Pandas have been eating bamboo for at least 6 million years

By Stephen Beech via SWNS

Pandas have been munching on bamboo for at least six million years, suggests a new fossil discovery.

It is the earliest evidence by more than 5.8 million years of a thumb-like sixth digit used by giant pandas and their ancestors to grip its favorite food.

Scientists say their findings, published in the journal Scientific Reports, indicate that the panda’s dedicated bamboo diet may have originated at least six million years ago.

In addition to five digits on their hands, modern giant pandas (Ailuropoda melanoleuca) have an enlarged wrist bone with a thumb-like structure that they use to manipulate bamboo.

Previous research documented evidence of the thumb-like structure to just 100,000 to 150,000 years ago.

Paleontologist Professor Xiaoming Wang and colleagues examined the wrist bone of an individual from the ancestral panda genus Ailurarctos that was discovered at Shuitangba, a site near the Chinese city of Zhaotong that dates back to the late Miocene period, around six to seven million years ago.

They compared the shape and size of the bone to previously published information on the wrist bones of modern giant pandas and Indarctos arctoides – an ancient bear that lived nine million years ago and may share the same common ancestor as giant pandas.

The researchers found that the modern giant panda’s thumb-like structure has the same distinctive shape as Ailurarctos’s wrist bone but not I. arctoides’s, which was larger, wider and more hooked.

Prof Wang, of the Natural History Museum of Los Angeles County , said: "This indicates that, while the thumb-like sixth digit was not present in I. arctoides or the common ancestor it shares with pandas, it has been present within the panda lineage – and used to grip bamboo - for at least six million years."

He said that although the sixth digit was present in both modern giant pandas and Ailurarctos, the researchers observed differences in its size and shape.

Prof Wang said: "The modern giant panda’s digit significantly shorter than Ailurarctos’s in relation to its body size and has a hook on the end of it and a flattened outer surface, while Ailurarctos’s does not."

The research team suggest that the hook may help modern pandas to better grasp bamboo, while the shorter length and flattened outer surface may assist with weight distribution when walking.

They said that the weight-bearing constraints could be the main reason that the giant panda’s thumb-like structure never evolved into a full digit.

Project co leader Dr. Denise Su, of the Institute of Human Origins at Arizona State University, said: "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.”

Through its long evolutionary history, the panda’s hand has never developed a truly opposable thumb and instead evolved the thumb-like digit from a wrist bone, the radial sesamoid.

While the false thumb in living giant pandas has been known for more than 100 years, how the wrist bone evolved was not understood due to a near-total absence of fossil records.

Prof Wang said: “Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries to quietly consuming bamboos, a plant plentiful in the subtropical forest but of low nutrient 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.”

The fossil Prof Wang and his colleagues discovered revealed a longer false thumb with a straighter end than its modern descendants' shorter, hooked digit.

Prof Wang said: “Panda’s false thumb must walk and ‘chew’. Such a dual function serves as the limit on how big this ‘thumb’ can become."

“Evolving from a carnivorous ancestor and becoming a pure bamboo-feeder, pandas must overcome many obstacles.

“An opposable ‘thumb’ from a wrist bone may be the most amazing development against these hurdles.”