The previously confirmed history of panda bamboo is only 100,000 years old. Today (July 1), the reporter learned from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (hereinafter referred to as the "Institute of Paleovertebrates of the Chinese Academy of Sciences") that researchers have traced the history of panda bamboo eating to 6 million years ago through the fossils of the false thumb and molars of the giant panda, the first panda.
Giant pandas grasp and chew bamboo. Photo by Sharon Fisher
Fossils of pseudo-thumbs and molars reveal the time when pandas ate bamboo
Giant pandas have many "stunts" that enable them to adapt to a life that specializes in eating bamboo. Among these "stunts", the giant panda's extra "thumb" is the most famous but also the most mysterious. In addition to the normal five fingers on most mammalian forelimbs, giant pandas have an 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. In recent decades, thanks to the recommendation of the famous evolutionary biologist Steven J. Gould, the thumb-like structure of giant pandas gaining grips has become a well-known example of evolutionary adaptation, emphasizing the correspondence between this unique structure and the unique habit of eating bamboo. The giant panda is a highly specialized member of the bear family, which has evolved into a focused plant-eating animal.
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 ins and outs of the false thumb, but the earliest previous records were found only in late Pleistocene sediments about 102,000-49,000 years ago in Shuanghe Cave, Guizhou.
Mammalian sesamoid bones are easily elongated during development, so why didn't giant pandas evolve a more slender radial sesamoid bone in order to grip bamboo more effectively? 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 first panda is known to have lived 8 million to 6 million years ago. The fossils of the pseudo-thumb and molars of the panda studied this time are from the scientific excavation of the Late Miocene site of Shuitangba jointly organized by the Institute of Paleovertebrates of the Chinese Academy of Sciences, the Institute of Archaeology of Yunnan Province and the Zhaotong Municipal Government in 2015. The new findings are the first to document the possible time and steps for pandas to evolve from omnivorous to bamboo-eating.
Why didn't the panda's pseudo-thumb evolve further and lengthen?
According to Deng Tao, a researcher at the Institute of Paleovertebrates of the Chinese Academy of Sciences, among carnivores, the molars of the bear family are the most complex, because they also chew a large number of non-meat foods. Among the bears, the panda has the most elaborate tooth pattern, with dense tooth tips on the crown, 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.
One of the most important characteristics 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 of flexors and extensors. 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.
Comparison of the hands of giant pandas (B, D, E), primitive bears (A) and humans (C). Photo: Institute of Paleovertebrate Vertebrate, Chinese Academy of Sciences
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.
Why didn't the panda's pseudo-thumb evolve further? The study believes that the radial sesamoid bone did not extend further, in order to balance the two functions of grasping the bamboo pole and bearing the weight. Unlike the toe line of running carnivores such as canines, cats, and hyenas, obese bears are in a toe manner, where the front and back paws touch the ground when walking. A more elongated radial sesamoid bone is more conducive to grasping bamboo, but inevitably conflicts with long-distance walking. The further enlargement of the radial sesamoid bone protrudes more prominently over the palmal surface, thereby interfering with walking, in layman's terms, the foot.
Therefore, the living giant panda balances the conflict between functions by bending sharply inward at the end of the pseudo-thumb into a hook and flattening on the outside. 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.
In addition to grasping bamboo, the pseudo-thumb also has a walking load-bearing function
Because the bamboo in the habitat is very abundant, giant pandas do not need to travel long distances every day to find food, and it is more important to eat more and eat faster.
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. Therefore, grasping the bamboo pole is more important for the panda than grasping more bamboo at once, so there is no strong requirement for the length of the pseudo-thumb.
Panda's humble pseudo-thumb is difficult to use to collect the usual omnivorous raw materials of the bear family, such as seeds, nuts, berries, and even dwarf grasses. This suggests that the only feeding target for enlarged radial sesamoid bones is bamboo, and that the pseudo-thumb is a key adaptation for effective bamboo harvesting in panda lineages.
The ecological restoration of pandas at the Zhaotong Shuitang Dam in Yunnan Province showed pseudo-thumb gripping function on the right and walking posture on the left. Mauricio Antón painted
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 in a walking style while grasping bamboo, and cannot be abandoned by either side. Weight-bearing restrictions 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 7 million to 6 million years ago. "The first panda between 8 million and 7 million years ago has not yet found pseudo-thumb fossils, so it is not known whether it has eaten bamboo at that time, but from the structure of its teeth, it has adapted to feed on plants." The researchers said.
Beijing News reporter Zhang Lu
Edited by Liu Mengjie Proofreader Wu Xingfa