"The skull was exceptionally smooth, without any signs of intentional cutting or hacking," he said. "That troubled me deeply."
Since the 1950s, Huang, 76, from the Institute of Vertebrate Palaeontology and Palaeoanthropology at the Chinese Academy of Sciences, has dug up, collected and examined just about every giant panda fossil fragment unearthed in China. Many were found at sites associated with ancient humans.
Most of the giant panda bone fossils, from leg bones to teeth, bear marks showing that they had been smashed and cracked with stone tools by early humans, suggesting that our ancestors had sucked out the marrow.
But Huang said the two-million-year-old skull, unearthed in a limestone cave, might tell a different story. At the same site, Huang had discovered the fossils of other animals and signs they had been eaten by humans.
How, he wondered, in those days of food scarcity, would early humans have restrained themselves from opening the skull and eating the tasty brain?
"In earlier times, pandas and humans may have been friends," he said. "Then we ate them. Now we keep them as pets."
The early giant pandas should have been easy prey. They were about a metre long, nearly a third shorter than their modern counterpart. Their teeth, though still retaining some of the characteristics of a meat eater, had wear patterns and unique muscle markings suggesting heavy chewing on bamboo.
Their legs and body structure limited their speed and, as a survival strategy, they spent most of the time in bamboo forests because predators such as the now extinct sabre-toothed tiger lost their speed and agility amid the thick bamboo.
Early humans in China, on the other hand, were quite weak, Huang said, drawing on his observations at Wushan. Although they could make some very primitive stone tools, their brains were not much larger than those of an ape and they may have behaved more like monkeys than modern humans. They may have spent most of the time in the trees to avoid predators. They did not have the intelligence or strength to launch collective hunts for other animals.
"Early humans and giant pandas stood on the same level of the food chain," Huang said. "They faced similar predators such as sabre-toothed tigers. That made it possible for them to live together harmoniously."
Wei Guangbiao, a researcher at the Three Gorges Museum in Chongqing who co-authored a book on the origins of the giant panda with Huang last year, said the early giant pandas may have benefited from their unique look.
Before the animal was found and named by Westerners, many farmers and hunters in Sichuan had regarded giant pandas as sacred, Wei said.
"Locals didn't hunt or eat giant panda because they were afraid that it might anger the spirits in the mountains," he said. "The black and white fur of the panda resembles the yin-yang symbol of Taoism. Such a colour combination is not camouflage. It is alien to the environment. You can imagine the shock of a farmer when he saw a large, black-and-white animal coming out of the bamboo. To them, it must have seemed like an animal that had come from another world."
Widespread and intensive hunting of giant pandas started at least half a million years ago, according to fossil records. A famous giant panda leg bone discovered in the 1930s by Pei Wenzhong, the man who discovered Peking Man (Homo erectus pekinensis), had been hacked by a flint. It was estimated to be about 500,000 years old.
At the time, humans had already learned to produce better stone tools with sharper and finer edges. Collective hunting was believed to have begun with some large animals, such as mammoths, on the menu .
Huang said giant pandas became hunted at more or less the same time throughout the country, from the south to northern regions beyond the Yellow River.
"Compared with other animals, giant pandas provided lots of meat," he said. "They were also relatively easy to kill. Though its fur was unique, the growth of the population had driven human hunters to hunt any animal within reach of their spears and arrows to feed their starving tribes.
"We turned friends into prey as soon as we obtained the strength."
Some have blamed climate change for driving the giant panda close to extinction, with a general decline in temperatures over the past few thousand years wiping out many bamboo forests. During the Tang dynasty (618-907), bamboo forests were still found north of the Yellow River, according to historical records.
Huang suspects giant pandas originated in Yunnan some eight million years ago and spread to the rest of the country by several routes. All branches of the species died out except those that remained in remote bamboo forests in such regions as Sichuan and Shaanxi. (South China Morning Post)