Giant pandas are dietary enigmas. Despite being part of the meat-eating order Carnivora, pandas typically practice a plant-based diet, eschewing salmon and seal meat at the bear family barbecue for shoots of bamboo. And because they lack multichambered stomachs to extract nutrients from the tough plant material, the pudgy bears eat around 30 pounds of bamboo each day to sustain themselves.
To shovel stalks into their mouths, pandas utilize a sixth, thumblike digit on their paws to clutch shoots like a human holding a churro. This pseudothumb comes in handy — pandas need a tight grasp as they gnaw at rigid 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 at the Natural History Museum of Los Angeles County. But the crude “thumbs” are more than capable of gripping bamboo.
Scientists have long been perplexed by this rudimentary thumb, which is actually a protruding 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 around 150,000 years old. But in a study published Thursday in the journal Scientific Reports, Dr. Wang and his colleagues posit that panda relatives have been utilizing pseudothumbs for millions of years.
In 2015, Dr. Wang was digging in an open-pit mine in southwestern China with a team of paleoanthropologists, when he came across fossilized bits of an ancient bear. A spoon-shaped bit of bone caught his eye. “Intuitively, I thought it was a fossilized panda thumb,” Dr. Wang said. Comparing the fossil with modern panda skeletons confirmed his hunch. After analyzing fossil teeth found nearby, the team deduced that the false thumb belonged to Ailurarctos, an ancestral panda that lived during the Miocene Epoch, six million to seven million years ago.
As the earliest example of a panda pseudothumb, the researchers expected the extra digit on Ailurarctos to be primitive, but the team found that it was noticeably larger than those found on modern pandas. However, living pandas probably have better grips. Unlike the fossilized bear’s straight thumbs, modern panda’s pseudothumbs are curved inward like a hook.
Pushing the origin of panda pseudothumbs back millions of years raises a perplexing question: Why have these nubs never developed into versatile, true thumbs? The emergence of a more bendable digit would make evolutionary sense.
In the new paper, Dr. Wang and his colleagues hypothesize that pseudothumb size is capped by how pandas plod. When they are not lounging, they walk on all fours. “We think the pseudothumb is an evolutionary balancing act,” Dr. Wang said. “You need it for grasping, but you also keep stepping on it.”
The scientists believe that if the bony protrusion grows too big, it could become a painful spur on the bottom of the paw. Modern panda pseudothumbs, which end in a flat surface and are cushioned by a fleshy pad, are slightly better adapted to carry the panda’s girth. Any larger digit could get 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 toward the rear end of the paw may have little impact on locomotion. Even if it did, he believes the benefits of an advanced thumb would outweigh the potential drawbacks for a sluggish animal that spends up to 16 hours per day eating.
“Usually when an anatomical trait produces some kind of trade-off in a species, the obtained benefits will widely exceed the possible detriment,” said Dr. Abella.
Whatever prevented the panda pseudothumb from taking the final leap, they are not the only mammals to sport extra, underdeveloped digits. Fossils reveal that the puma-sized Miocene predator Simocyon batalleri had pseudothumbs, which were passed on to the red panda. Certain primates like the aye-aye lemur have an extra thumblike digit as well. Some primitive bears, such as the 9-million-year-old Indarctos arctoides, which may be a panda ancestor, also possessed bulky wrist bones.
But Ailurarctos appears to be among the first to put these enlarged bones to use. According to Dr. Wang, this trait allowed the prehistoric panda to thrive in a jungle crowded with ancient elephants, deer and apes. Despite being low in nutrients and packed with indigestible fibers, fast-growing shoots of bamboo were available in bulk and little-used. And with the help of their extra “thumb,” pandas have been snacking ever since.