Wendy Zukerman, Asia Pacific reporter

(Image: Pete Oxford/Nature Picture Library/Rex Features)

The giant panda may have taken longer to go vegetarian than previously thought. As recently as 2 million years ago the panda's ancestors may have been tucking into meat as well as chewing on bamboo - despite evidence that they'd long lost a taste for flesh.

Consensus has it that the giant panda's ancestors began replacing flesh with bamboo 7 million years ago, and were committed vegetarians by the 2 million year mark. Indeed, a genetic study last year suggested that the giant panda lineage lost the ability to taste flesh 4.2 million years ago, meaning that even if meat was available at a later date the bears would have been less inclined to eat it.

But the new study suggests that old habits die hard.

Borja Figueirido at the University of Malaga, Spain, and colleagues compared the skulls of the living giant panda with two extinct panda species: the pygmy panda (Ailuropoda microta) that plodded the Earth 2 million years ago and the 100,000-year-old Ailuropoda baconi. They also analysed the skulls of 171 living bear species that don't eat bamboo.

By comparing 38 "landmarks" on the skulls - points that change slightly among different bear species such as the top of nasal bones and the space between the cheek bone and braincase - the team gleaned information about their likely dietary preferences.

They found that the skulls of the giant panda and Ailuropoda baconi were so similar - and so distinct from the other bears - that both likely chewed the same food: bamboo.

But the 2-million-year-old pygmy panda hadn't completed the adaptation. Its cranial cavity was smaller than that of Ailuropoda baconi and the living giant panda, and so housed smaller chewing muscles. The pygmy probably had a weaker bite than today's giant panda and would not have been able to break thick bamboo stems, Figueirido speculates.

Mike Archer, an evolutionary biologist at the University of New South Wales, Australia, who was not involved in the work, thinks the skull study suggests that the pygmy panda may have supplemented its diet with meat - like other bears.

However, he adds that the pygmy panda may be "a deviant cousin" of today's giant panda rather than a direct ancestor. So even if it ate meat, its contemporaries on the giant panda branch of the family tree might not have done so. "We need a better fossil record," he says.