An ancient fossil reveals the earliest panda to survive solely on bamboo and the evolution of false panda thumbs.
When is a thumb no longer a thumb? When a giant panda's elongated wrist bone is used to grasp bamboo. The panda's hand has never developed a truly opposable thumb, instead of evolving a thumb-like digit from a wrist bone, the radial sesamoid, over its long evolutionary history. Despite being bears, this unique adaptation allows these bears to subsist entirely on bamboo (members of the order Carnivora, or meat-eaters).
Xiaoming Wang, Curator of Vertebrate Paleontology at the Natural History Museum of Los Angeles County, and colleagues report on the discovery of the earliest bamboo-eating ancestral panda with this "thumb" in a new paper published in Scientific Reports. Surprisingly, it outlives its modern descendants.
While the celebrated false thumb in living giant pandas (Ailuropoda melanoleuca) has been known for more than a century, the evolution of this wrist bone was unknown due to a near-complete lack of fossil records. A fossil false thumb from an ancestral giant panda, Ailurarctos, discovered at the Shuitangba site in Zhaotong, Yunnan Province in south China and dating back 6-7 million years ago, provides scientists with 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 understand the evolution of this unique structure.
"Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries for quietly consuming bamboos, a plant abundant in the subtropical forest but of low nutrient value," says Dr. Xiaoming Wang of the NHM Vertebrate Paleontology Department. "Perhaps the most important adaptation to consuming a prodigious amount of bamboo is tightly holding bamboo stems in order to crush them into bite sizes."
How to Walk and Chew Bamboo Simultaneously
This discovery may also help to answer a long-standing panda mystery: why are their false thumbs so underdeveloped? 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 become longer?
"The panda's false thumb must walk and 'chew,'" Wang says. "Such a dual function limits how large this 'thumb' can become."
Wang and his colleagues believe that the shorter false thumbs of modern pandas are an evolutionary compromise between the need to manipulate bamboo and the need to walk. A modern panda's hooked second thumb allows them to manipulate bamboo while carrying their impressive weight to the next bamboo meal. After all, the "thumb" serves 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," says Denise Su, associate professor at Arizona State University's School of Human Evolution and Social Change and research scientist at the Institute of Human Origins, and co-leader of the project.
"From a carnivorous ancestor to a pure bamboo-feeder, pandas must overcome many obstacles," Wang explains. "The most amazing development against these obstacles may be an opposable 'thumb' from a wrist bone."
The authors of this article are affiliated with the Natural History Museum of Los Angeles County, Los Angeles, CA, USA; the Chinese Academy of Sciences' Institute of Vertebrate Paleontology and Paleoanthropology, 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; and the Yunnan Institute of Cultural Relics and Archae
The National Science Foundation of the United States of America, the Yunnan Natural Science Foundation, the National Natural Science Foundation of China, the governments of Zhaotong and Zhaoyang, and the Institute of Vertebrate Paleontology and Paleoanthropology provided funding.
Reference: DOI: 10.1038/s41598-022-13402-y