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Earliest gibbon fossil unlocks clues about the history of apes
A newly discovered fossil has helped to fill a long-elusive gap in the evolution of primates.
Researchers identified remains as belonging to an early gibbon species, which roamed the forests of southern China more than seven million years ago.
New evidence from China is helping to piece together the evolutionary history of apes, filling in a massive gap in the fossil record. The fossil, which includes part of an upper jaw and seven isolated teeth, was discovered in the Yuanmou area of the biodiversity-rich Yunnan Province in southwestern China.
A new study published in the Journal of Human Evolution suggests it belongs to an early hylobatid, the family of primates that includes 20 species of living gibbons found throughout Southeast Asia.
This small ape, called Yuanmoupithecus xiaoyuan, shines a new light on the evolution of hylobatids, which are poorly understood mainly due to the rarity of their fossils.
Nicole Barber, a PhD student who studies primate evolution at the Museum, says, 'It's really exciting that this fossil pushes our understanding of this group of primates back by a few million years.'
'The majority of gibbon fossils date no further back than two or three million years, so something from the Late Miocene that is seven or eight million years old is really exciting. There are almost no fossils from that far back, even though molecular evidence indicates that there were hylobatids around during this time.'
An excavation in Yunnan, China where Yuanmoupithecus remains have been found. Image © Terry Harrison/NYU’s Department of Anthropology
Why do we know so little about gibbon evolution?
Gibbons are a group of around 20 species of primates that, alongside chimpanzees, bonobos, gorillas and humans, form the apes. Historically referred to as 'lesser apes', gibbons are smaller than great apes and have smaller social groups.
These agile mammals are native to the forests of southern Asia and are perfectly adapted to life in the trees. Their elongated arms, strong legs and hook-shaped hands allow them to grasp, swing and jump between branches. The evolution of the early hylobatids is poorly understood as fossil remains are rare. Most specimens consist of isolated teeth and jawbone fragments that date back no more than two million years.
However, genetic studies indicate that the hylobatids diverged from great apes between 22 and 17 million years ago, which means there is a 10-million-year gap in the fossil record.
There are a few different reasons why this might be the case. Large primates are not particularly numerous, meaning there are fewer individuals that could get fossilised, while the environment in which they live could also be playing a role.
'There are lots of different factors that can determine the rarity of a fossil,' says Nicole. 'One possible explanation is preservation bias. For instance, things may fossilise much better in a sealed cave environment than in the kind of rainforest habitat where we find gibbons today.'
'It is often the case that if something has lived in a habitat that isn't friendly to preservation, you're not going to find many fossils from that particular group.'
How do we know this is currently the earliest fossil gibbon?
The fossils of Yuanmoupithecus were originally thought to be related to an early family of primates from east Africa that lived during the Miocene around 14-21 million years ago.
The most recent discovery of fossil teeth and the upper jaw of a two-year-old infant has helped to provide critical evidence to establish the phylogenetic relationship of the Yuanmou fossil ape, mainly that it seems more closely related to hylobatids.
'When you find a fossil, and you're trying to place it somewhere in a group or lineage, the more of that fossil you have, the more traits you can observe and the more confidently you can hypothesise that it belongs on this particular branch or in this particular group,' explains Nicole.
Researchers compared the teeth and lower face of Yuanmoupithecus to modern-day gibbons. They appeared to be very similar, but some features were more primitive in the fossil species. The size of the molar teeth helped the researchers estimate that it was similar in size to today's gibbons, weighing around six kilograms.
Fossils from another primate called Kapi ramnagarensis were previously thought to be an early species of hylobatid based on a single isolated fossil molar from India. However, this study found it not to be a hylobatid but from a more primitive group of primates more distantly related to modern-day apes.
Lar gibbons are believed to be extinct in the wild in China. Image © Piya Harnsilawat/Shutterstock
The future of gibbons in China
The present-day Yunnan province is one of China's most biologically diverse regions. Twenty-five species of primates are native to China, many of which can be found in Yunnan.
However, most of these species are poorly known and thought to be sliding silently towards extinction as human activities in the area continue to grow.
The lar gibbon and white-cheeked gibbon were previously found in Yunnan but have not been spotted in over a decade. These two gibbon species have now been declared extinct in the wild in China.
'Studying the evolution of primates is important because it helps us understand how species can change or adapt to different pressures,’ says Nicole.
'We don't necessarily know how living species are going to respond to changing circumstances, so understanding how those evolutionary processes have worked in the past can inform how we think about species today.'
