Their ancestors, like most bears, ate a much wider diet that included meat, and it was thought that modern pandas' exclusive diet has evolved relatively recently. According to a new study, pandas' passion for bamboo may have originated at least 6 million years ago - possibly due to the plant's wide, year-round availability.
Modern pandas Ailuropoda melanoleuca have developed a special sixth finger, a thumb of sorts that allows them to grasp bamboo stalks and strip the leaves, because they have only survived on low-nutrient bamboo.
The study author Xiaoming Wang, curator of vertebrate paleontology at the Natural History Museum of Los Angeles County, said that 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.
Wang and his team identified evidence of pandas having an extra finger and thus an all-bamboo diet in the form of a fossilized digit dating back 6 to 7 million years. The fossil, unearthed in Yunnan Province in southwest China, belongs to a panda ancestor known as Ailurarctos. The Shuitangba fossil site in Yunnan, China is an artist's reconstruction of the giant panda ancestor Ailurarctos. The new research was published Thursday in the journal Scientific Reports. The persistence of this distinctive morphology over millions of years suggested that it plays an essential function for survival, although the sixth digit of the giant panda is not as elegant or dexterous as human thumbs. The scientists who were involved in the study said that this fossilized structure was longer than those of modern giant pandas, which have a shorter, hooked sixth finger. The study says that bacteria help pandas get the most out of being picky eaters, while Wang and his colleagues think modern pandas' shorter sixth digit is an evolutionary compromise between the need to manipulate bamboo and the need to walk and carry their hefty bodies. 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 enough to be useful without being big enough to get in the way, 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.