Gibbons are a kind of ape that have a small body and long arms. They move very quickly through the trees, which makes it difficult for scientists to interpret the evolutionary history of these species.
Now, a partial upper jaw and seven isolated teeth found near a village in the southwestern Chinese province of Yunnan have added bite to a suggestion that the earliest known gibbons hung out there approximately 7 million to 8 million years ago, as researchers report in the October issue of the Journal of Human Evolution.
According to paleoanthropologist Xueping Ji of the Kunming Natural History Museum of Zoology in China and colleagues, those fossils, along with 14 teeth previously found at the same site and a site nearby, belong to an ancient hylobatid species called Yuanmoupithecus xiaoyuan. This conclusion was reached by paleoanthropologist Xueping Ji and colleagues. Hylobatids are a family of apes that are found in tropical forests from northeastern India to Indonesia. Within this family, there are around 20 species of gibbons that are still alive, as well as a gibbon with black fur that is named the siamang.
Since first presenting the species in a Chinese paper in 2006, Ji’s group has operated on the assumption that Y. xiaoyuan was an old gibbon. But there was a lack of more fossils, thus that hypothesis could not be confirmed.
The recently discovered upper jaw piece comprises four teeth, including a partially erupted molar that enabled researchers identify it as the remains of a child who passed away before reaching the age of 2. The fragment was found by a local villager and brought to Ji during fieldwork roughly a decade ago.
The team says that comparisons with modern apes and fossils of ancient primates establish Y. xiaoyuan as the oldest known gibbon and cast doubt on a report that was published two years ago claiming that a molar tooth that was discovered in northern India and was approximately 13 million years old came from a hylobatid. According to the experts, the fossil that was discovered in India and belonged to a species that has been given the name Kapi ragnagarensis represents an ancient group of South Asian monkeys that were not closely linked to apes as we know them today.
Previous DNA research conducted on live monkeys revealed that the divergence between hylobatids and other apes occurred in Africa between 22 and 17 million years ago. Terry Harrison, a paleoanthropologist at New York University and a collaborator of the research, notes that it is unknown when the ancestors of gibbons first arrived in Eurasia. There is a gap in the fossil record of about 10 million years between the estimated period when hylobatids developed in or near Africa and evidence of Y. xiaoyuan in Asia. This gap is due to the fact that hylobatids first appeared in or near Africa.
There is evidence, both from fossils and genetic testing, that all living gibbon species descended from the same progenitor around 8 million years ago, when Y. xiaoyuan was still active. Harrison thinks that it is possible that Y. xiaoyuan is the ancestor of all gibbons that came after it and that this is the case. If this is not the case, he believes that Y. xiaoyuan was closely connected to an ancient gibbon progenitor.
According to Ji’s research team, the bumps and depressions on the chewing surfaces of Y. xiaoyuan as well as other tooth and jaw traits seem quite similar to those of live gibbons. The researchers believe that some characteristics of the extinct species were the forerunners of slightly different characteristics seen in present gibbons.
They estimate that Y. xiaoyuan weighed around six kilos, which is comparable to the weight of gibbons living now based on the sizes of its teeth. According to Harrison, the molar structure of Y. xiaoyuan shows that it mostly consumed fruits, much as the majority of gibbon species living today.
According to paleoanthropologist David Alba of the Institut Català de Paleontologia Miquel Crusafont in Barcelona, the research conducted by Ji’s group “makes a very solid argument that Y. xiaoyuan is a hylobatid.”
However, the evolutionary status of the K. ragnagarensis species is still unknown due to the fact that just one tooth belonging to that species has ever been discovered, according to Alba, who was not involved in the current research.