The earliest fossil evidence of a thumb-like sixth digit used by giant pandas and their ancestors to grip bamboo is presented in a study published in Scientific Reports. The findings indicate that the panda’s dedicated bamboo diet may have originated at least six million years ago.
In addition to five digits on their hands, modern giant pandas (Ailuropoda melanoleuca) have an enlarged wrist bone with a thumb-like structure that they use to manipulate bamboo. Previous research documented evidence of the thumb-like structure to just 100 to 150 thousand years ago.
Xiaoming Wang and colleagues examined the wrist bone of an individual from the ancestral panda genus Ailurarctos that was discovered at Shuitangba, a site near the city of Zhaotong in Yunnan Province, China and dates back to the late Miocene period (approximately six to seven million years ago). They compared the shape and size of this bone to previously published data on the wrist bones of modern giant pandas and Indarctos arctoides – an ancient bear that lived nine million years ago and may share the same common ancestor as giant pandas.
The researchers found that the modern giant panda’s thumb-like structure has the same distinctive shape as Ailurarctos’s wrist bone but not I. arctoides’s, which was larger, wider and more hooked. This indicates that, while the thumb-like sixth digit was not present in I. arctoides or the common ancestor it shares with pandas, it has been present within the panda lineage – and used to grip bamboo – for at least six million years.
Although the sixth digit was present in both modern giant pandas and Ailurarctos, the researchers observed differences in its size and shape. The modern giant panda’s digit significantly shorter than Ailurarctos’s in relation to its body size and has a hook on the end of it and a flattened outer surface, while Ailurarctos’s does not. The authors propose that the hook may help modern pandas to better grasp bamboo, while the shorter length and flattened outer surface may assist with weight distribution when walking. These weight bearing constraints could be the main reason that the giant panda’s thumb-like structure never evolved into a full digit, they add.
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