Hong Kong, 1935: a young palaeontologist picks his way through the backstreets, ducking in and out of apothecary shops. He's looking for dragon teeth, the Chinese name for old animal teeth used in traditional medicines. In a dusty drawer of trinkets, his eyes fall on a large molar unlike that of any living animal, and he instantly knows his search is over. The tooth belongs not to a dragon but an ape, and if its teeth are anything to go by, it was huge.
So begins the story of the discovery of a truly fantastic beast, the greatest of all great apes. According to some estimates, it stood 3.5 metres tall, weighed more than 500kg, and stalked the nightmares of the earliest humans to reach China. Its name? Gigantopithecus.
Eighty years after Ralph von Koenigswald stood dumbstruck in a Hong Kong pharmacy, fossils of the giant ape remain sparse. A jawbone fragment described earlier this year is just the fourth ever found. Those four pieces and several thousand teeth are our only evidence it even existed. But from these scraps, we are slowly piecing together an image of this real-life King Kong, how it lived and why it eventually vanished from the face of the planet.
Von Koenigswald was born in Berlin, Germany. From an early age, he yearned to hunt for evidence of humanity's ancestors. That meant one thing: a journey to Southeast Asia, home to the oldest and most primitive human fossils at the time. After a stint looking for Homo erectus in Indonesia, von Koenigswald's attentions turned to orangutans and their poorly studied ancestors. His particular stroke of luck was born of the realisation that he could take his search to Asian apothecary shops, where ground-up teeth are an important part of many traditional medicines.
"I began to hunt for fossils in the Chinese drugstores in Java," he later wrote. "I discovered that I had made a grave mistake in simply inquiring about 'teeth'. I should have asked for 'dragon teeth', since that was the name of the 'drug' I sought. When I finally learned the correct name and obtained a prescription, I succeeded in finding these teeth in every Chinese drugstore in every Chinese community."
Subjected to von Koenigswald's expertise in extinct fauna, the so-called dragon teeth proved almost systematically to be fossils of ancient mammals, from horses to large giraffes.
In 1935, he headed for Hong Kong - a decision that would lead him to one enormous molar and change his career.
"He took one look at that tooth," says Russell Ciochon, a palaeoanthropologist at the University of Iowa, in the United States, "and knew it was ape. And it was huge." At roughly 2.5cm across, its grinding surface was easily twice as big as a typical human molar.
For the next few years, von Koenigswald scoured pharmacies for more evidence of the extinct behemoth, with little success.
"The rarity of this giant form is obvious," he wrote. By 1939, he had examined thousands of fossil teeth but discovered only three more belonging to Gigantopithecus.
Still, each new find added weight to the idea that he was dealing with a new species. He was categorical that the four molars had belonged to four separate individuals. He also noticed a trend that gave him clues about when the apes had lived. In shopkeepers' trays, the molars were always found mixed in with other teeth in a similar state of preservation. Von Koenigswald knew the owners of these other teeth well: giant pandas, tapirs, bears and the extinct, elephant-like stegodon. The giant ape was surrounded by middle Pleistocene animals. It was about a million years old.
Then came the war. Japan's imperial army rolled into Java in 1942 and von Koenigswald was interned in a prisoner-of-war camp. The teeth he had gone to such lengths to collect saw out the Japanese occupation in a milk bottle buried in a neighbour's garden on the island. For von Koenigswald's colleagues in the US, the wait was interminable. Franz Weidenreich at the American Museum of Natural History, in New York, was particularly exasperated. He had big ideas about Gigantopithecus and was itching to share them with the scientific world.
In 1945, with von Koenigswald still a prisoner, Weidenreich pushed ahead with the publication of his theory. He had noticed that smaller breeds of dog often have relatively large brains and small faces compared with larger ones. Humans also have much larger brains and relatively smaller faces than chimps, gorillas and orangutans, something that had puzzled researchers for years. Weidenreich believed it all made sense if our family tree included some unusually large ancestors. As the millennia passed, our bodies shrank but our brains remained large. In other words, we were descended from Asian giants, and Gigantopithecus was one of them.
"No one gives the idea much credibility now," says Ciochon. "If von Koenigswald had not been in prison, that episode probably would never have happened." But it's not the craziest theory ever concocted and, for a brief time, Weidenreich's idea that humans evolved from an enormous Asian ape was in the scientific mainstream. Then the war ended, von Koenigswald was released, and he gently but firmly plucked Gigantopithecus out of the human evolutionary tree and plonked it back with the other apes.
The downgrading didn't diminish scientific interest. Far from it: Chinese authorities, determined to find fossils of the giant ape in their proper geological context, launched a series of field expeditions in the 1950s. The quest took them to southern China's fantastical karst landscapes, where limestone sugarloaf mountains loom over rice paddies. They met with farmers who had long collected Gigantopithecus teeth to sell on the lucrative medicinal market, and were directed to caves carved into the sheer cliff faces.
In Liucheng cave, in the Guangxi Zhuang autonomous region, no fewer than 1,000 Gigantopithecus teeth were recovered, together with fragments of three giant jawbones. From the size of the jaw fragments, the researchers deduced that Gigantopithecus might have stood 3.5 metres tall. In truth, there's little to go on. Ciochon believes the ape was more like 2.5 metres tall.
Von Koenigswald named the Chinese species Gigantopithecus blacki. Its oldest reliably dated remains are 2 million years old. Another, older and slightly smaller species has also been found in northern India: G. giganteus, known from an 8.6-million-year-old jawbone and teeth.
The idea that the enormous apes might have been part of our family history stuck around until quite recently. In the 80s, Bruce Gelvin, of California State University, argued that if it were not a direct ancestor, it might have been a very early cousin species, much like the African Australopithecus species. Around the same time, a team that included Ciochon found more than a dozen Gigantopithecus teeth with a couple of stone tools in Longgupo cave, in Wushan county, Chongqing. And two years ago, a Chinese team raised the possibility that Gigantopithecus was the toolmaker.
Ciochon rejects the idea. If they did make tools, he says, these would probably have been made of organic material such as sticks, bamboo or leaves, and in any case tools are not necessarily evidence of an intelligent ancestor. There are at least three living species of non-human primate that use stone tools: chimpanzees, bearded capuchins and long-tailed macaques. Ciochon says the stone tools at Longgupo were probably left much later, by early humans.
There's another reason to doubt the image of Gigantopithecus as a cave-dwelling toolmaker. The caves its teeth were found in were not caves when the ape was alive. At the time, the deep valleys had not yet been carved into the karst landscape to produce the iconic limestone mountains. Ground level was much higher up, and what we now see as lofty caves dug out of cliff faces were small sinkholes and underground fissures. They were home to porcupines, not giant apes.
Those porcupines offer an answer to one of the enduring questions about Gigantopithecus: how come it left so many teeth and so few bones? Porcupines need calcium to make their quills, explains Ciochon. They are known to gather bones, drag them back into their underground lairs and gnaw on them until there's nothing left. You could say that Pleistocene porcupines ate most of the evidence for the world's biggest ape.
It's lucky, then, that we can reconstruct a fair amount about an animal from its teeth and jaws alone.
"Even from tiny pieces of dental tissue, we learn about the diet, ecology and life history," says Kornelius Kupczik, at the Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany. What emerges is a picture of Gigantopithecus as a forest-dwelling herbivore. Analysis of carbon and other elements in the fossil teeth reveals no signs of a meat diet: the ape was vegetarian.
Gigantopithecus also suffered from unusually poor dental health. Something like 10 per cent of the teeth that have been found have cavities, and the recently discovered fourth jawbone shows that some individuals lost a tooth or two during their lives. Both features suggest the ape ate lots of sugary fruit. Earlier this year, Hervé Bocherens, of the University of Tubingen, in Germany, published a study of Gigantopithecus teeth that argued we should think of the ape as an overgrown orangutan confined to the forest floor by its great size.
The shape of its dental roots suggests another possibility, say Kupczik and Christopher Dean, professor of anatomy at University College London. It had a bite powerful enough to chew through tough foods such as bamboo. Other lines of evidence, including food residues on some teeth, also suggest bamboo was on its menu. In other words, perhaps Gigantopithecus was more like a giant panda than an oversized orangutan.
Either diet leaves a central puzzle unsolved. Both species that Gigantopithecus has been compared with - giant pandas and orangutans - lived alongside it in Asian forests. Why did they survive and the ape die out?
"The extinction of Gigantopithecus blacki is still mysterious," says Zhang Yingqi, at the Chinese Academy of Sciences, in Beijing.
On present evidence, it seems it may have vanished about 320,000 years ago. This is the age of the youngest teeth yet found, which Zhang and his colleagues described in 2013.
We now know that those last apes might well have been the grandest of the bunch.
Last year, Zhang, Ciochon and their colleagues collaborated on a study of teeth from a number of sites. They were able to confirm that the last Gigantopithecus had exceptionally large teeth, even by its own standards. The biting surfaces were 1.5 times larger than those of earlier members of the species.
"This does not necessarily mean their body size increased accordingly, although I prefer to believe so," says Zhang. If he's right, then Gigantopithecus was larger than it had ever been just before vanishing.
Zhang and his colleagues recently noticed something else about those last teeth. The biting surfaces were not just larger, they were also distinctly more complex, which might hint that the giant ape was facing a drastic change in its menu before its extinction, perhaps because the region's plants were changing.
Bocherens agrees with this assessment. His team's work suggests that while Gigantopithecus ate any leaves, shoots and fruits it could lay its hands on, it drew the line at grasses and other vegetation from the savannah. That doomed the species, says Bocherens, because climate change during the Pleistocene meant grasslands got bigger at the expense of forests, where the apes lived.
That might not be the full picture, though. Homo erectus arrived in Southeast Asia about 1.7 million years ago and could have rubbed shoulders with Gigantopithecus for a million years. If Homo erectus hunted the apes, or competed with them for resources, this might have added to their problems. Gigantopithecus may even have been one of the first species that humans pushed towards extinction. Then again, maybe not - for here, too, the Gigantopithecus story is plagued by open questions.
In the mid-1990s, Ciochon and others found Gigantopithecus and Homo erectus teeth together in Chinese and Vietnamese caves, and declared that the two apes had had a "long co-existence". But Ciochon now thinks the Homo erectus teeth belonged to another ape, and there were no hominins in the cave.
"I think today most people agree humans and Gigantopithecus were not living side by side," he says. "They were certainly both present in China 1 million years ago, but I don't think they were inhabiting the same areas."
Gigantopithecus, he says, was confined to dense forests, whereas early humans were far more likely to live and hunt in open grasslands. That's not to say they never, ever met.
"Let's not forget that humans may kill for other purposes than feeding," says Bocherens. "We cannot rule out that humans hunted Gigantopithecus even if they did not feed on the same resources." Proof would come with the discovery of Gigantopithecus bones with butchery marks. "This is probably a bit too much to ask," he concedes.
We've learned more about Gigantopithecus in recent years than von Koenigswald could have hoped as he stood in that Hong Kong pharmacy 80 years ago. Yet still the story of our largest fellow ape eludes us.
"We need to find its face," says Zhang. Such a momentous discovery would at last tell us what Gigantopithecus really looked like.