The “amazing” feature of the giant panda was developed at least six million years ago

An artist reconstructs Ailurarctos from Shuitangba. The holding function of its false thumb (shown in the right individual) has reached the level of the modern panda, while the radial sesamoid may be slightly more prominent than its modern counterpart while walking (seen in the left individual). Credit: Illustration by Mauricio Anton

eat bamboo? It’s all on the wrist.

When isn’t a thumb really a thumb? When the carpal bone is elongated from the giant panda that is used to grasp bamboo. Through its long evolutionary history, the panda’s hand has never developed a truly hostile thumb. Instead, it developed a thumb-like finger from the carpal bone, the radial sesamoid. This unique adaptation helps these bears to live entirely on bamboo despite being bears (members of the order Carnivores, or carnivores).

In a new research paper published today (June 30, 2022), scientists report the discovery of the oldest ancestral panda that ate bamboo and which had this ‘thumb’. Surprisingly, she is taller than her modern descendants. The research was conducted by the Los Angeles County Museum of Natural History, curator of vertebrate fossils Xiao Ming Wang and colleagues.

While the famous false thumb in contemporary giant pandas (giant panda melanoleuca) has been known for more than 100 years, and how this carpal bone evolved is not understood due to the almost complete absence of fossil records. Fossil false thumb of giant panda ancestors, Ilorarctus, dating back 6-7 million years ago, was discovered at the Shuitangba site in Zhaotong City, Yunnan Province in southern China. It gives scientists a first look at the early use of this extra number (sixth) – and the first evidence of a bamboo diet in panda ancestors – helping us better understand the evolution of this unique structure.

Chengdu panda eat bamboo. Credit: Image reproduced with permission from Sharon Fisher

“Deep in the bamboo forest, giant pandas exchanged a bitumen diet of meat and berries to quietly consume bamboo, an abundant plant in the subtropical forest but of low nutritional value,” says NHM’s curator of vertebrate paleontology Dr. Xiaoming Wang. “Holding the bamboo stalks tightly in order to crush them to their bite sizes is perhaps the most important adaptation to consuming a massive amount of bamboo.”

How to walk and chew bamboo at the same time

The discovery could also help solve a longstanding puzzle of pandas: Why do their false thumbs seem to be underdeveloped? As the grandfather of the modern panda, Ilorarctus They may have been expected to have a less developed pseudo-thumb, but fossil Wang and colleagues discovered a longer false thumb with a straighter end than the shorter, hooked figure of its modern descendants. So why did the false panda’s thumb stop growing to get a taller figure?

“The lying panda’s thumb should walk and ‘chew,’” says Wang. “This dual function acts as a limit on how big this ‘thumb’ can be.”

Holding a panda versus walking (the white bone is the pseudo-thumb). Credit: Courtesy of the Los Angeles County Museum of Natural History

Wang and colleagues believe that the modern panda’s shorter false thumb is an evolutionary compromise between the need to manipulate the bamboo and the need to walk. The hooked tip of the second thumb allows modern pandas to manipulate the bamboo while allowing them to carry their impressive weight to their next bamboo meal. After all, the “thumb” does double duty as the radial sesamoid – a bone in the animal’s wrist.

“Five to six million years should be enough time for pandas to develop a longer pseudo-thumb, but it appears that the evolutionary pressure of needing to travel and carry its own weight has kept the ‘thumb’ short — strong enough to be useful without being big enough,” says Dennis Su, associate professor. In the College of Human Evolution and Social Change and Research Scientist at the Human Origins Institute at Arizona State University, and co-leader of the project that retrieved the panda specimens:

“Pandas evolved from carnivorous ancestors and became a pure bamboo feeder, and they must overcome many obstacles,” says Wang. “The resistible ‘thumb’ of the wrist bone may be the most amazing advance against these obstacles.”

Reference: “Ancient Giant Panda’s False Thumb Points to Conflicting Demands for Movement and Feeding” by Xiaoming Wang, Dennis F. Su, Nina Jablonsky, Shuibing Jie, Jay Kelly, Lawrence J. Flynn, and Tao Ding, June 30, 2022, Scientific Reports.
DOI: 10.1038 / s41598-022-13402-y

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; Penn State University, University Park, Pennsylvania, USA; Kunming Institute of Zoology, Chinese Academy of Sciences, Kunming, Yunnan, China; Yunnan Institute of Cultural Relics and Archeology, Kunming, Yunnan, China; Harvard University, Cambridge, Massachusetts, USA.

Funding was provided by the National Science Foundation of the USA, the Yunnan Natural Science Foundation, the National Natural Science Foundation of China, the Zhaotong and Chaoyang Governments, and the Institute of Vertebrate Paleontology and Paleoanthropology.