Gibbons are the sister group of the great apes and therefore close relatives of humans. But when and how they got to Asia was still unclear. Now researchers in India have discovered a fossil record of the oldest known ancestor of the gibbons and Siamangs. It is a 13 million year old molar that belonged to a monkey similar in many respects to today’s gibbons. The find closes a large gap in the fossil family tree of the so-called small great apes and proves that their ancestors came from Africa to Asia at least five million years earlier than has been shown to date.
Gibbons, including the genus of the Siamangs, form the family of the Hylobatidae or small great apes. Today they occur exclusively in Southeast Asia and are all severely threatened. Typical of these tree-dwelling primates are their disproportionately long arms, which they use to swing in the treetops. Genetic and molecular analyzes suggest that the gibbons separated from their sister group, the great apes, around 20 million years ago. But where this happened and when these monkeys got from Africa to Asia is still unknown. “The origins of the hylobatids are still shrouded in mystery,” say Christopher Gilbert of the City University of New York and his colleagues. “Although they belong to one of the most species-rich groups of monkeys and were once distributed across East and Southeast Asia, their fossil record is woefully incomplete.” The oldest fossil evidence of an ancient hylobatid, the Yuanmoupithecus, is around seven to eight million years old teeth that were found in the Chinese province of Yunnan.
Tooth from a prehistoric gibbon
Gilbert and his team have now discovered another fossil gibbon tooth in Ramnagar in northern India. As the researchers climbed a small hill on which a fossil primate jawbone had already been found the year before, Gilbert took a break and noticed something small, shiny in a pile of earth. Upon closer inspection, the find turned out to be a tooth. “We knew immediately it had to be a primate tooth, but it was different from any other primate previously found in the area,” says Gilbert. “Based on the shape and size, we suspected that it might have come from a gibbon ancestor, but that seemed almost too good to be true.” Gibbon fossils had never been discovered in the Ramnagar area before. To identify the tooth more precisely, the paleontologists examined it with the help of microtomography and compared its characteristics with those of today’s living and extinct primate species. Dating revealed that the fossil was around 13 million years old.
“What we found was really overwhelming. It undoubtedly indicates a close relationship between this 13 million year old tooth and the gibbons, ”says co-author Alejandra Ortiz of Arizona State University in Tempe. According to the comparative analyzes, the tooth is even more similar to those of gibbons living today than that of Yuanmoupithecus, although it is a good five million years older. “Even if we only have this one tooth from him so far and we have to be careful, this is a unique discovery,” says Ortiz. “Because this find moves the oldest known evidence of gibbons by at least five million years into the past and thus provides us with a much-needed insight into the early stages of their evolutionary history.”
Migration from Africa at the same time as the orangutan ancestors?
The prehistoric gibbon, baptized Kapi ramnagarensis, is the oldest known ancestor of the hylobatids living today – and the earliest evidence of the presence of this primate group in Asia. The find shows for the first time that the ancestors of the gibbons and the orangutans, which belong to the great apes, were apparently present in Asia around the same time. This also gives new insights into when these primates came to Asia from Africa. “While much of the evolution of the hylobatids remains unknown, it is now likely that they came to Asia from Africa in the late Miocene – probably at the same time as some great apes,” say Gilbert and his colleagues. Thanks to the fossil tooth of Kabi ramnagarensis, it is now clear that this migration must have taken place 13 million years ago.
Source: Christopher Gilbert (City University New York) et al., Proceedings of the Royal Society B: Biological Sciences, doi: 10.1098 / rspb.2020.1655