Researchers from Aarhus University and the Max Planck Institute estimate in a new study that men and women in East Asia over the last 40,000 years on average have waited at least 3-5 years longer to have children than men and women in Europe and America have.

The researchers have done this by using the fragments of Neanderthal DNA that are scattered in the genomes of modern non-African humans as molecular clocks.

“This new way of using genomic data enabled us to retrieve information about our human life traits buried in the past, which complements what can be learned from archaeology about our history,” says Professor Mikkel Heide Schierup, leader of the project. 

The study has just been published in Nature Communications. Its first author is PhD student Moisès Coll Macià from BiRC. He points out that the difference in the age of the parents was probably greater than the 3-5 years – because if the change happened, for example, during the last 10,000 years, the signal is probably diluted over the 40,000-year period covered by the study.

The researchers estimate that the average age was around 32 years for Asian parents and 28 years for Europeans. That is a conservative estimate, however, as it can only be established after further research with more data.

How does the molecular clock work?

Facts: The researchers analyzed non-African genomes because there is no Neanderthal DNA in African genomes. This is because only the Homo sapiens who emigrated from Africa back in the day had children with Neanderthals. The study examines at the evolution over the last 40,000 years, because that is how long ago, the Neanderthals disappeared, and thus also how long ago the they mixed their genomes with modern humans.

The results obtained about generation intervals are reflected in the accumulation of genetic changes in different parts of the world.

“Older parents transmit different mutations than younger ones to their children. In this study, we find that populations estimated to have older parents from their Neanderthal legacy also have mutations suggesting older parenthood” says Coll Macià.  

These mutational differences also allowed the researchers to tease apart whether changes in generation interval is due to changes in the fathers’ age at reproduction, the mothers’ age at reproduction or both.

“For instance, we see that east Asian populations tended to have older fathers than mothers, while European populations had similar ages for both,” says Coll Marcià”. 

Why this difference?

The authors speculate that the differences in generation length historically are responses to changes in the environment.

Differences in climate, but also technological and cultural developments in human societies, might have made living conditions more or less favorable to reproduce and thus played an important role in deciding which was the best time to have descendants.  

“Furthermore, in a harsh environment, you also grow up more slowly and thus become sexually mature later. And in some cultures, men need to gain status before they are attractive enough to marry and have children. It makes good sense, that in ancient times you only had children at the age of 30,” Mikkel Heide Schierup explains.

He predicts that in the future, scientists will be able to use the wealth of ancient and modern human genome sequences to make a much finer map of changes in the length of generations, that can be related to environmental and cultural conditions.