A fossilized skull of the earliest giant panda has been discovered revealing that two million years ago the ancient animal was a pygmy.

It is the first skull of the extinct species to be found and provides a clear idea of what the ancient creature would have looked like.

It dates back at least two million years and the remains show that, at three feet (one meter) long, it was little more than half the size of the modern giant panda, which is five feet long.

However, the animal known as Ailuropoda microta, meaning pygmy giant panda shows strong similarities to modern pandas.

The fossil revealed that the bear had heavy wear patterns on its teeth and defined muscle scars, suggesting it had the very powerful chewing mechanism required for a diet of bamboo shoots.

The team does not know if A. microta carried the same distinctive black and white markings as its relative.

The skull of the giant panda's earliest known ancestor also provides more clues into its evolution.

"Bears are generally carnivorous or omnivorous, and then you have pandas - they have gone in a completely different direction, they are committed vegetarians," Russell Ciochon, an anthropologist at the University of Iowa, US, and an author on the PNAS paper, said.

"Early on in the evolutionary history of pandas, they must have invaded this bamboo niche and begun to eat bamboo.

"Given the food source they were eating was very prevalent, then they must have become more and more specialized. It probably has been exploiting this kind of environment for many millions of years."

A. microta is thought to have lived in a moist lowland tropical forest habitat where bamboo was one of the most dominant plant types.

Today, the giant panda lives in the upland bamboo forests of Sichuan. However, half of the panda's mountainous bamboo habitat was lost between 1974 and 1988 and the animal is listed as Endangered on the IUCN Red List of Threatened Species.

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