Do you have bamboo? It’s all about the wrist.
When is the thumb really not a thumb? When it is the elongated wrist bone of a giant panda that is used to grip bamboo. In its long evolutionary history, the panda hand has never developed a truly opposable thumb. Instead, he developed a thumb-like digit from the carpal bone, the radial sesamoid bone. This unique adaptation allows these bears to feed entirely on bamboo despite being bears (members of the carnivore or meat eater order).
In a new article published today (June 30, 2022), scientists report the discovery of such a “thumb” in an ancient bamboo-eating panda. Surprisingly, it is longer than its modern descendants. The study was conducted by Xiaoming Wang, curator of vertebrate paleontology at the Los Angeles County Museum of Natural History, and colleagues.
While the famous false thumb in modern giant pandas (Ailuropod melanoleuca) has been known for over 100 years, it was not clear how this carpal bone had evolved due to the almost complete absence of the fossil record. Fossil false thumb of a giant panda ancestor, Ailuarktos, dated 6–7 million years ago, was discovered at the Shuitangba site in Zhaotong City, Yunnan Province in southern China. This gives scientists a first look at the early use of this extra (sixth) toe – and the earliest evidence for a bamboo diet in panda ancestors – helping us better understand the evolution of this unique structure.
“Deep in the bamboo forest, giant pandas have switched from an omnivorous diet of meat and berries to quietly eating bamboo, a plant that is abundant in the subtropical forest but low in nutritional value,” says NHM curator of vertebrate paleontology Dr. Xiaoming Wang. “Holding the bamboo stems tightly to crush them into pieces is perhaps the most important adaptation to consuming massive amounts of bamboo.”
How to walk and chew bamboo at the same time
The discovery may also help solve the panda’s puzzle: why do their false thumbs seem so underdeveloped? As an ancestor of modern pandas, Ailuarktos one would expect them to have even less developed false “thumbs”, but the fossil discovered by Wang and colleagues showed a longer false thumb with a straighter end than its modern descendants have a shorter, hooked toe. So why did pandas’ false thumbs stop growing to make the finger longer?
“The panda’s false thumb is supposed to walk and chew,” says Wang. “This dual function serves as a limit to how big that ‘thumb’ can get.”
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 the modern panda’s second thumb allows them to manipulate the bamboo, allowing them to carry their impressive weight until their next bamboo meal. After all, the “thumb” serves the dual function of the radial sesamoid bone, the bone in the animal’s wrist.
“Five to six million years should be long enough for a panda to develop longer false thumbs, but it seems that evolutionary pressure to travel and carry its weight has made the “thumb” short—strong enough to be useful. but not be big enough. to get in the way,” says Denise Su, assistant professor in the School of Human Evolution and Social Change and researcher at Arizona State University’s Human Origins Institute and co-leader of the panda specimen discovery project.
“Evolving from carnivorous ancestors to becoming bamboo’s pure feeders, pandas have to overcome many obstacles,” says Wang. “The separation of the thumb from the carpal bone may be the most amazing achievement in overcoming these obstacles.”
Reference: “Giant panda’s earliest false thumb suggests conflicting locomotion and feeding needs”, Xiaoming Wang, Denise F. Su, Nina G. Jablonsky, Xueping Ji, Jay Kelly, Lawrence J. Flynn, and Tao Deng, June 30, 2022 ., Scientific reports.
The authors of this article are affiliated with the Los Angeles County Museum of Natural History, Los Angeles, California, 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 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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