Ecological restoration of pandas in Zhaotong Shuitang Dam, Yunnan: the individual on the right shows the pseudo-thumb grip function, and the individual on the left shows the walking posture. Mauricio Antón drawing
Beijing, July 1 (China News Network) (Reporter Sun Zifa) Why do giant pandas that belong to the carnivorous order only eat bamboo? How did its skillful grasping of bamboo "stunts" originate and evolve? These problems are not only of concern to the academic community, but also quite curious to the public.
Deng Tao, a researcher at the Institute of Paleovertebrates of the Chinese Academy of Sciences, was interviewed in Beijing to introduce the latest research results on the origin and evolution of giant panda feeding habits. Photo by Sun Zifa, a reporter from China News Service
The history of giant panda bamboo eating dates back 6 million years
Wang Xiaoming, a visiting researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (Institute of Paleovertebrates of the Chinese Academy of Sciences), a researcher at the Natural History Museum of Los Angeles, USA, Deng Tao, a researcher at the Institute of Paleovertebrates of the Chinese Academy of Sciences, and Ji Xueping, a researcher at the Kunming Institute of Zoology of the Chinese Academy of Sciences, in collaboration with their scientific research counterparts, have recently completed the research findings on the key fossils of the early Miocene (about 7 million to 6 million years ago) of the early Miocene panda at the site of Zhaotong Shuitang Dam in Yunnan Province, namely pseudo-thumb (also known as the sixth finger) and molar fossils. The panda has the earliest enlarged radial sesamoid bone, which has formed a "thumb" function of the opposite hand.
However, since the late Miocene, the panda's "thumb" has not been further enlarged, because it must also take into account the load-bearing function of walking in a walking style (the front and back soles of the feet touching the ground) while grasping the bamboo, and cannot be abandoned in either side. This weight-sharing restriction may be the main reason why pandas' pseudo-thumbs never evolved into full fingers, but this humble "thumb" suggests that the giant panda's habit of eating bamboo specifically originated at least 6 million years ago.
This paper on the important achievements of paleontological research, published online in the international academic journal "Scientific Report" on the night of June 30, Beijing time, puts forward important fossil evidence for the evolution of the Archaeopteryx-giant panda lineage that predates the Late Pleistocene, and also provides key evidence for answering whether the panda has begun to eat bamboo, and whether the false thumb can help the panda grasp the bamboo pole.
Schematic diagram of the pseudo-thumbs of giant pandas (A, C) and pandas (B, D). Photo courtesy of Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
Giant panda pseudo-thumb "stunt" is the most famous and mysterious
Researcher Deng Tao, co-author of the paper, said in an exclusive interview with a reporter from China News Agency on the day of the publication of the paper that the giant panda, which belongs to the carnivorous order, is a highly specialized member of the bear family, and it has evolved into a focused plant-eating animal. The giant panda is different from its "brothers" in that it has many "stunts" that allow it to adapt to a life that specializes in eating bamboo.
Among these "stunts", the giant panda's extra "thumb" is arguably the most famous but also the most mysterious. In addition to the normal five fingers on most mammalian forelimbs, giant pandas have a strongly enlarged wrist bone, the radial sesamoid bone, which acts as a sixth finger, forming a "thumb" that can be held in opposition to grasp the bamboo.
The panda's pseudo-thumb has long attracted the attention of naturalists, and in recent decades, thanks to the introduction of the famous evolutionary biologist Steven J. Gould, the thumb-like structure of the giant panda gaining a grip has become a famous example of evolutionary adaptation, and the unique link between this unique anatomy and the equally unique habit of eating bamboo has attracted attention.
Deng Tao pointed out that although the giant panda's pseudo-thumb is very famous, to understand the ins and outs of the pseudo-thumb, fossils are the most important evidence, and the earliest records were only found in the Late Pleistocene sediments of about 102,000-49,000 years ago in Shuanghe Cave, Guizhou. If grasping bamboo is a major function of this feature, since the sesamoid bone of mammals is easily elongated during development, why didn't giant pandas evolve a more elongated radial sesamoid bone, a sesamoid bone that more resembles a true counter-grip thumb in order to grip bamboo more efficiently? This question was not previously answered because of the lack of relevant fossil evidence in the Echizoite-Giant Panda lineage, which preceded the Late Pleistocene.
Comparison of giant pandas (B, D, E) with primitive bears (A) and human (C) hands. Photo courtesy of Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
Giant panda grips are fundamentally different from human grips
Deng Tao said that this study is based on the new discovery of the fossils of the pseudo-thumb and molars of the giant panda at the site of the Late Miocene Pond Dam in the Zhaotong Basin of Yunnan in 2015, and based on the earliest known enlarged radial sesamoid bone of the panda, its pseudo-thumb shows the intermediate morphology, and for the first time recorded the possible time and step of the panda's evolution to eat bamboo.
Among carnivores, the molars of the bear family are the most complex because they also chew large amounts of non-meat foods. Among bears, the panda has the most delicate tooth pattern, with many very pronounced tip of the folds, which are conducive to chewing hard bamboo with the help of strong mandibles and temporomandibular joints. The tooth morphology of the first panda has reached the level of complexity of modern giant pandas, or even more complex. "Has the panda started eating bamboo?" The pseudo-thumb that can help grasp the bamboo pole is another key piece of evidence."
Deng Pointed out that one of the most important features of humans and their primate relatives is the evolution of a thumb that can be held against other fingers for precise grip, which requires not only joint flexibility, but also complex interactions between flexor and extensor muscles.
The grip of the giant panda is fundamentally different from that of humans, and its radial sesamoid bone forms a functional complex with the first metacarpal bone and the scapula by fixing the joints, which are linked to the other metacarpal bones, rather than being able to move independently of other fingers like the human thumb. This passive grip system is far less effective than the active grip system of humans, but it is enough to provide giant pandas with the gripping ability needed to eat bamboo. In addition, from an evolutionary point of view, even if the radial sesamoid bone is initially only slightly enlarged, it can form a simple but useful grip function, moderately preventing the bamboo from slipping off the finger.
The radial sesamoid bone of the living giant panda has a sharp inward curved hook near the end, and its function is to rely on a single bone to form a claw. In contrast to the human thumb, which has two knuckles, the end section of the human thumb can be bent for easy grip.
Detailed drawing of giant panda grasping and chewing bamboo. Photo by Sharon Fisher
Pseudo-thumb evolution takes into account both grasping a bamboo pole and taking on weight
Deng Tao said that the radial sesamoid bone of the first panda lacks a curved hook at the end, indicating that its evolutionary process is divided into two steps: the pseudo-thumb is initially simply elongated, and later a finer end curved hook appears, accompanied by a slight shortening of the end. Whether in absolute or relative length, the pseudo-thumb of the panda exceeds its present offspring, but lacks the end curved hook of the giant panda.
"This raises the question that longer fingers will certainly enhance the ability to grasp bamboo poles, so why didn't the panda's pseudo-thumb stretch further?"
The research team believes that the radial sesamoid bone has not been further extended, and that the pseudo-thumb is a functional reconciliation between grasping the bamboo pole and taking on the weight. Unlike the toes of running predators such as canines, cats, and hyenas (where the front and back toes land), obese bears are in a walking style. Strongly elongated radial sesamoids are more conducive to grasping bamboo, but inevitably clash with long walking, thus impairing the dual functions of the inner surface of the pseudo-thumb for grasping and the outer surface for load-bearing. Any further enlargement of the radial sesamoid bone will result in a more pronounced prominence on the palmar surface, which interferes with walking, "in layman's terms, the foot".
Therefore, the living giant panda balances the conflict between functions by sharply bending the end of the pseudo-thumb inward into a hook and flattening on the outside. In living ursopheae, only giant pandas have a large fleshy pad to cushion the radial sesamoid bone, indicating that the weight-bearing function of this bone is important.
Because the habitat is very rich in bamboo, giant pandas do not need to travel long distances every day to find food, so eating more and eating fast is more important. When a panda tears a bamboo pole with its teeth, clinging to the bamboo pole may be the most useful function of the pseudo-thumb. Bamboo poles are very hard, especially when completely wooden in winter, requiring the panda's hands to have considerable grip to twist and pull in order to cooperate with the chin for powerful nibbling and tearing. Because grasping a bamboo pole is more important to pandas than grasping more bamboo, there is no strong requirement for pseudo-thumb length evolution.
Evolutionary relationship between pseudo-thumb and molars and bamboo-eating habits of the panda family. Photo courtesy of Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
Pseudo-thumbs are a key adaptation for effective bamboo harvesting
Deng Tao believes that the simple pseudo-thumb of the panda is difficult to use to collect the usual omnivorous raw materials of the bear family, such as seeds, nuts, berries, and even dwarf grasses, which indicates that the only feeding target of the enlarged radial sesamoid bone is bamboo, and the pseudo-thumb is a key adaptation for effectively collecting bamboo in the panda lineage. Although its digestive system appears to be inefficient, the giant panda's strategy of "eating the sea" has allowed it to successfully expand to large parts of southern China and Southeast Asia and become an important member of the Pleistocene giant panda-saber-toothed elephant fauna.
Deep in China's bamboo forests, giant pandas eat its bamboo quietly and live in isolation and solitude, away from the dominance of their carnivorous "relatives" in the food chain.
Pandas' transition from extensive omnivorous recipes to single-mindedly eating bamboo is supposed to require multiple changes based on anatomy, physiology, and genetics. However, even after at least 6 million years of focusing on eating bamboo, these shifts are still limited and focused on feeding skills, while the digestive system of giant pandas is still a carnivorous configuration.
After the late Miocene, the fact that the panda family's pseudo-thumbs were not further extended shows that they have gained enough grip to deal with bamboo. "That is to say, it is enough to grasp a bamboo pole or a handful of bamboo, and at the same time to support the fat body to walk, the giant panda evolved pseudo-thumb is not long or short." Deng Tao concluded. (End)