Giant pandas evolved their most confusing traits at least 6 million years ago
Giant pandas have six hand indices to help catch bamboo.
As we all know, giant pandas are a group of discerning diners. They chew only bamboo and spend 15 hours a day eating up to 99 pounds (45 kilograms) of stuff.
But their ancestors, like most bears, ate a much broader diet, including meat, and it is believed that the exclusive diet of modern giant pandas was only recently evolved. However, a new study has found that giant pandas' particular passion for bamboo may have originated at least 6 million years ago — possibly due to the plant's widespread availability throughout the year.
A model of a giant panda clawing bamboo.
In order to survive on low-nutrient bamboo alone, modern giant pandas (Ailuropoda melanoleuca) have developed a peculiar sixth finger that allows them to easily grasp bamboo stems and peel off leaves.
"Clinging to bamboo stems to crush them into a single bite may be the most critical adaptation to consuming large amounts of bamboo," study author Xiaoming Wang, director of vertebrate paleontology at the Natural History Museum of Los Angeles County, said in a statement.
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The panda fossil record is sparse, which means how bears evolved this confusing trait, a puzzling trait that has long plagued biologists but isn't really understood. Previous studies have found evidence of this thumb-like structure about 100,000 to 150,000 years ago.
Wang and his team found earlier evidence that pandas had an extra finger — and therefore a whole-bamboo diet — in the form of fossil numbers dating back 6 million to 7 million years. The fossil was unearthed in Yunnan Province, southwestern China, and belongs to an ancestor of giant pandas called Ailurarctos.
The picture shows the giant panda ancestor Erulaktos reconstructed by the artist from the fossil site of Shuitangba in Yunnan, China.
The new study was published Thursday in the American journal Scientific Reports.
The study notes that while the sixth number of giant pandas is not as elegant or dexterous as a human thumb, the persistence of this "unique form" over millions of years suggests that it plays a vital role in survival.
But what's particularly puzzling for the scientists involved in the study is that this fossil structure is longer than that of modern pandas, which have a shorter, hooked sixth finger.
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Wang and his colleagues argue that the shorter sixth finger of the modern giant panda is an evolutionary compromise between the need to manipulate bamboo and the need to walk and carry its heavy bodies.
"Five to six million years should be enough for pandas to grow longer false thumbs, but the evolutionary pressure that seems to require travel and bear its weight makes the 'thumbs' short — strong enough to be useful without getting in the way," said denise Su, co-author of the study, an associate professor at the School of Human Evolution and Social Change and a research scientist at the Institute of Anthropology. Originated at Arizona State University, in a statement.