With its cuddly physique and black saucer eyes, the panda may be the most famous exotic animal in the world. But surprisingly little is known about the creature's evolutionary history. That may change, thanks to the discovery of the oldest panda skull yet found. Among other insights, the 2-million-year-old fossil indicates that pandas have been tucking into bamboo for a very long time.
Pandas have proven a problematic creature to pigeonhole. In the early 20th century, biologists surmised that pandas were members of a herbivorous raccoonlike species called procyonids. But in the 1950s, anatomical studies indicated that pandas were instead a type of bear--a finding that was confirmed 2 decades later with genetic data. Still, there has been virtually no record of how pandas evolved over 6 million years from a carnivorous Miocene ursid to the modern panda's precursor, the 1 million-year-old Ailuropoda baconi.
The new skull helps fill this evolutionary hole. It was discovered 6 years ago in a limestone cave in southern China by two paleontologists from the Chinese Academy of Sciences in Beijing. The teeth and the shape of its bones indicate that the owner was a precursor of the modern giant panda, says anthropologist Russell Ciochon of the University of Iowa, Iowa City, a co-author of the new study. In particular, the thick-enameled teeth, short canines, and molars with multiple cusps for grinding tough fibers, suggest that the unique vegetarian diet of today's pandas was adopted more than two million years ago.
The all-bamboo diet enabled the bears to rapidly expand in size. The fossil skull belonged to a creature measuring less than a meter, says Ciochon, whereas today's pandas are closer to 1.5 meters. "Once you make the shift to vegetarianism and become very specialized, then you get bigger."
The skull "fills an important evolutionary gap," says Blaire Van Valkenburgh, a vertebrate paleontologist at the University of California, Los Angeles. "It's a very nice transitional form."
Whether the ancient panda's distinctive black-and-white coloration was the same as today's panda is unknown, says Ciochon. But given the other resemblances, it wouldn't be surprising, he says. Tropical animals, as these were, tend to have strong coloration so they can be seen through the foliage by their own kind. The team reports its findings online this week in Proceedings of the National Academy of Sciences.