Giant pandas are food riddles. Despite being part of the carnivorous order Carnivora, pandas usually practice a plant-based diet, avoiding salmon and seal meat at the bear family barbecue for bamboo shoots. And because they don’t have multi-chambered stomachs to get nutrients from the tough plant material, the chubby bears eat about 30 pounds of bamboo each day to support themselves.
To scoop stems into their mouths, pandas use a sixth, thumb-like digit on their legs to grab shoots like a human holding a churro. This pseudo-thumb is handy – pandas need a firm grip when gnawing on stiff bamboo. “It’s not nearly as good of a thumb as ours, so they can’t make tools or complex movements,” said Xiaoming Wang, a paleontologist with the Natural History Museum of Los Angeles County. But the rough “thumbs” are more than capable of gripping bamboo.
Scientists have long been amazed at this vestigial thumb, which is actually an excellent extension of the panda’s wrist bone. But a lack of fossilized panda paws has made it difficult to decipher when the strange trait originated. For years, the earliest evidence was only about 150,000 years old. But in a study published Thursday in the journal Scientific Reports, Dr. Wang and his colleagues found that panda relatives have been using pseudothumbs for millions of years.
In 2015 Dr. Wang was digging with a team of paleoanthropologists at an open-cast mine in southwestern China when he found fossilized pieces of an ancient bear. A spoon-shaped piece of bone caught his eye. “Intuitively, I thought it was a fossilized panda thumb,” said Dr. Cheek. Comparing the fossil to modern panda skeletons confirmed his suspicion. After analyzing fossil teeth found nearby, the team concluded that the false thumb belonged to Ailurarctos, an ancestral panda that lived during the Miocene, six million to seven million years ago.
As the earliest example of a panda pseudothumb, the researchers expected the extra digit on Ailura arctos to be primitive, but the team found it was noticeably larger than that on modern pandas. Live pandas, however, probably have better grip. Unlike the straight thumbs of the fossilized bear, the pseudo-thumbs of modern pandas are curved inward like a hook.
Pushing the origins of panda pseudothumbs back millions of years raises a perplexing question: Why did these studs never develop into versatile, real thumbs? The emergence of a more pliable cipher would make evolutionary sense.
In the new article, Dr. Wang and his colleagues argue that pseudothumb size is limited by how pandas slog. When not lounging, they walk on all fours. “We think the pseudothumb is an evolutionary balancing act,” said Dr. Cheek. “You need it to grab, but you also keep kicking it.”
The scientists believe that if the bony prominence grows too large, it can become a painful spur on the underside of the leg. Modern panda pseudothumbs, which terminate in a flat surface and are cushioned by a fleshy toad, are slightly better adapted to carry the panda’s girth. Any bigger digit can be crushed.
Not every researcher is sold on this reasoning. Juan Abella, a paleontologist at Spain’s Catalan Institute of Paleontology who helped discover the earliest known panda ancestor, says the location of the extended wrist bone at the back of the paw may have little impact on locomotion. Even if it did, he believes the benefits of an advanced thumb outweigh the potential drawbacks for a sluggish animal that spends up to 16 hours a day feeding.
“Usually, when an anatomical feature causes some kind of compromise in a species, the benefits gained will far outweigh any potential drawbacks,” said Dr. abella.
Whatever kept the panda pseudothumb from making the final jump, they’re not the only mammals with extra, underdeveloped digits. Fossils reveal that the Miocene predator Simocyon batalleri the size of a puma had pseudo-thumbs, which were passed on to the red panda. Certain primates such as the aye-aye lemur also have an extra thumb-like digit. Some primitive bears, such as the 9-million-year-old Indarctos arctoides, which may be an ancestor of the panda, also possessed sizable wrist bones.
But Ailurarctos appears to be one of the first to use these enlarged bones. According to Dr. Wang, this trait allowed the prehistoric panda to thrive in a jungle teeming with ancient elephants, deer, and monkeys. Despite being low in nutrients and packed with indigestible fiber, fast-growing bamboo shoots were available in bulk and underutilized. And with the help of their extra “thumb”, pandas have been snacking ever since.