The discovery of panda fossils in China dating back millions of years has helped scientists solve the mystery of how the giant species evolved a “false thumb,” which helps this vegetarian bear eat the bamboo that makes up most of its diet. forms, and how it became the only devoted vegetarian in the bear family.
Researchers said Thursday they discovered fossils about 6 million years old near the town of Zhaotong in Yunnan province of an extinct panda called Ailura arctos that bore the oldest known evidence of this makeshift extra digit — actually a greatly enlarged wrist bone called the radial sesamoid.
It closely resembled the false thumb of modern pandas, but is slightly longer and lacks the inward hook present at the tip in the extant species that provides an even greater ability to manipulate bamboo stems, shoots and roots while feeding.
The false thumb is an evolutionary adaptation to increase the existing five actual digits of the panda’s hand. A bear’s hand lacks the opposable thumb that humans and various primates have that allow for grasping and handling objects with the fingers. The false thumb has a similar function.
“It uses the false thumb like a very rough, opposable thumb to grip bamboo, a bit like our own thumbs, except it’s on the wrist and is much shorter than human thumbs,” said Xiaoming Wang, a paleontologist at the University of Groningen. Los Angeles County Natural History Museum. lead author of the study published in the journal Scientific Reports†
Ailura arctos was an evolutionary precursor to the modern panda, smaller but with anatomical features suggestive of a similar lifestyle, including a bamboo diet. The modern panda’s false thumb has some advantages over the earlier version.
“The crochet false thumb offers a tighter hold of the bamboo and at the same time the less protruding point – because of the curved hook – makes it easier for the panda to walk. Think of the false thumb as on every step time the panda is walking. And that’s why we think that’s why the false thumb in modern pandas has gotten shorter, not longer,” Wang said.
The panda’s firm grip on bamboo counteracts the jarring action of the mouth to quickly break food into bite-sized chunks, Wang added.
The researchers initially found an Ailurarctos arm bone in 2010, then discovered teeth and the false thumb in 2015, giving them a much better understanding of the animal. To date, the oldest known evidence of this thumb-like structure has been dated to fossils from about 102,000 to 49,000 years ago in the same panda species living today.
The false thumb allows pandas to hold bamboo to eat, but not rotate the food as a real thumb would allow.
“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 grasping. The panda’s false thumb is much less effective than the human thumb, but it is sufficient to provide the giant panda with the grasping ability to eat bamboo,” said paleontologist and co-author Tao Deng of the Chinese Academy of Sciences in Beijing.
Pandas, one of the world’s eight bear species, once lived in much of Asia. They now mainly live in temperate forests in the mountains of southwestern China, with an estimated wild population of less than 2,000.
A panda’s diet is 99% vegetarian, although they sometimes eat small animals and carrion. Due to their inefficient digestive system, pandas consume large amounts to meet their nutritional needs — 12 to 38 kilograms (26 to 84 pounds) of bamboo while feeding for up to 14 hours a day.
The false thumb was not present in another closely related bear that lived about 9 million years ago, the researchers said.
“This is an amazing innovation — transformation of a small bone into an element that is useful for a particular purpose,” said Harvard University paleobiologist and co-author Lawrence Flynn.
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