According to the latest figures, the average age of a woman who now gives birth in Belgium is 30.6 years. And the co-parent, i.e. the father or co-mother, is on average 34 years old. The age gap where the mother is the youngest parent seems to have always been there, or at least for the last 250,000 years. This is reported by a team of American researchers in the journal The progress of science.
The age at which people have children has fluctuated throughout history. The elders are mothers and fathers today. But also around 38,000 years ago, parents were on average almost 30 years old when they had a child. This “average” includes all children born during that time period, regardless of whether they were their parents’ first child or not.
After that peak 38,000 years ago, the average age of parents fell again. About 6,400 years ago, parents were the youngest on average. Then the mothers and fathers were on average 25 years old at the time of conception. This period of young parents roughly coincides, the researchers say, with the emergence of the first civilizations. Then the age increased again. This upward trend continues to this day.
It’s up to the father
But behind the parents’ average age, there are notable age differences between mothers and fathers. The fathers have always been older than the mothers. Throughout history, mothers’ ages have averaged around 23 years and fathers’ ages around 31 years, says evolutionary geneticist Maarten Larmuseau (KU Leuven), who was not involved in the study.
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“If the average age of parents rose or fell, for a long time it was mainly due to the fathers and not the mothers, according to this study. It was therefore the fathers who moved up through time and had children at a younger or later age. We suspect that cultural norms also play a role, such as how important it is for a man to have a certain status and independence before starting a family. So probably much more than just biological fertility played a role in the past.’
It is only in the very new generations that the age gap between men and women becomes smaller, because women get older when they have children and are therefore closer in age to men, for example because women want to have their careers on track before they have children.
But how can you find out the age of the fathers and mothers who fathered a child thousands (or even hundreds of thousands) of years ago, at a time when there were no birth records?
The researchers were very ingenious in their analysis. In short, scientists make clever use of errors or ‘mutations’ that happen to creep into a person’s genetic code during life. The older men and women are, the more mutations their sperm and egg cells contain. And it is precisely these mutations that they pass on to the children they father. The parents’ age therefore leaves traces in the child’s DNA. The number of mutations in a child’s DNA may even be directly related to the age of its parents.
The researchers trained a computer model with DNA from Icelandic families and data on the age at which male and female relatives had children. The computer model thus ‘learned’ to recognize mutation patterns in the children’s DNA and to link these patterns to the mothers’ and fathers’ ages.
But it is important that the DNA of a child born today does not only contain the mutations passed on to them by their own mother or father. All carry mutations that distant ancestors have passed on to the next generation of ancestors. Human DNA therefore also contains mutations that have lasted for thousands or even hundreds of thousands of years. With powerful computers fed millions of human genomes, it is possible to estimate how many generations ago an ancestor must have passed DNA to the next generation.
For this new study, the researchers ran their trained computer model on such ‘old’ mutations in human DNA. The computer model was able to work out what age the mother and father must have been when these errors crept into the human DNA.
“It’s spectacular that we now know what the average age was for mothers and fathers 250,000 years ago when they had children,” says Larmuseau. ‘Earlier estimates only went back 40,000 years. For evolutionary research, it is very important to know how much time there was between two generations before that. In our own research, for example, we can already find out how many generations ago something must have happened in human evolution. If we now also know exactly how many years a human generation has lasted, we can say how long ago it was.’
‘This new research is important to be able to make that conversion. Although of course it is also very interesting to know how old our distant ancestors were on average when they had children’.