Giant Panda’s “Amazing” Feature Developed at Least Six Million Years Ago
Eating bamboo? It’s all in the wrist.
When is a thumb not really a thumb? When it’s an elongated wrist bone of the giant panda that is used to grasp bamboo. Through its lengthy evolutionary history, the panda’s hand has never developed a truly opposable thumb. Instead, it evolved a thumb-like digit from a wrist bone, the radial sesamoid. This unique adaptation helps these bears subsist entirely on bamboo despite being bears (members of the order Carnivora, or meat-eaters).
In a new paper published today (June 30, 2022), scientists report the discovery of the earliest bamboo-eating ancestral panda to have this “thumb.” Surprisingly, it’s longer than its modern descendants. The research was conducted by the Natural History Museum of Los Angeles County’s Curator of Vertebrate Paleontology Xiaoming Wang and colleagues.
While the celebrated false thumb in contemporary giant pandas (Ailuropoda melanoleuca) has been known for more than 100 years, it was not understood how this wrist bone evolved due to a near-total absence of fossil records. A fossil false thumb from an ancestral giant panda, Ailurarctos, dating back 6–7 million years ago was uncovered at the Shuitangba site in the City of Zhaotong, Yunnan Province in south China. It gives scientists a first look at the early use of this extra (sixth) digit–and the earliest evidence of a bamboo diet in ancestral pandas–helping us better understanding the evolution of this unique structure.
“Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries to quietly consuming bamboos, a plant plentiful in the subtropical forest but of low nutrient value,” says NHM Vertebrate Paleontology Curator Dr. Xiaoming Wang. “Tightly holding bamboo stems in order to crush them into bite sizes is perhaps the most crucial adaptation to consuming a prodigious quantity of bamboo.”
How to Walk and Chew Bamboo at the Same Time
This discovery could also help solve an enduring panda mystery: why are their false thumbs so seemingly underdeveloped? As an ancestor to modern pandas, Ailurarctos might be expected to have even less well-developed false“thumbs,” but the fossil Wang and his colleagues discovered revealed a longer false thumb with a straighter end than its modern descendants’ shorter, hooked digit. So why did pandas’ false thumbs stop growing to achieve a longer digit?
“Panda’s false thumb must walk and ‘chew’,” says Wang. “Such a dual function serves as the limit on how big this ‘thumb’ can become.”
Wang and his colleagues think that modern panda’s shorter false thumbs are an…
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