Scientists have found the earliest fossil evidence of a thumb-like sixth digit used by giant pandas to grip bamboo stems, an advance that suggests the bear’s vegetarian diet may have originated at least six million years ago.

A greatly enlarged wrist bone called the radial sesamoid was discovered in fossils in southwest China’s Yunnan province dating back to about six million years, according to research published in the journal Scientific Reports.

The findings shed light on how the giant panda species developed a “false thumb” which enabled it to to grip and break heavy bamboo stems, turning it into the only dedicated vegetarian in the bear family, said researchers, including those from the Chinese Academy of Sciences in Beijing.