Neanderthal genes tell us about how old our ancestors were when they had children

A new study suggests that generation intervals have fluctuated during the past 40,000 years of human evolution in contrast to what has been commonly assumed. The results indicate that human life history can change appreciably in response to external and cultural factors

The authors from Aarhus University in Denmark and the Max Planck Institute for Evolutionary Anthropology in Germany used Neanderthal fragments scattered in non-African genomes as molecular clocks to estimate generation intervals in Eurasian and American populations.

“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 research team report in Nature Communications on 7. September, that humans in populations in Europe reproduced on average at a younger age than populations from east Eurasia and America over the past 40,000 years.

“We estimate a difference of 3 to 5 years between the mean generation interval among populations. We believe that this difference was probably more dramatic. If the change happened during the last 10,000 years for example, we are probably diluting the signal over the 40,000 years period we study,” says PhD student Moisès Coll Macià, first author of the study.

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”.

So why did the lengths of generations differ historically around the world?

The authors speculate that this was probably a response 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.

“In the future, we will be able to use the wealth of ancient and modern human genome sequences appearing at a fast rate to make a fine map of changes to age of human reproduction, that we can relate to environmental and cultural conditions,” professor Schierup suggests.