Researchers have discovered panda fossils in China which have helped them solve the mystery of how the bears developed a "false thumb" and become the only dedicated herbivores in the bear family.
These fossils are six million years old, and have been discovered in southwest China's Yunnan province. The fossils included a greatly enlarged wrist bone called a radial sesamoid, which is a sesamoid bone found bilaterally. A sesamoid bone is a small bone commonly found embedded within a muscle or tendon near joint surfaces, in the knee, hand, wrist, and foot.
The greatly enlarged wrist bone is the oldest known evidence of the modern giant panda's (Ailuropoda melanoleuca) false thumb that allows it to grip and break heavy bamboo stems, according to a new study published in the Scientific Reports.
All About A Giant Panda’s False “Thumb”
The giant panda is a member of the order Carnivora. The extra "thumb" of the giant panda is arguably the most celebrated peculiarity of the bear that enables it to adapt to life as a dedicated bamboo feeder.
The giant panda has a greatly enlarged wrist bone, the radial sesamoid, in addition to the normal five digits in the hands of mammals. The radial sesamoid acts as a sixth digit, an opposable "thumb" for manipulating bamboo.
Study Reports Earliest Enlarged Radial Sesamoid In Ancestral Panda
In the new study, researchers report the earliest enlarged radial sesamoid, already a functional opposable "thumb", in the ancestral panda Ailurarctos from the late Miocene site of Shuitangba in Yunnan Province, China. Ailurarctos were an ancient relative of the giant panda that lived in China six to eight million years ago, and are now extinct.
Why Has The “Thumb” Not Enlarged Further?
According to the study, the "thumb" has not enlarged further because it must be balanced with the constraints of weight bearing while walking in a plantigrade posture. Plantigrade foot posture is one in which the heel contributes to support of body weight during walking, running and standing. In other words, plantigrade posture is one in which the surface of the whole foot touches the ground during locomotion.
The “Thumb” Performs A Dual Function
The authors noted in the study that this morphological adaptation in panda evolution reflects a dual function of the radial sesamoid for both bamboo manipulation and weight distribution. The constraints of weight bearing could be the main reason why the panda's false thumb never evolved into a full digit. The crude "thumb" suggests that the origin of the panda's dedicated bamboo diet dates back to as early as six to seven million years ago.
So Far, There Was Lack Of Evidence About How The Extra Digit Evolved
Though the panda's false thumb had been known for about a century, the lack of fossil evidence had left questions unanswered about how and when the extra digit, which is not seen in any other bear, evolved.
The researchers found the enlarged radial sesamoid along with a broken canine and a partial humerus from the late Miocene site in the Zhaotong Basin, Yunnan Province. The false thumb found in China was longer than that found in modern pandas, but without a sharp bend inward on the distal end to form a hook.
Ursid is a plantigrade carnivore of the family Ursidae which comprises the spectacled bear, the black, brown and sun bears, and various extinct species that gave rise to the giant panda of the family Ailuropodidae.
Eating Habits Of Pandas
According to the study, pandas traded the high-protein, omnivorous diet of their ursid ancestors for bamboo, a woody grass of high fibre and low nutrition. However, bamboo had a year-round availability in South China and Southeast Asia millions of years ago. Depending on the season, pandas eat prodigious quantities of bamboo, up to 45 kilograms a day. They spend 15 hours a day eating.
Moreover, the panda's short digestive tract, which it inherited from its carnivoran ancestors, is poorly suited for extracting nutrients, absorbing less than 20 per cent of digestible dry matter.
The paper said that while the giant panda's false thumb is not the most elegant or dexterous, the persistence of the distinctive morphology for the last six million years suggests that it has fulfilled an essential function for the survival of the lineage.
What Is The Purpose Of Small Muscles Between Radial Sesamoid And First Metacarpal?
The paper also said that small muscles between the radial sesamoid and the first metacarpal (one of the five bones between the wrist and the fingers) serve as a cushion for the bamboo stems grasped between the radial sesamoid and the phalanges.
Though this passive system of gripping is far less effective than that of humans, it nonetheless offers the panda the tightness of grip it needs for bamboo feeding.
According to the study, such a simple passive mechanism of grasping can be functionally useful from an evolutionary point of view, even with a slight initial enlargement of the radial sesamoid.
The study said that natural selection would be effective from the early stages of enlargement. This means that even a small, protruding lump at the wrist can be a modest help in preventing bamboo from slipping off bent fingers.
The authors noted in the study that the giant panda is a striking example of a highly specialised member of the bear family (Ursidae) that has become a dedicated herbivore.
The study also said that the false thumb in Ailurarctos shows an intermediate morphology, and thus, documents for the first time the likely timing and steps in the evolution of bamboo feeding in plants.
The authors concluded that the hook and the fleshy pad, which cushions the radial sesamoid, evolved over time because the false thumb had to bear the burden of considerable body weight.