There was something strange about the assemblage of Homo erectus fossils found at Zhoukoudian - the famous 750,000 - 200,000 year old site in China popularly known as Dragon Bone Hill. Despite the abundance of skulls and teeth, there were hardly any remains of the hominins from below the neck. Where were the bodies?
The majority of Homo erectus fossils from Zhoukoudian were discovered and studied by an international team of scientists during the 1920's and 1930's. (Unfortunately most of the specimens were lost with the outbreak of WWII, but casts of these early discoveries remain.) They were just what paleoanthropologists had been hoping to find - evidence that human evolution had primarily taken place in Asia (a hypothesis later overturned by discoveries in Africa) - but the dearth of postcranial remains was puzzling. Clearly something must have happened to bias the fossil record so that skulls were more likely to be preserved than bodies.
At this time many paleoanthropologists believed that, as the English philosopher Thomas Hobbes once put it, the lives of early humans as "nasty, brutish, and short." The heavy brows and robust bones of hominins like Neanderthals and Homo erectus testified of a time when strength and savagery were more important to survival than intelligence or culture. No doubt these prehistoric people were just as brutal to each other as they were to the animals they hunted, and so the horrifying act of cannibalism seemed like a plausible explanation for what the scientists found at Zhoukoudian.
The cannibalism hypothesis was most fully articulated by the German anatomist Franz Weidenreich. According to him, the Zhoukoudian cave sites held the fossilized aftermath of a grisly slaughter of over 40 individual Homo erectus, perhaps the victims of a more "advanced" kind of human. This was not just idle speculation. Many of the Homo erectus skulls were marked by nicks, pits, and cuts, and Weidenreich thought that this kind of damage could only have been inflicted by tool-wielding humans. If true, this meant that the cave at Dragon Bone Hill might have been the lair of stone-age murderers:
The Sinanthropus [=Homo erectus] remains must have been brought into the cave as parts already severed from the body; perhaps they represent trophies or, more probably, the hunting spoil of head hunters.
Other authorities who had worked at the same site, such as Pei Wenzhong, disagreed. Rather than being a haven for murderous Homo erectus, the cave appeared to be a hyena den, and even Weidenreich eventually admitted that at least some of the damage done to the skulls could have been inflicted by carnivores. Even so, the idea of bloodthirsty "ape-men" was publicly popular, and the work of anthropologist Raymond Dart, ethologist Konrad Lorenz, and writer Robert Ardrey affirmed that our species had been born from a violent, "blood bespattered" past. The atrocities of WWII, especially, made it easy to believe that our species was still living with the violent legacy of our dark origins, and despite the intellectual progress of our species there was still a shadowy part of ourselves that drove us to bloodshed.
As popular as this vision of our past was, however, anthropologists were developing a less grisly interpretation of human prehistory. Hunting and meat-eating were important during particular times, but they were not the driving forces behind our evolution. Eventually Dart's murderous australopithecines and Weidenreich's bloodthirsty Homo erectus faded from view, but this still left the mystery of why the Zhoukoudian cave mainly contained fragments of skulls. With the cannibal hypothesis discarded, and the idea that there was some sort of cave collapse which shattered the bodies of the hominins in doubt, there did not seem to be a satisfying solution to the mystery.
But what about the hyenas? The idea that the cave was a hyena den populated by the immense, extinct species Pachycrocuta brevirostris had seemingly been forgotten, but in 2000 Noel Boaz, Russell Ciochon, Xu Qinqi, and Liu Jinyi reopened the debate. In a paper published in the journal Acta Anthropologica Sinica (and later expanded in a 2004 Journal of Human Evolution paper) they argued that rather than being the dominant hunters in the area, Homo erectus were often prey for the large carnivores.
The bone fragments found in the cave, as well as the damage done to them, were consistent with damage a Pachycrocuta might inflict. Of the few Homo erectus limb bones found at the site, for example, most were represented by shafts; the ends had been broken or gnawed off to get at the marrow inside. They are indicators that the hyenas, much like their living relative the spotted hyena (Crocuta crocuta), obliterated the carcasses of whatever animals they captured. Once the flesh was gone they would begin to crack open and even consume the bones (as evidenced by tooth-marked, acid-etched bones found in the cave). No wonder so little was left of the hominins.
The Homo erectus heads, however, presented the hyenas with a unique problem. The jaw muscles and the tongue were the only easily-available parts worth eating, but inside the skull was the large, fatty brain. It would have no doubt been a delicacy for hyenas, and given what is known about the way living hyenas eat Boaz and colleagues attempted to reconstruct how Pachycrocuta opened Homo erectus skulls.
The hyenas probably started with what was most easily available. After stripping the jaw muscles from the skull they would have easily been able to crack off the lower jaw and eat the tongue. By this point a catastrophic amount of damage would have been done to the Homo erectus head, thus explaining why the faces of the skulls found in the cave are missing.
Getting at the brain, however, was a little trickier. As reconstructed by the scientists, the hyenas clamped onto the front of the skull, braced the bottom against the ground, and pressed downward. With enough pressure this would have cracked open the skull so that they brain could be extracted, and whatever scraps of bone survived this ordeal would be left to litter the cave.
The individual hominins we once considered to be our proud, if murderous, ancestors were really "food refuse" of giant hyenas. How so many Homo erectus fell prey to the hyenas over thousands of years is unknown, but it is possible that the cave was a sort of natural trap. The hyenas would simply have to wait for some animal to fall or injure itself at which point they could dine at their leisure. The presence of scorched bones and tools suggest that Homo erectus inhabited the cave at some point for some period of time, but for much of its history it appears that the cave was a hyena den.
It might be expected that such an ancient "crime scene" would be rare, but new research has revealed that many important hominin fossils bear tell-tale signs of predation. The group of Australopithecus afarensis known as the "First Family", for instance, may represent a massacre at the hands of carnivores, and the "Taung Child" specimen of Australopithecus africanus was scored by the talons of a large bird of prey. Even the fossils of the recently-described early hominin Orrorin bear bite marks, and several months ago paleontologists identified the "horned" crocodile (given the fitting moniker Crocodylus anthropophagus) which may have left tooth marks on Homo habilis footbones from Olduvai Gorge. It is chilling to consider the fate of such individuals, but through their messy feeding habits some predators may have accidentally helped some of our ancient relatives enter the fossil record.
Noel T. Boaz, Russell Ciochon, Xu Qinqi, and Liu Jinyi (2000). Large Mammalian Carnivores as a Taphonomic Factor in the Bone Accumulation at Zhoukoudian Acta Anthropologica Sinica, 19, 224-234
Boaz, N. (2004). Mapping and taphonomic analysis of the Homo erectus loci at Locality 1 Zhoukoudian, China Journal of Human Evolution, 46 (5), 519-549 DOI: 10.1016/j.jhevol.2004.01.007
Accusing our ancestors to have been bloodthirsty killers really is a poor attempt at excusing the horrors of WWII, and others. Which really could never have happened without a specific social and economic organisation with mass industrial production. As Gould (quotung Condorcet) loved to say, this is really "using nature as an accomplice".
All of which reminds me: I've never seen anyone with serious anthro/evo credentials respond to Bruce Chatwin's speculation in The Songlines  that enlarged human brains are - at least in part - the result of interaction between Homo habilis and predatory leopards (whose teeth, he reported, exactly match the holes in early hominid skulls found in South African caves).