BLOOMINGTON, Ind. — Nowadays, impatient mothers and fathers waiting for grandchildren often remind their adult children that they had already settled down at their age. Interestingly, however, while it may seem like people are waiting longer to have kids, human history shows things actually haven’t changed much at all. Research out of Indiana University suggests humans have largely been conceiving and having kids around the same ages for the past 250,000 years.
Scientists from Indiana University report the average age that humans conceive children over the past 250,000 years is 26.9 years old. Importantly, fathers tend to be older (30.7 years old on average) in comparison to mothers (23.2 years old on average). That being said, though, the age gap between moms and dads has shrunk somewhat over the most recent 5,000 years. Researchers say the most recent estimates of maternal age average 26.4 years old.
Besides that somewhat recent trend among mothers, study authors note parental age has not increased steadily from the past. In fact, it appears parental age dipped roughly 10,000 years ago, likely due to rapid population growth sparked by the birth of civilization.
These findings were made possible via a new method developed at IU that uses DNA mutations. The research team posits these findings can help us better understand the environmental challenges experienced by our ancestors while simultaneously allowing us to potentially 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 had children from the types of DNA mutations they left to their children. We then applied this model to our human ancestors to determine what age our ancestors procreated,” says study co-author Matthew Hahn, Distinguished Professor of biology in the College of Arts and Sciences and of computer science in the Luddy School of Informatics, Computing and Engineering at IU Bloomington, in a media release.
“These mutations from the past accumulate with every generation and exist in humans today,” adds study co-author and IU post-doctoral researcher Richard Wang. “We can now identify these mutations, see how they differ between male and female parents, and how they change as a function of parental age.”
Closer look at DNA reveals history of human conception over time
The DNA children inherit from their parents contains about 25 to 75 new mutations, allowing scientists to compare parents and their kids, and then classify according to the type of mutation. While observing mutations across thousands of kids, study authors noticed a pattern: The mutations a child displayed depended on the ages of their mother and father.
Earlier genetic strategies for assessing historical generation times hinged on the compounding effects of either recombination or mutation of modern human DNA sequence divergence from ancient samples. Those results, however, were always averaged across both men and women, and covered only about 40,000 to 45,000 years into the past.
Study authors constructed a new model that makes use of de novo mutations. This is a genetic alteration that initially occurs in one family member as a result of a variant or mutation in a germ cell received by one of the parents, or that appears in the fertilized egg during early embryogenesis. The model allowed them to separately estimate both male and female generation times at various points in time over the past 250,000 years.
Originally, the research team had not set out to reach these findings. They were developing a larger investigation regarding the amount of mutations passed on from parents to their children. The age-based mutation patterns just happened to be noticed while they were attempting to understand differences and similarities between these pattens in humans as opposed to numerous other mammals, like 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 concludes. “Our genomes, the DNA found in every one of our cells, offer a kind of manuscript of human evolutionary history. The findings from our genetic analysis confirm some things we knew from other sources (such as the recent rise in parental age), but also offer a richer understanding of the demography of ancient humans. These findings contribute to a better understanding of our shared history.”
The study is published in Science Advances.