Fossil finds put gibbons in Asia as early as 8 million years ago

Small-bodied, long-armed apes called gibbons swing rapidly through the trees, far outpacing scientists’ attempts to decipher these creatures’ evolutionary story.

Now, a partial upper jaw and seven isolated teeth found near a southwestern Chinese village have added bite to a suggestion that the earliest known gibbons hung out there about 7 million to 8 million years ago, researchers report in the October Journal of Human Evolution.

Those fossils, as well as 14 teeth previously found at the same site and a nearby site, belong to an ancient hylobatid species called Yuanmoupithecus xiaoyuan, say paleoanthropologist Xueping Ji of the Kunming Natural History Museum of Zoology in China and colleagues. Hylobatids, a family of apes that includes about 20 species of living gibbons and a black-furred gibbon called the siamang, inhabit tropical forests from northeastern India to Indonesia.

Ji’s group has presumed that Y. xiaoyuan was an ancient gibbon since introducing the species in a 2006 Chinese publication. But additional fossils were needed to check that suspicion.

The newly discovered upper jaw piece — found by a local villager and given to Ji during fieldwork around a decade ago — contains four teeth, including a partly erupted molar that helped researchers identify it as the remains of an infant that died before reaching age 2.

Comparisons with modern apes and fossils of ancient primates peg Y. xiaoyuan as the oldest known gibbon and cast doubt on a two-year-old report that a roughly 13-million-year-old molar tooth found in northern India came from a hylobatid, the team says (SN: 9/8/20). The fossil found in India, assigned to a species dubbed Kapi ragnagarensis, represents an extinct group of South Asian primates that were not closely related to present-day apes, the scientists say.

Prior DNA analyses of living primates suggested that hylobatids diverged from other apes in Africa between 22 million and 17 million years ago. But it’s a mystery when gibbon ancestors arrived in Eurasia, says paleoanthropologist and study coauthor Terry Harrison of New York University. A gap in the fossil record of about 10 million years exists between the estimated time when hylobatids emerged in or near Africa and evidence of Y. xiaoyuan in Asia.

Genetic evidence also indicates that gibbon species today shared a common ancestor around 8 million years ago, when Y. xiaoyuan was alive. “It could be that [Y. xiaoyuan] is the ancestor of all later gibbons,” Harrison says. If not, Y. xiaoyuan was closely related to a modern gibbon ancestor, he suspects.

Bumps and depressions on chewing surfaces and other tooth and jaw features of Y. xiaoyuan look much like those of living gibbons, Ji’s team says. Some traits of the fossil species were precursors of slightly different traits in modern gibbons, the researchers suggest.

Based on molar sizes, they estimate that Y. xiaoyuan weighed about six kilograms, similar to gibbons today. Molar structure indicates that Y. xiaoyuan focused on eating fruits, like most gibbon species today, Harrison says.

Ji’s group “makes a very good case that [Y. xiaoyuan] is a hylobatid,” says paleoanthropologist David Alba of Institut Català de Paleontologia Miquel Crusafont in Barcelona.

But the evolutionary status of K. ragnagarensis remains unsettled because only a single tooth from that species has been found, says Alba, who did not participate in the new study.