Pandas evolved their most perplexing feature at least 6 million years ago

It is well known that giant pandas are picky eaters. They solely eat bamboo, and each day they may consume up to 99 pounds (45 kilograms) of it in 15 hours.

Though it was believed that the present pandas' specialized diet originated relatively recently, their forebears, like the majority of bears, consumed a far wider diet that included meat. According to a recent research, pandas may have developed a special fondness for bamboo at least 6 million years ago, probably as a result of the plant's extensive distribution and year-round availability.

Modern pandas (Ailuropoda melanoleuca) have an unusual sixth finger, a type of thumb that enables them to readily grab bamboo stalks and peel the leaves, allowing them to thrive primarily on low-nutrient bamboo.

"Perhaps the most important adaptation to ingesting a prodigious amount of bamboo is to tightly grip bamboo stems in order to smash them into bite sizes," said research author Xiaoming Wang, curator of vertebrate paleontology at the Natural History Museum of Los Angeles County, in a statement.

Since there are few fossils of pandas, it is difficult to understand how the bears acquired this puzzling trait that has long confounded researchers. According to earlier studies, this thumb-like feature existed between 100,000 and 150,000 years ago.

Wang and his colleagues discovered fossilized digits from a finger that dates back 6 to 7 million years, providing considerably earlier evidence that pandas had an additional finger and, thus, only consumed bamboo. The fossil was found in Yunnan Province in southwest China and belongs to the Ailurarctos panda ancestor.

The new study was released in the journal Scientific Reports on Thursday.

The sixth digit of the giant panda is not as graceful or dexterous as a human thumb, but the persistence of this "distinctive morphology" through millions of years suggests that it serves a crucial survival role, according to the study.

Evolutionary conciliation

The fact that this prehistoric structure was lengthier than that of current giant pandas, who have a shorter, hooked sixth finger, puzzled the study's experts more than anything else.

According to Wang and his colleagues, the shortened sixth digit of modern pandas is the result of an evolutionary trade-off between the requirement to carry and move their bulky bodies while walking and manipulating bamboo.

"Five to six million years should be enough time for the panda to develop longer false thumbs," said study coauthor Denise Su, an associate professor at the School of Human Evolution and Social Change and research scientist at the Institute of Human Origins at Arizona State University, in a statement. "However, it seems that the evolutionary pressure of needing to travel and bear its weight kept the 'thumb' short — strong enough to be useful without being big enough to get in the way."