The length of a specific generation can help understand a lot about the biology and social organization of humans. Researchers Matthew Hahn and Richard Wang at Indiana University can determine the average age that women and men had children throughout human evolutionary history with a new method they developed using DNA mutations.
The researchers said this work can help us understand the environmental challenges experienced by our ancestors and may also help us in predicting the effects of future environmental change on human societies.
Through their research on modern humans, the researchers noticed they could predict the age at which people had children from the types of DNA mutations they left to their children. They then applied this model to their own human ancestors to determine what age their ancestors procreated.
According to the study, the average age that humans had children throughout the past 250,000 years is 26.9 years old. Fathers were consistently older, at 30.7 years on average, than mothers, at 23.2 years on average, but the age gap has shrunk in the past 5,000 years, with the study’s most recent estimates of maternal age averaging 26.4 years. The shrinking gap seems to largely be due to mothers having children at older ages.
Other than the recent uptick in maternal age at childbirth, the researchers found that parental age has not increased steadily from the past and may have dipped around 10,000 years ago because of population growth coinciding with the rise of civilization.
Wang says these mutations from the past accumulate with every generation and still exist in humans today. They can identify these mutations, see how they differ between male and female parents, and how they change as a function of parental age.
Children’s DNA inherited from their parents contains roughly 25 to 75 new mutations, which allows scientists to compare the parents and offspring, and then to classify the kind of mutation that occurred. When looking at mutations in thousands of children, the IU researchers noticed a pattern: The kinds of mutations that children get depend on the ages of the mother and the father.
Previous genetic approaches to determining historical generation times relied on the compounding effects of either recombination or mutation of modern human DNA sequence divergence from ancient samples. But the results were averaged across both males and females and across the past 40,000 to 45,000 years.
Hahn, Wang and their co-authors built a model that uses de novo mutations — a genetic alteration that is present for the first time in one family member as a result of a variant or mutation in a germ cell of one of the parents or that arises in the fertilized egg during early embryogenesis — to separately estimate the male and female generation times at many different points throughout the past 250,000 years.
The researchers were not originally seeking to understand the relationship of gender and age at conception over time; they were conducting a broader investigation about the number of mutations passed from parents to children. They only noticed the age-based mutation patterns while seeking to understand differences and similarities between these pattens in humans versus other mammals, such as cats, bears and macaques.
The story of human history is pieced together from a diverse set of sources: written records, archaeological findings, fossils, etc., Wang says. Genomes, the DNA found in every one of our cells, offer a kind of manuscript of human evolutionary history. The researchers say their findings confirm some things they knew from other sources (such as the recent rise in parental age), but also offer a richer understanding of the demography of ancient humans – helping contribute to a better understanding of the shared history.