp increase in parental age, particularly in the industrialized world, has been one of the most striking demographic changes of the past century and reflects the drastic increase in people’s life expectancy. Still, this shift may not be as unique as people think, according to new evidence.
It seems that during the Ice Age, the parents (especially the fathers) were much older than most of the farming age.
Fossils do not reveal parental age unless a mother died in childbirth. The subject therefore seemed to be something that anthropologists could only speculate about prehistory.
But that changed when Professor Matthew Hahn of Indiana University announced in Science Advances that the age of our ancestors’ parents at conception is encoded in our DNA. Every child has between 25 and 75 new (de novo) mutations in their DNA, and many have already appeared in their ancestors. Most are harmless, or at least do little harm. But it does provide a record of where we came from. Hahn and co-authors note that the types of mutations change as parents age.
“Through our research on modern humans, we realized that we could predict the age at which children were born from the types of DNA mutations that humans left to their children,” Hahn said in a statement. “We then applied this model to our human ancestors to determine at what age our ancestors reproduced.”
Lead author Dr. “These mutations from the past accumulate with each generation and exist in humans today,” says Richard Wang. The authors say they’ve also managed to track which mutations came from which parent thousands of generations ago.
Hahn, Wang and co-authors discovered that our species conceived at an average age of 26.9 years during its existence. The average is 30.7 years for the father and 23.2 years for the mother. Of course, the table they published shows a much wider distribution and variation.
The results confirm that, on average, fathers are almost always older than mothers, although the size of the difference varies quite significantly between periods. It’s relatively easy to explain the recent change.
But when people are expected to die from diseases that have become rare in their 40s or 50s, there is a strong incentive to have children early. Increasing life expectancy makes waiting to have a child less risky, effective contraception makes waiting easier, and increased educational opportunities for women also make waiting financially advantageous.
But the reasons for the earlier transition to parenthood that accompanied the spread of agriculture, or the spike in paternal ages before the Last Glacial Maximum, about 38,000 years ago, are not so clear. Surprisingly, until relatively recently, generation times for Asian and European populations were shorter than six years compared to their African counterparts. Larger specimens may provide more detailed breakdowns by region.
Although mutations in DNA sequences have previously been used to estimate parental ages, the methods used only allowed averaging over thousands of generations and did not discriminate by parental sex. Hahn and Wang claim that their method is much more precise, using the distribution of the 25 million de novo mutations identified by the 1,000 Genomes Project to estimate when they arise.
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