Artist’s reconstruction of Ailurarctos by Shuitangba. The grasping function of its false toe (shown in the right individual) reached the level of modern pandas, while the radial sesamoid may have protruded slightly more than its modern counterpart during walking (seen in the left individual). Credit: Illustration by Mauricio Anton
Eating bamboo? It’s all in the wrist.
When is a thumb not really a thumb? When it is an elongated bone in the giant panda’s wrist that is used to grasp bamboo. In its long evolutionary history, the panda hand never developed a truly opposable thumb. Instead, a thumb-like finger evolved from a wrist bone, the radial sesamoid. This unique adaptation helps these bears to feed entirely on bamboo, even though they are bears (members of the order Carnivora, or carnivores).
In a new paper published today (June 30, 2022), scientists report the discovery of the earliest ancestral bamboo-eating panda to have this ‘thumb’. Surprisingly, it is longer than its modern descendants. The study was conducted by Los Angeles County Museum of Natural History curator of vertebrate paleontology Xiaoming Wang and colleagues.
Although the famous false thumb of modern giant pandas (Ailuropoda melanoleuca) has been known for more than 100 years, it was not understood how this wrist bone evolved due to the almost complete absence of fossils. A fossilized false thumb from a giant panda ancestor, Ailurarctos, dating to 6-7 million years ago, was discovered at the Shuitangba site in Zhaotong City, Yunnan Province in southern China. This gives scientists the first look at the early use of this extra (sixth) digit – and the earliest evidence of a bamboo diet in panda ancestors – helping us better understand the evolution of this unique structure.
A panda in Chengdu eats bamboo. Credit: Photo reproduced with permission from Sharon Fisher
“Deep in the bamboo forest, giant pandas replaced an omnivorous diet of meat and berries with a quiet consumption of bamboo, a plant that is abundant in subtropical forests but has little nutritional value,” says NHM’s Curator of Vertebrate Paleontology e- p Xiaoming Wang. “Holding bamboo stalks tightly to crush them into bites is probably the most important adaptation to consuming huge amounts of bamboo.”
How to walk and chew bamboo at the same time
This discovery may also help solve an enduring mystery about pandas: Why are their false thumbs so seemingly underdeveloped? As an ancestor of modern pandas, Ailurarctos might be expected to have even less developed false “thumbs,” but the fossil discovered by Wang and his colleagues reveals a longer false thumb with a straighter end than the shorter, hooked toe of his modern descendants. So why did pandas’ false thumbs stop growing to achieve a longer finger?
“The panda’s fake thumb needs to walk and ‘chew,'” says Wang. “Such a dual function serves as a limit to how big this ‘thumb’ can get.”
Panda grip vs walking (the white bone is the fake thumb). Credit: Courtesy of the Natural History Museum of Los Angeles County
Wang and his colleagues think that the shorter false thumbs of modern pandas are an evolutionary compromise between the need to manipulate bamboo and the need to walk. The curved tip of the modern panda’s second toe allows them to manipulate the bamboo while allowing them to carry their imposing weight to the next bamboo dish. After all, the “thumb” does double duty as the radial sesamoid, a bone in the animal’s wrist.
“Five to six million years should have been enough time for the panda to develop longer false thumbs, but it seems that the evolutionary pressure of having to travel and carry its weight has kept the ‘thumb’ short – strong enough to be useful without being big enough to get in the way,” says Dennis Su, an associate professor in the School of Human Evolution and Social Change and a researcher at Arizona State University’s Institute of Human Origins and co-leader of the project that recovered the panda specimens.
“Evolving from a carnivorous ancestor and becoming pure bamboo, pandas have to overcome many obstacles,” says Wang. “The opposable ‘thumb’ of wrist bone may be the most amazing development against these obstacles.”
Reference: “Earliest giant panda false thumb suggests conflicting requirements for locomotion and feeding” by Xiaoming Wang, Dennis F. Su, Nina G. Jablonsky, Xueping Ji, Jay Kelly, Lawrence J. Flynn and Tao Deng, 30 June 2022, Scientific Reports .DOI: 10.1038/s41598-022-13402-y
The authors of this article are affiliated with the Natural History Museum of Los Angeles County, Los Angeles, CA, USA; Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Arizona State University, Tempe, AZ, USA; Pennsylvania State University, University Park, Pennsylvania, USA; Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; Yunnan Institute of Cultural Relics and Archaeology, Kunming, Yunnan, China; Harvard University, Cambridge, Massachusetts, USA.
Funding was provided by the US National Science Foundation, Yunnan Natural Science Foundation, National Natural Science Foundation of China, Zhaotong and Zhaoyang Governments, Institute of Vertebrate Paleontology and Paleoanthropology.
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