The Skull-Crushing Hyenas of Dragon Bone Hill

[Author's note: This post gets a little bit graphic, so those who are made squeamish by taphonomy might want to skip this one.]

There was something funny about the assemblage of Homo erectus fossils found at Dragon Bone Hill in Zhoukoudian, China. There were plenty of teeth and skulls but scarcely any post-cranial remains. 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 the early discoveries remain.) They were just what paleoanthropologists had been hoping to find, but the dearth of "Sinanthropus" (as these Homo erectus fossils were originally called) bodies was puzzling. It made sense that early humans might live in a cave but what had happened to bias the fossil record in favor of skulls and teeth?

To borrow from Thomas Hobbes, at this time paleoanthropologists viewed the lives of early humans as "nasty, brutish, and short." The heavy brows and robust bones of hominins like Neanderthals and Homo erectus testified to the cruel, primitive conditions under which these people lived. With such a bleak vision of the past, the horrifying act of cannibalism fit right in with what the scientists at Zhoukoudian found.

The German anatomist Franz Weidenreich most fully articulated this interpretation. The Zhoukoudian cave sites held the fossilized aftermath of a grisly slaughter of over 40 individual "Sinanthropus", perhaps by another tribe or an even more advanced type of human. For evidence Weidenreich pointed to damage inflicted on the hominin fossils which he thought could only have been made by tool-wielding murderers. This made sense of the preponderance of skulls and teeth, as Weidenreich outlined in a presentation of his case;

The Sinanthropus 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 workers at the same site, like Pei Wenzhong, disagreed. They thought that the site was a hyena dwelling and not a haven for headhunters. Indeed, even Weidenreich later became more uncertain of his earlier conclusions, stating the at least some of the damage could have been caused by carnivores, but the idea of bloodthirsty "ape-men" was publicly popular even if it was scientifically criticized. Anthropologist Raymond Dart, ethologist Konrad Lorenz, and playwright Robert Ardrey popularized our violent, "blood bespattered" past in their respective works during the 1950's and 1960's. In the wake of the atrocities of WWII it made sense that violent acts committed today could be traced back to our ancient, cannibalistic ancestors. Murder and war were just the current manifestations of our proclivity towards violence that had been a part of humanity from the start.*

*[While the evidence used in this argument has changed, whether early hominins had a more "demonic" or "peaceful" existence is still hotly debated.]

As popular as this vision of our past was publicly, however, anthropologists had a less grisly interpretation of our past. Hunting and meat eating were important, yes, but our ancestors preyed on other animals, not on each other. Eventually Dart's murderous australopithecines and Weidenreich's bloodthirstry Homo erectus faded from view, but this still left the mystery of why the Zhoukoudian cave mainly contained fragments of skulls. Perhaps there was some sort of cave roof collapse that shattered the bodies, but even this idea was not entirely satisfying.

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 the cave was dominated by giant hyenas that tore hapless Homo erectus limb from limb.

Indeed, not only are the remains of Pachycrocuta found in the cave, but the type of damage seen on the Homo erectus bones (and other fossils) is consistent with what is observed in modern hyena dens. Limb bones, for instance, are scarce and represented mainly by shafts; the ends were broken or gnawed off. Thus postcranial remains are rare because the giant hyenas obliterated them, tearing apart the carcasses and cracking the bones, and some of the bones were even swallowed (as shown by rare acid-etched fragments also bearing tooth marks).

Given the destructive power of these hyenas it might be expected that the Homo erectus skulls would be cracked into splinters, too, but there is very little meat on a hominin head. The main fleshy parts of a head are the jaw muscles, tongue, and the brain, the latter organ being something hyenas relish. Given what is known about the way modern hyenas eat and the damage to the Homo erectus skulls, then, Boaz and colleagues were able to reconstruct what might have happened to the poor hominins.

The first step might have been stripping the large jaw muscles off the face. After this was done there would be little worth consuming on the outside of the skull, so the hyenas probably cracked off the lower jaw and ate the tongue. By this point a catastrophic amount of damage would have been done to the face of the hominin, explaining why so many Homo erectus from Zhoukoudian are missing faces.

As stated above, however, the large brain of Homo erectus would have been a lipid-rich treat that the hyenas certainly would have tried to get at. To do this it seems that the hyenas bit down on the skull and pressed, using it as a kind of fulcrum, until a large enough hole was broken open through which they could extract the brain. At this point there wouldn't be anything else left on the skull worth eating and the remaining skull fragments would be left to litter the cave floor. Given the violence of this skull processing by the hyenas it is surprising that any skull material was collected for Homo erectus at all!

As Boaz, et al. stated, 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; the rest of the skeleton might have proven elusive because a predator destroyed the rest. 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.