An artistic reconstruction of Ailurarctos from Shuitangba. The grasping function of its false thumb (shown in right person) has reached the level of modern pandas, while its radial sesamoid may be slightly more protruded when walking than its modern counterpart (shown in left person). Photo credit: Illustration by Mauricio Anton
eat bamboo? It’s all in the wrist.
When is a thumb actually not a thumb? When it is an elongated giant panda wrist bone used for grasping bamboo. During its long evolutionary history, the panda’s hand never evolved 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 carnivores).
In a new article published today (June 30, 2022), scientists report discovering the earliest bamboo-eating panda ancestor with this “thumb.” Surprisingly, it is longer than its modern descendants. The research was conducted by Los Angeles County Museum of Natural History Curator of Vertebrate Paleontology, Xiaoming Wang, and colleagues.
While the famous false thumb in contemporary giant pandas (Ailuropoda melanoleuca) has been known for more than 100 years, due to the almost total lack of fossil records, how this wrist bone evolved has not been understood. A fossil false thumb from an ancestral giant panda, ailurarctos, which formed 6-7 million years ago, was uncovered at the Shuitangba site in the city of Zhaotong, Yunnan Province, southern China. It gives scientists a first glimpse of the early use of this extra (sixth) digit – and the earliest evidence of a bamboo diet in ancestral pandas – and helps us better understand the evolution of this unique structure.
Chengdu panda eating bamboo. Photo Credit: Photo reproduced with permission from Sharon Fisher
“Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries for quiet consumption of bamboo, a plant that is abundant in the subtropical forests but has little nutritional value,” says NHM Vertebrate Paleontology Curator Dr. Xiaoming Wang. “Holding bamboo stalks in order to chop them into bite-sized sizes is perhaps the most important adaptation to eating a prodigious amount of bamboo.”
How to walk and chew bamboo at the same time
This discovery could also help solve an ongoing panda mystery: Why are their false thumbs seemingly so underdeveloped? As the ancestor of modern pandas, ailurarctos One might expect them to have even less well-developed false “thumbs,” but the fossil Wang and his colleagues discovered showed a longer false thumb with a straighter end than the shorter, hooked finger of its modern descendants. So why did pandas’ fake thumbs stop growing to achieve a longer finger?
“The panda’s false thumb has to walk and ‘chew,'” says Wang. “Such dual function serves as a limit on how big that ‘thumb’ can get.”
Panda grasping versus walking (white bone is the wrong thumb). Photo credit: Courtesy of the Natural History Museum of LA County
Wang and his colleagues believe that the modern panda’s shorter false thumbs are an evolutionary compromise between the need to manipulate bamboo and the need to walk. The hooked tip of a modern panda’s second thumb allows them to manipulate bamboo while allowing them to carry their impressive weight to the next bamboo meal. Finally, the “thumb” performs a dual role as the radial sesamoid—a bone in the animal’s wrist.
“Five to six million years should be enough for the panda to evolve longer false thumbs, but it appears that evolutionary pressures to travel and carry one’s weight have kept the ‘thumb’ short – strong enough to be useful without being big enough to get in the way,” says Denise Su, associate professor in the School of Human Evolution and Social Change and research scientist at Arizona State University’s Institute of Human Origins and co-leader of the project investigating the panda recovered specimens.
“In the process of evolving from a carnivorous ancestor to a pure bamboo eater, pandas face many obstacles,” says Wang. “An opposable ‘thumb’ from a wrist bone is perhaps the most amazing development against these hurdles.”
Reference: “False Thumb of Earliest Giant Panda Suggests Conflicting Locomotion and Feeding Requirements” by Xiaoming Wang, Denise F. Su, Nina G. Jablonski, Xueping Ji, Jay Kelley, Lawrence J. Flynn, and Tao Deng, June 30, 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, Arizona, 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 USA National Science Foundation, Yunnan Natural Science Foundation, National Natural Science Foundation of China, Zhaotong and Zhaoyang governments, Institute of Vertebrate Paleontology and Paleoanthropology.