【New knowledge】
◎Our reporter Lu Chengkuan
Unlike other carnivorous "brothers", giant pandas have many "stunts" that enable them to adapt to the life of eating bamboo exclusively.
Among these "stunts", the giant panda's "thumb" is the most famous and mysterious.
On July 1, "Scientific Reports" published a new study on the evolution of the giant panda's pseudo-thumb.
Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (hereinafter referred to as the Institute of Paleoanthropology, Chinese Academy of Sciences) and other units found that the proto-Panda developed an enlarged pseudothumb—the radial sesamoid bone 6 million years ago.
It is equivalent to a human thumb, which can grasp bamboo; at the same time, if the pseudo-thumb is too long, the panda will slap its feet when walking, so the "thumb" is not long or short, and it has not evolved into a complete finger.
"This is the earliest enlarged pseudo-thumb discovered so far. This simple 'thumb' shows that the giant panda's habit of eating bamboo exclusively originated at least 7 million to 6 million years ago." Institute of Paleo-Spine, Chinese Academy of Sciences Chang Deng Tao pointed out.
The giant panda's "thumb" has long attracted the attention of biologists.
Pseudo-thumbs that giant pandas can grasp has become a well-known example of evolutionary adaptation in recent decades.
By studying the fossils of the early Miocene found in Zhaotong, Yunnan, 7 million to 6 million years ago, the researchers found that the pseudo-thumb of the early panda showed an intermediate shape, which recorded the possible time of the evolution of the panda to eat bamboo for the first time. and steps; at the same time, the tooth morphology of the primordial panda has reached the level of complexity of the modern giant panda, or even more complex, which is also conducive to chewing hard bamboo.
One of the most important features of humans and their primate relatives has been the evolution of a thumb that can grip with other fingers.
Although giant pandas can also grasp, their grasping is fundamentally different from that of humans.
"Pseudo-thumbs of giant pandas do not move independently of other fingers like human thumbs, it is a passive grasp, far less effective than human's active grasp, but this passive grasp is enough to provide food for pandas The grasping ability required for bamboo." Deng Tao explained.
The radial sesamoid bone of the living giant panda has a sharply inwardly curved hook near the terminal end, however, the radial sesamoid bone of the primitive panda lacks the terminal hook.
This suggests a two-step evolutionary process: The pseudothumb initially simply elongated, and later developed a more refined terminal hook, accompanied by a slight shortening of the terminal.
"This raises the question, why haven't giant pandas developed a more elongated pseudo-thumb, a thumb that is more like a true pair-grip thumb, in order to grasp bamboo more effectively? This question has remained unanswered. "Deng Tao said.
After in-depth research, the researchers believe that the pseudo thumb did not extend further, which may be the result of the balance between grasping the bamboo pole and bearing the weight.
"Bears walk in a plantar-walking style with the front and back of the feet on the ground. If the pseudothumb grows too long, it will protrude significantly from the palm surface, and the foot will stump when walking." Deng Tao said.
Therefore, the living giant panda balances the conflict between grasping the bamboo pole and bearing weight by sharply bending the end of the pseudothumb inward into a hook shape and flattening the outside.
At the same time, the simple pseudo-thumb of pandas is difficult to collect the common omnivorous raw materials of bears, such as seeds, nuts, berries, dwarf grass, etc., which indicates that the feeding target of the enlarged pseudo-thumb is bamboo, and the pseudo-thumb is a panda pedigree The evolution of key adaptations for efficient bamboo harvesting in China.