The concept of generation is diverse and the duration of a generation is also given in science with very different information. Evolutionary geneticists now focus on certain mutations, develop a model from this and determine different numbers for women and men.
In the history of mankind, men were on average a good seven years older than women when their first child was born, 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. US researchers report this in the journal “Science Advances” on the basis of genome analyses. 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.
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 migratory movements and mixing of different populations such as Homo sapiens with groups that have since died out, such as Neanderthals, which 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 one of the reasons why the determination of the interval is quite complex. In an earlier study, a group led by David Reich from Harvard University analyzed old genome 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.
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 realized that we could infer the age at which people had children from the nature of DNA mutations,” Hahn explains. “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 first child 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 at the first birth has increased. The variance over time was therefore much greater for men than for women, at times the age at first fatherhood was well over 30 years.
The mean generation interval was at its greatest about 1400 generations ago – this corresponds to about 38,000 years – i.e. 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 yields a longer generation length for males than for females across all time intervals examined,’ the team writes. 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 sound. 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. Whether the frequency of the examined mutations actually depends mainly on the age of the parents has not been clarified.
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 Twitter. In a preprint that has not yet been reviewed, they question the basic assumptions of the Indiana team.
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.
