When young baboons play in the trees in Kenya’s Amboseli National Park, you can’t tell from looking at them that the lineage of their ancestors split around 1.4 million years ago. Since then, Anubis baboons in the savannahs between Mali in the west and Ethiopia down to northern Tanzania in East Africa and the steppe baboons further south on the grasslands between Kenya and Zambia and Angola have gone their separate ways in evolution – but not always.
When representatives of the two species meet in Kenya, for example, they repeatedly produce offspring with each other despite this long separation. If these later mate with animals of one of the two parent species, healthy monkeys are born again. In the offspring, however, most of the genome of the other parent species disappears over time, following the same pattern that most human DNA traces have disappeared from Neanderthals. This is what Jenny Tung from Duke University in Durham, USA, and her team report in the journal Science.
“This result is also very interesting for our research,” explains evolutionary geneticist Kay Prüfer, who studies the genome of Neanderthals and other human lineages at the Max Planck Institute for Evolutionary Anthropology (EVA) in Leipzig. Neanderthals also sometimes shared children with modern humans before they became extinct. But these hybridizations took place tens of thousands of years ago, while Tung and team are able to observe the female offspring of Anubis and Steppe baboons in Amboseli National Park, holding their young in their arms.
The animals have also been scientifically observed nearby since 1971. So Tung can draw on more than half a century of field research. “She and her team were therefore also able to observe what happens to the genome of both species over time,” explains Prüfer, who has to rely on individual finds himself. Since the genetic material in these bones is also slowly broken down over the years and millennia, only a few finds remain.
Tung, on the other hand, was able to examine the genomes of 443 baboons for the Science study that were born in the Amboseli region between 1969 and 2016. In this region mainly steppe baboons live, which only occasionally encounter Anubis baboons.
However, these meetings have left clear traces in the genome: In the animals examined, the group found an average of just over 30 percent Anubis baboon DNA that came from two different waves of encounters between the two species. The Anubis portion is from a pre-1971 encounter wave and a second that began in 1982 and could be observed directly.
The hybrids born after mating of both species had no disadvantages compared to steppe or Anubis baboons and they also reproduced as well as their parents. This observation was already known from other monkey species.
A similar relationship apparently once existed between modern humans, Neanderthals, and another lineage, the Denisovans. Then why don’t the baboon species or the human lineages permanently mix and merge into one species?
Tung and her team found that sections of the baboons’ genome that differed between the two species disappeared. The hereditary characteristics of the steppe baboons, which make up the majority in the Amboseli National Park, mostly remained.
This can possibly be explained by preferences in the choice of partner: If steppe baboons with an Anubis portion prefer to mate with steppe baboons, the Anubis portion in the genome will shrink over time. “In the baboons, these mechanisms result in patterns similar to those in the human genome, where only part of the Neanderthal genome has been preserved,” says EVA researcher Prüfer, who was not involved in Jenny Tung’s Science study.
However, nobody currently knows exactly what these mechanisms look like. Perhaps Tung will find out at the EVA in Leipzig, where she is currently setting up a research group as the newly appointed director.
Her studies on the still-existing apes, relatively close relatives of humans, could yield further surprises, such as genes that come from the rarer parent and still survive because they offer an advantage. For example, from the extinct Denisovans, a sister group of Neanderthals in east and south-east Asia discovered at the EVA, a very special heritage has survived after hybridizations with Neanderthals and modern humans in Tibet: it enables today’s inhabitants of the highlands, even with the strong to remain efficient in the reduced oxygen supply.
