The fossil record of the hylobatids is poorly known, being largely restricted to fossil and subfossil remains from the Pleistocene and Holocene of China and Southeast Asia.
Recently, a team of scientists discovered the earliest gibbon fossil in the Yuanmou area of Yunnan Province in southwestern China. This discovery could help fill a long-elusive evolutionary gap in the history of apes.
The new study mainly focused on centers on hylobatids, a family of apes that includes 20 species of living gibbons. Hylobatids fossil remains are scarce. Most specimens are isolated teeth and fragmentary jaw bones found in southern China and Southeast Asia cave sites dating back no more than 2 million years ago.
The fossil discovered- is of a small ape called Yuanmoupithecus xiaoyuan. Scientists confirmed this after analyzing the teeth and cranial specimens of Yuanmoupithecus, including an upper jaw. The ape was less than 2 years old when it died.
Yuanmoupithecus was close in size to modern-day gibbons, with a body weight of roughly 6 kilograms—or about 13 pounds—according to the size of the molar teeth.
Terry Harrison, a professor of anthropology at New York University and one of the paper’s authors, said, “The teeth and the lower face of Yuanmoupithecus are very similar to those of modern-day gibbons, but in a few features, the fossil species was more primitive and points to it being the ancestor of all the living species.”
Xueping Ji of the Kunming Institute of Zoology and the lead author of the study found the infant’s upper jaw during his field survey. The fossil was identified as a hylobatid after its comparison with modern gibbon skulls in the Kunming Institute of Zoology.
In 2018, he invited Harrison and other colleagues to work on the specimens stored in the Yunnan Institute of Cultural Relics and Archaeology and the Yuanmou Man Museum that had been collected over the past 30 years.
Harrison said, “The remains of Yuanmoupithecus are extremely rare, but with diligence, it has been possible to recover enough specimens to establish that the Yuanmou fossil ape is indeed a close relative of the living hylobatids.”
Scientists also demonstrated that the Kapi ramnagarensis, believed to be an earlier hylobatid species based on a single isolated fossil molar from India, is not a hylobatid after all. It is a member of a more primitive group of primates that are not closely related to modern-day apes.
Harrison said, “Genetic studies indicate that the hylobatids diverged from the lineage leading to the great apes and humans about 17 to 22 million years ago, so there is still a 10-million-year gap in the fossil record needs to be filled. With the continued exploration of promising fossil sites in China and elsewhere in Asia, it is hoped that additional discoveries will help fill these critical gaps in the evolutionary history of hylobatids.”