Neanderthal DNA in the genome of non-Africans shows how old their ancestors were on average when they had children. These ages are surprisingly similar to current averages, although they differ significantly across continents.
55,000 years ago, our own species, Homo sapiens, met our distant Neanderthal relatives somewhere in southern Europe or the Middle East.
The encounter was sexual and resulted in children, and that means that there is between 1% and 2% Neanderthal DNA in the genomes of all people of non-African origin.
Researchers have now used this DNA to determine the average age at which the ancestors of non-Africans had children over the past 40,000 years.
The results surprisingly show that the average generational interval – the average age at which people had children – was longer in Asia than in Europe but also that people were relatively old when they had children.
This discovery provides new insight into the lives of early humans.
“By knowing about changes in the generational interval, anthropologists can use this in inferring how life and culture were tens of thousands of years ago. A shorter generational interval tells one story, and a longer generational interval tells a different story. In addition, differences in the cultural contexts may possibly explain why people in one place had children early whereas people in another place had children later,” explains a researcher behind the study, Mikkel Heide Schierup, Professor, Bioinformatics Research Centre, Aarhus University.
The research has been published in Nature Communications. Moisès Coll Macià, Postdoctoral Fellow at the Bioinformatics Research Centre, is the first author.
Studied the length of Neanderthal DNA fragments
The researchers started with the DNA in non-African genomes originating in part from the Neanderthals.
The children resulting from the sexual encounters of non-African ancestors with the Neanderthals 55,000 years ago carried large quantities of Neanderthal sequences in their genome.
Over time and because of recombination, the Neanderthal DNA in non-African genomes is cleaved into smaller pieces, with clear predictability through the generations.
By measuring these fragment lengths and comparing these with the speed of the recombination clock, researchers can determine relatively accurately how many generations have passed since the Neanderthals and the non-African ancestors mated.
Then dividing the number of years by the number of generations provides the average age at which non-African ancestors had children.
People had children later in Asia than in Europe
The average generation interval of the populations in Europe is 28 years versus 32 years in Asia.
Mikkel Heide Schierup says that he is not surprised that the average generational interval was so high. We might otherwise base our assessment on the situation in the modern era.
“150 years ago in Europe, people had good access to food and had children early. In the 19th century, we had record low generational intervals. 500 or 50,000 years ago, the situation was very different, because people on average had children later. This resembles current norms, in which people also have children around the age of 30 years,” he says.
The great apes, which resemble humans in many respects, also have long generational intervals.
Chimpanzees average 25 years old when they give birth, even though, like humans, they can reproduce at earlier ages.
However, Mikkel Heide Schierup was surprised that the averages differed so much: 28 years in Europe and 32 in Asia.
“We had expected a measurable difference but not that big,” he explains.
Larger studies will map people globally
The researchers studied 250 archaic sequences, and they now want to examine many more to improve the resolution in their data.
They will map changes in generational intervals by analysing up to 15,000 full genome sequences from around the world, thus assisting archaeologists and anthropologists in discovering much more about non-African ancestors.
Mikkel Heide Schierup says that he imagines more time would pass between each child for families living in very harsh environments, because the families would have more difficulty in caring for two infants at a time, and this would increase the generational interval.
Conversely, a group of people living in the same secure location with good access to food would have shorter generational intervals.
“In harsh environments, men might have needed social standing before women were interested in having children with them. Men being required to demonstrate their hunting skills might have extended the generational interval,” says Mikkel Heide Schierup.
Men and women differed greatly
In their upcoming study, the researchers will also examine differences between fathers and mothers.
Anthropologist colleagues of Mikkel Heide Schierup have discovered a historical and prehistorical tradition among Australian Aborigines that women had children at a much younger age than the fathers, perhaps up to 15 years.
The Neanderthal DNA may confirm this once the researchers analyse it.
The researchers have already found that the reason the generational interval is longer in Asia is probably that the fathers were older when they had children. Conversely, mothers being older might explain the longer generational interval in the Americas.
“Being able to determine these things is very exciting. We can start outlining the history of non-African ancestors and determine how the agricultural revolution affected the generational interval. Being able to settle down and no longer needing to travel from place to place to search for food changed the way these ancestors lived. This probably affected the generational interval and therefore also the fragment lengths of DNA from the Neanderthals. I am very excited about this type of study because we obtain information that we otherwise could not get,” concludes Mikkel Heide Schierup.