A study revealing the average age at conception did not

age at conception

image: Graphs showing the average age at conception for men versus women over the past 250,000 years.
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Credit: Han, Wang, et al. Indiana University

Bloomington, Indiana – A given generation length can tell us a lot about the biology and social organization of humans. Now, researchers at Indiana University can determine the average age of women and men who have given birth to children throughout human evolutionary history with a new method they developed using DNA mutations.

The researchers said that this work can help us understand the ecological challenges our ancestors faced and may also help us predict the effects of future environmental change on human societies.

“Through our research on modern humans, we noticed that we could predict the age at which people would have children from the types of DNA mutations they left to their children,” said a co-author of the study. Matthew Hahn, Distinguished Professor of Biology in the College of Arts and Sciences and Computer Science in the Luddy School of Informatics, Computing, and Engineering at IU Bloomington. “We then applied this model to our human ancestors to determine the age at which our ancestors gave birth.”

According to the study, Posted today in Science advances And co-authored by Richard Wang, a postdoctoral researcher at IU, the average age at which humans have had children for the past 250,000 years is 26.9. Furthermore, fathers have been consistently older, by an average of 30.7 years, than mothers, by an average of 23.2 years, but the age gap has narrowed in the past 5,000 years, with the study’s most recent estimates of maternal age at an average of 26.4 years. The diminishing gap appears to be largely due to mothers having children at an older age.

Other than the recent increase in maternal age at birth, the researchers found that parental age has not increased steadily over the past and may have declined about 10,000 years ago due to population growth coinciding with the rise of civilization.

“These mutations from the past accumulate with each generation and are found in humans today,” Wang said. “We can now identify these mutations, see how they differ between parents, and how they change according to the age of the parents.”

Children’s DNA inherited from their parents contains approximately 25 to 75 new mutations, which allows scientists to compare parents and offspring, and then classify what type of mutation occurred. Looking at the mutations in thousands of children, the IU researchers noticed a pattern: The types of mutations children get depend on the age of the mother and father.

Previous genetic approaches to determining historical generation times relied on the compounding effects of recombination or mutation in the divergence of modern human DNA sequences from ancient samples. But the results have been averaged across both males and females and over the past 40,000 to 45,000 years.

Han, Wang, and their colleagues built a model that uses de novo mutations—a genetic change that is first present in a family member as a result of a change or mutation in the germ cell of one or more parents. It originates in a fertilized egg during early embryonic development—estimating the generation times of males and females separately at many different points throughout the past 250,000 years.

The researchers were not originally seeking to understand the relationship between sex and age at conception over time; They were conducting a broader investigation of how many mutations are passed from parents to children. They only noticed age-based patterns of mutations as they sought to understand the differences and similarities between these inverses in humans versus other mammals, such as cats, bears, and macaques.

“The story of human history has been pieced together from a variety of sources: written records, archaeological finds, excavations, etc.,” Wang said. “Our genomes, the DNA found in each of our cells, provides a kind of codex for the history of human evolution. Our findings from our genetic analysis confirm some of the things we knew from other sources (such as the recent rise in parental age), but also offer a richer understanding of of ancient human demographics. These findings contribute to a better understanding of our shared history.”

Other contributors to this research are Samer El-Saffar, a graduate student at IU at the time of the study, and Jeffrey Rogers of Baylor College of Medicine.

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