In the history of mankind, men were on average a good seven years older than women when they gave birth, according to a study. During the past 250,000 years, the average age of the fathers at this point in time was 30.7 years and that of the mothers 23.2 years.
The US researchers report in the journal “Science Advances” based on genome analysis. Accordingly, the average duration of a generation was almost 27 years – albeit with deviations over the millennia. However, independent experts view the results with skepticism.
molecular clock
The length of generations over the past millennia is of interest for many scientific questions: It can depend on the form of society, for example – i.e. it can vary between hunters and gatherers, farmers and cattle breeders. In addition, a dating of those developments that are reflected in the genome can be derived from this. These include migration movements and mixing of different populations such as des homo sapiens with now extinct groups such as Neanderthals, who are counted in generations and can thus be limited in time.
However, the generation time depends on many factors. These include environmental conditions, demographics and culture. This is another reason why the determination of the interval is quite complex. In ar previous study A group led by David Reich from Harvard University analyzed old genetic material sequences from around 45,000 years ago and then tracked the further development of these gene segments. Based on average mutation rates – the so-called molecular clock – they came up with a general generation interval for this period of 26 to 30 years, averaged for men and women, in the specialist journal PNAS in 2016.
In the current study, the team led by evolutionary geneticists Richard Wang and Matthew Hahn from Indiana University in Bloomington wanted to significantly refine this calculation. To do this, the researchers focused on so-called de novo mutations, which occur for the first time in a generation, especially during cell division during embryonic development.
older fathers
The researchers assume that the relative frequency of certain such mutations depends primarily on the sex and age of the parents. A research group had previously determined this ratio in a study of more than 1,500 Icelanders and their parents. Wang, Hahn and colleagues compared those results with other genetic data from thousands of people from all over the world and developed a model from them.
“Through our research on modern humans, we noticed that we could infer the age at which people had children from the type of DNA mutations,” explains Hahn. “We transferred this model to our human ancestors in order to determine the age of conception of these ancestors.”
According to this, the age difference between fathers and mothers at the birth of their offspring was 7.5 years on average over the past 250,000 years. This gap has repeatedly changed and narrowed over the past 5000 years – also because the age of the mothers has increased. The variance over time was therefore much greater for men than for women, and at times the average age at paternity was well over 30 years.
Before the Ice Age
The mean generation interval was greatest about 1400 generations ago – that corresponds to about 38 000 years – before the beginning of the last ice age. At that time it was almost 30 years. According to the study, it was at its lowest at just under 25 years ago, around 250 generations ago: According to the researchers, this time around 6,400 years ago roughly coincides with the rise of more complex societies. Since then, the interval has increased significantly again.
“Our model shows a longer generation length for males than for females across all time intervals examined,” writes the team. This applies not only to the course of history, but also today to all societies worldwide. One reason for this could be that men are much longer fertile than women, so they can take more time.
Harald Ringbauer from the Max Planck Institute for Evolutionary Anthropology in Leipzig considers the study to be scientifically clean. The team from Indiana enjoys an excellent reputation in professional circles. “The crux of the study, however, lies in the basic assumption,” emphasizes the archaeogeneticist. It has not been clarified whether the frequency of the mutations examined actually depends mainly on the age of the parents.
The renowned population geneticists Molly Przeworski from Columbia University in New York and Priya Moorjani from the University of California in Berkeley commented on the discussion about the study by Wang and Hahn, which was also conducted on the short message service Twitter. In a not yet reviewed preprint they challenge the Indiana team’s core assumptions.
In it, they write that the range of mutations studied depends not only on the age of the parents, but also significantly on other factors – including diet, environmental conditions such as UV radiation and possibly even genetic developments that have changed the frequency of such mutations over time.
In other words, the basic assumption of the group around Wang and Hahn is too simple to be able to derive such far-reaching conclusions. “Przeworski and Moorjani’s paper paints a more complex picture,” says Ringbauer. The validity of the results of Wang and Hahn’s team may become apparent in the coming years, as the number of available datasets is constantly increasing.
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