Pandas have been eating bamboo for at least 6 million years, according to international experts who claim to have discovered the oldest fossil of a panda with a thumb-like bone on its paw.

According to popular belief, pandas also evolved a thumb for a superior grip, notably to grasp at bamboo straws.

The panda’s thumb is actually a modified finger that grew from the radial sesamoid bone in its wrist rather than a true opposable thumb.

Because of this unique adaptation, these bears can eat only bamboo even though they are bears (members of the order Carnivora, or meat-eaters).

Xiaoming Wang, curator of vertebrate paleontology at the Natural History Museum of Los Angeles County, and colleagues describe the discovery of the first bamboo-eating ancestral panda with this “thumb” in a recent publication published in Scientific Reports.

It’s longer than its contemporary descendants, which is surprising.

Even though people have known about the famous false thumb of living giant pandas (Ailuropoda melanoleuca) for more than 100 years, they didn’t know how this wrist bone evolved because there were almost no fossil records.

The fossil false thumb of an ancestral giant panda, Ailurarctos, was discovered in the City of Zhaotong, Yunnan Province, south China, and dates back 6–7 million years. This provides scientists with the first look at the early use of this extra (sixth) digit, as well as the earliest evidence of a bamboo diet in ancestral pandas, which aids in our understanding of the evolution of this unique structure.

The NHM Vertebrate Paleontology Curator, Dr. Xiaoming Wang, explains why giant pandas who lived deep in the bamboo forest switched from an omnivorous diet of meat and berries to silently eating bamboos, a plant abundant in the subtropical jungle but with little nutritional value. 

“Tightly holding bamboo stems in order to crush them into bite sizes is perhaps the most crucial adaptation to consuming a prodigious quantity of bamboo.” 

Walking and Chewing Bamboo

This could also help answer a long-standing question about pandas: why do their false thumbs look so small? The false “thumbs” of Ailurarctos, the ancestor of modern pandas, might have been even less developed, but the fossil Wang and his colleagues uncovered showed a longer false thumb with a straighter end than its current relatives’ shorter, hooked digit. So why didn’t pandas’ false thumbs continue to lengthen in order to get a longer digit?

Wang explains, “Panda’s false thumb must walk and ‘chew’. Such a dual function serves as the limit on how big this ‘thumb’ can become.”

The shorter fake thumbs of the modern panda, according to Wang and his colleagues, are the result of an evolutionary compromise between the requirement to manipulate bamboo and the need to move around. A modern panda’s second thumb’s hooked tip allows them to manage bamboo and carry their remarkable weight to the next bamboo feast. After all, the animal’s “thumb” also serves as the radial sesamoid, a bone in the wrist.

According to Denise Su, co-leader of the project, “Five to six million years should be enough time for the panda to develop longer false thumbs, but 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.”

Pandas had to overcome numerous challenges to evolve from a carnivorous predecessor and become a pure bamboo feeder, according to Wang. The most astonishing advancement against these obstacles may be an opposable “thumb” that develops from a wrist bone.

Image Credit: Getty

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