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Scientists have discovered a new way to identify the average ages when men and women reproduced throughout human evolutionary history.
By studying DNA mutations in modern humans, they discovered a window that let them peek 250,000 years back in time.
“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,” says study co-author Matthew Hahn, a genomicist at Indiana University Bloomington.
“We then applied this model to our human ancestors to determine what age our ancestors procreated.”
They found that, over the past 250,000 years, the average age for humans to have children is 26.9 years. (For context, 300,000 years ago is also roughly when our species first appeared.)
The average Homo sapien father has always been older than the average Homo sapien mother, the study found, with men becoming parents at 30.7 years old, versus 23.2 years for women.
But the age gap has dwindled in the last 5,000 years, the researchers add, noting the study’s most recent estimates suggest the average age when women become parents is now 28 years. This trend seems driven largely by women having children at older ages, they suggest.
Aside from the recent rise in maternal age, however, the study found remarkable consistency in the average age of new parents throughout our species’ existence. It has not increased steadily since prehistory, the team reports, although it has fluctuated over time.
The average age at conception seems to have fallen about 10,000 years ago, and since that would roughly coincide with the advent of agriculture and the dawn of civilization, the researchers say it might be related to rapid population growth at the time.
Recorded history only goes back a few thousand years at best, and broad, population-level information like this is difficult to glean from archaeological evidence alone.
But secrets of our ancestors also lurk within each of us today, and that’s how Hahn and his colleagues stumbled upon a way to determine parental age so far back in time.
The new study seizes on the discovery about de novo mutations – DNA alterations that debut in one family member, appearing spontaneously rather than being inherited through the family tree.
While working on another project involving these novel genetic changes and parents of known ages, the researchers noticed an interesting pattern. Based on data from thousands of children, the pattern and numbers of novel mutations forming in parents before being passed on to their kids depend on each parent’s age at conception.
This let the researchers estimate separate male and female generation times across 250,000 years.
“These mutations from the past accumulate with every generation and exist in humans today,” says study co-author and Indiana University phylogeneticist 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.”
Previous research has also used genetic clues to estimate generation length over time, but it has typically relied on comparisons between modern DNA and ancient samples that were averaged across sexes and across the past 40,000 to 45,000 years, the researchers note.
“The story of human history is pieced together from a diverse set of sources: written records, archaeological findings, fossils, etc.,” Wang says.
“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, but also offer a richer understanding of the demography of ancient humans.”
The study was published in Science Advances.