Source: CCTV News
The carnivorous giant panda, unlike its siblings, 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 oppositely to grasp the bamboo.
Recently, the scientific research team of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences cooperated with peers to publish a paper on the evolution of the sixth finger of the panda, that is, the "pseudo-thumb", in the international journal "Science Report". The key pseudo-thumb and molar fossils of the panda are from the scientific excavation of the late Miocene site of Shuitangba jointly organized by Deng Tao, Ji Xueping and the Institute of Paleovertebrates, the Yunnan Provincial Institute of Archaeology and the Zhaotong Municipal Government in 2015. The results of the study showed that the first panda had the earliest enlarged radial sesamoid bone, which had 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 on foot while grasping bamboo, and cannot be abandoned by either side. The restriction of sharing weight may be the main reason why the panda's pseudo-thumb never evolved into a full finger, but this simple "thumb" suggests that the giant panda's habit of eating bamboo specifically originated at least 7-6 million years ago.
△ The individual on the right side of the ecological restoration of the First Panda in Zhaotong Pond Dam in Yunnan Shows the pseudo-thumb grip function, and the individual on the left shows the walking posture
Although the panda's pseudo-thumb is very famous, it is only a rather rudimentary small, flat structure that once puzzled early anatomists. Fossils are the most important evidence to understand the origins of the pseudo-thumb, but the earliest previous records were found only in late Pleistocene sediments about 102,000 to 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. The results released a new discovery at the Site of the Late Mirmiocene Pond Dam in the Zhaotong Basin of Yunnan Province, which showed an intermediate morphology based on the earliest known enlarged radial seed bone of the panda, and the pseudo-thumb showed an intermediate morphology, which for the first time recorded the possible time and step of the evolution of the panda 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 a bamboo pole is another key piece of evidence.
△ Anatomy of the pseudo-thumbs of giant pandas (A, C) and pandas (B, D).
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 moon bone by fixing the joints, which is linked to the other metacarpal bones, and cannot be independent of other finger movements 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.
Comparison of giant pandas (B, D, E) with primitive bears (A) and human (C) hands
Longer fingers will certainly enhance the ability to grasp bamboo poles, so why didn't the panda's pseudo-thumb stretch further? The study believes that the radial sesamoid bone did not extend further, and it was the pseudo-thumb that took into account the functional reconciliation between grasping the bamboo pole and bearing the weight. Unlike the toe-like pattern of running carnivores such as canines, cats, and hyenas, obese bears are plantar, where the front and back paws touch the ground when walking. The strongly elongated radial sesamoid bone is more conducive to grasping bamboo, but inevitably conflicts with long walks, thus impairing the dual function of the pseudo-thumb – the inner surface is used for grasping and the outer surface is used for load bearing. Any further enlargement of the radial sesamoid bone will result in a more pronounced prominence on the palmal surface, which interferes with walking, in layman's terms, the foot. Thus, the living giant panda balances the conflict between functions by bending sharply inward at the end of the pseudo-thumb into a hook. Of the extant bears, only giant pandas have a large fleshy cushion to cushion the radial sesamoid bone, indicating that the weight-bearing function of this bone is important.
△ Giant pandas grasp and chew bamboo
Pandas' transition from widespread omnivorous to single-mindedly eating bamboo has logically required 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, while also supporting the fat body to walk, the pseudo-thumb is not long or short.
(CCTV reporter Shuai Junquan)