Most human anatomists and paleoanthropologists today would agree that the evolution of the brain and the chewing apparatus of hominids is of major importance in explaining the anatomical changes that we see documented in the hominid fossil record. The only problem is that neither of these explanations is sufficient to account for the unique attributes of the strange skull form of Homo erectus. We believe that a third function contributed to the evolution of the Homo erectus skull, and it is protection.
When people today sustain head injuries they are much more likely to die when their skull has been fractured. What might seem to a casual observer like a relatively minor break can in fact tear blood vessels that tightly adhere to the inside surface of the bone and cause intracranial bleeding. The buildup of blood inside the skull pushes on the brain. Coma and, eventually, death can result.
A common type of fracture seen on modern skulls is an "eggshell" fracture. Concussion by a heavy or strongly wielded blunt object can depress a section of the cranial vault, cracking but not disjointing the bone. The bone springs back to nearly its original shape after the impact—pulled by the attachments of skin, underlying scalp, and muscle coverings—but the damage is done. Branches of the arteries supplying the fibrous coverings of the brain, the meninges, begin to bleed. This blood accumulates as a hematoma in the space between the inside of the skull and the outer meningeal covering, the dura mater. As the hematoma expands and begins to compress the patient's brain, sometimes hours after the injury, neurological symptoms become progressively severe, culminating in loss of consciousness, coma, and death.
In the days before emergency rooms, X rays, and intracranial surgery, people hit hard on the head and suffering from intracranial bleeding survived as best they could. This usually meant not very well. Even if an individual managed to regain consciousness and survive an extradural hematoma, there are frequently residual and significant neurological deficits. Partial paralysis, gait problems, lack of eye-hand coordination, speaking difficulities, or any number of cognitive function disruptions can result. For active Plio-Pleistocene hominids, one could hardly imagine a more debilitating condition, and we might reasonably surmise that traits that would reduce the chances of cranial fracture would provide a significant selective advantage to those individuals possessing them.
Australian researcher Peter Brown has investigated skull thickness in modern and historical Australian aboriginal populations.3 These peoples show the thickest cranial bones of any members of living Homo sapiens. Brown has hypothesized that their thick skull vaults may have evolved by way of their traditional method of settling conflicts.
A man or woman with a bone to pick with another member of the group follows a fixed behavioral code for resolving the conflict, challenging their adversary to a duel with the "nulla-nulla" (a heavy wooden club). Once the bout begins, it continues until one of the combatants wins either by knockout or TKO (his or her adversary is disabled and cannot continue). There are no wins on points or split decisions. Occasionally, entire communities became involved. One ethnographic report of the South Australian Narrinyeri tribal group reported that some one hundred people involved in a general melee were "earnestly endeavoring to knock each other's brains out."4
Brown found evidence of healed depressed fractures on the frontal or parietal bones of an amazing 59 percent of female crania and 37 percent of male crania in a sample of 430 Aboriginal crania that he studied. These results mean that depressed eggshell-type fractures occurred in these people and that they survived. But undoubtedly, many others did not. Brown concludes that "behavior of this type must have rigorously selected against those individuals with thinner bones in their cranial vaults and favoured thickened frontal and parietal bones where the blows most frequently occurred."5
If Brown is correct and skull thickness evolved in Australians as a result of generations of head bashing, is the model useful for understanding the evolution of pachyostosis and the unique bony excrescences of Homo erectus? We believe that it is.
The anatomical aspects of the Homo erectus skull that are least explicable in terms of the first functions of brain size increase and chewing apparatus decrease are best explained in terms of the third function—an evolved defense against trauma. We examine each trait in turn.
Experiments that we have undertaken on the strength of modern human bone as a function of thickness have clearly shown that the thicker the bone, the better it is able to withstand forces that would break it. In human biological terms, the thicker a cranial bone, the less likely it will bend in and crack like an eggshell, rending delicate blood vessels and brain tissue underneath. This general protective function of thickened cranial bone is the best explanation for why the Homo erectus skull is constructed of bone that is almost twice as thick as that of most modern humans.
The thickness of Homo erectus cranial bone is anatomically distinguishable from pathologically thickened modern human cranial bone. Diseases, like malaria that affect the blood and cause the blood-forming bone marrow
A photograph taken by G. Elliot Smith in Beijing comparing cranial thicknesses of the parietal skull bones, viewed from the front, in modern Chinese Homo sapiens (top) with Zhoukoudian Homo erectus (bottom).
to increase in size, can increase bone thickness.6 The resultant bone is like Swiss cheese, composed mostly of large, marrow-filled spaces, with a very thin skin of compact bone on the outside. Homo erectus bone, on the other hand, is like armor-plating. Franz Weidenreich observed that the skull vault bones from Longgushan had thick, solid layers of bone on their inside and outside surfaces. They sandwiched between them the marrow-containing trabecular bone and were, in aggregate, thicker than this softer inside layer.
In a fascinating recent study, Greek anatomist Antonis Bartsiokas investigated the microscopic structure of one of the earliest Homo sapiens from Africa, the Omo Kibish I skull from Ethiopia. He found that the thickness of this skull vault was within the range of Homo erectus and similarly showed the thickened inner and outer bony armor plating typical of Homo erectus, and unlike most Homo sapiens. The microscopic structure of the thickened bone also showed a different arrangement—the individual structural elements, the "osteons," were flattened and pressed together. Bartsiokas hypothesized that "perhaps this osteonal morphology is an adaptive means of strengthening the skull against head injuries."7 So far, Homo erectus fossil skull bone has not been similarly investigated, but the possibility exists that even its microscopic structure will make sense in terms of a defensive adaptation to protect against blunt head trauma.
In addition to the general thickness of its bones, the Homo erectus skull also has a number of unique bony structures. Franz Weidenreich gave these Latin names—torus supraorbitalis, torus angularis, torus occipitalis, and crista sagittalis—better referred to as the sagittal keel. The first three bony thickenings form a ring starting above the eyes, extending back around the head above the ears, and meeting on the back of the head. The sagittal keel forms a thickened bony mound from the middle of the forehead extending back over the crown of the head to meet the horizontal ring of bone in the back. The characteristic flattening of the Homo erectus skull falling away to both sides of the sagittal keel provides additional strength to the skull vault.
A forensic review of man's inhumanity to man provides ample evidence of how important these bony adaptations would have been to a hominid routinely subjected to blunt trauma to the skull. An American surgeon named E. R. LeCount classified the types of fractures that occur when people are hit hard on the head.8 A heavy blow falling directly on the top of the head tends to cave in the bone protecting the channel of venous blood draining along the midline of the brain known as the superior sagittal sinus. If this structure is damaged there is bleeding into the space between the brain and its outer covering, the dura mater. A so-called subdural hematoma (blood collecting under the dura mater) can compress the brain, causing defects of function, and it is potentially fatal. LeCount hypothesized that the strongly constructed midline of the human skull is adapted to protect against this type of damage. In most Homo erectus this adaptation appears in exaggerated form as the sagittal keel, a low, rounded thickening of bone running from the front of the skull to the back.
However, blows do not usually rain down from above on the heads of antagonists in a physical disagreement, but are instead delivered more or less at eye level. The battered skulls of Bosnian and Croatian victims of genocide, for example, uniformly show damage to the regions around the eyes, on the sides of the head, behind the ears, and at the back of the head.9 This pattern of damage is exactly the location of the ring of tori as seen in the Homo erectus skull. LeCount saw the same areas in the modern human skull as protection against the most common injuries resulting from blunt trauma to the head.
Another surgeon, René Le Fort of France, studied the pattern of facial fractures in modern people, and his conclusions are also instructive.10 Le Fort classified the types of fractures that he observed. A Le Fort Type 1 fracture is one that results from a blow to the upper face that breaks the bone surrounding the eye socket. Direct blows to the brow frequently break the orbit, the bone forming the roof of the bony space that holds the eyeball and its muscles. Homo erectus has a remarkably straight roof to the orbit, an anatomical peculiarity that until now has defied a functional explanation. Extending back to the strong base of the skull from the heavy supraorbital tori this trait would have helped Homo erectus individuals avoid Le Fort Type 1 breaks. Weidenreich himself, in his posthumous publication on the Javan Ngandong crania,11 suggested that the massively projecting ridges of bone above the eyes of Homo erectus likely had a protective function. Most recently, John Hawks of the University of Wisconsin used three-dimensional simulations of impact trauma to the head and concluded that having a larger supraorbital torus significantly enhanced protection of the upper face and eyes.12
The straight and relatively unangled face to neurocranium hafting that is characteristic of Homo erectus would have prevented Le Fort Type II and III fractures, very debilitating breaks that result from separation of the facial skeleton from the braincase. A strong blow to the reinforced Homo erectus face would have resulted in soft tissue damage, including perhaps less serious fracture of anterior parts of the maxilla, but fractures mobilizing the teeth or the zygomatic arches would have been reduced. Anterior blows to the face would also have resulted in fracturing incisors particularly, and these teeth in Homo erectus also show a reinforcing thickening of enamel on their lingual sides that could have prevented loss of dental function. Homo erectus shows a high incidence of such "shovel-shaped incisors."
An old boxing adage warns away those potential pugilists with a "glass jaw." Indeed, fracture of the mandible is a serious injury in those on the receiving end of a barroom punch to the chin or lower face. A broken jaw renders active chewing painful and difficult, if not impossible, and today it requires surgery and wiring together of the broken sections of bone. Clearly a jaw fracture would have been a life-threatening event for a Homo erectus individual, who without surgery would not have been able to eat solid food. We suspect that a large number of Homo erectus unfortunately died this way. The anatomy of the Homo erectus mandible shows a unique thickening of bone on the side of the jaw exactly where it most commonly breaks from trauma. Weidenreich in his monograph on the Longgushan mandibles named this thickening of bone on the inside of the mandible the torus mandibularis. At first thought to be pathological, the torus mandibularis makes most anatomical sense as another defense against trauma to the lower face.
A point of Homo erectus anatomy that is particularly convincing to us regarding the protective pachyostosis hypothesis, concerns the course of an artery that can be traced on the inside of the skull. The middle meningeal artery is a branch off the maxillary artery and runs up inside our temple. Its vulnerability to damage is the reason that the baseball batting helmets have a little protective flange extending down on the side of the head facing the pitcher. The bone overlying the middle meningeal artery at a region of intersecting sutures known as the pterion is particularly thin in modern humans. It is partially protected by the overlying chewing muscle, the temporalis, but a good shot in the temple is likely to break the bone here and tear the middle meningeal artery. An arterial tear is even more dangerous than damage to a venous sinus because the blood in an artery is under higher pressure and can bleed out more rapidly. Usually damage at the pterion results in a large amount of blood pooling on the outside of the dura mater (an extradural hematoma) and rapid loss of consciousness or coma.
The Homo erectus skull is not particularly thick at the pterion, and if this region is susceptible to damage in modern humans, we would expect it to have been even more so in Homo erectus. Observations by Franz Weidenreich on the unusual anatomy of the middle meningeal arteries in Homo erectus, until now unexplained, may provide the answer.
In modern humans the middle meningeal artery divides into a large branch that runs forward on the inside of the skull under the pterion, and a smaller posterior branch that runs backward. But in Homo erectus, the anterior branch is miniscule compared to the quite large posterior branch. Weidenreich was so struck by this anomaly in Homo erectus that he devoted an entire paper to it.13 We think that this anatomical trait of Homo erectus is a result of natural selection to withstand the effects of breakage in this area of the skull. If for developmental and structural reasons (because perhaps it is a convergence point of skull sutures) the region of pterion could not easily be thickened during evolution—especially as the cranial vault was expanding with a larger brain—it makes sense that Homo erectus adapted to minimizing bleeding in this area should this artery be torn. By redirecting blood flow to a posterior course, under stronger skull vault bone, the chances of incurring a fatal and debilitating epidural hematoma would have been substantially reduced. This region of the temporal bone also underlies the large temporalis chewing muscle that can partially cushion a blow to the head at this point.
The back of the skull of Homo erectus suggests to us that blows from behind were a major factor in human evolution. The torus occipitalis overlies and protects the confluence of venous sinuses inside the back of the skull, branches of the posterior cerebral artery supplying the brain, the occipital lobe of the cerebrum, and the cerebellum. Damage here can result in blindness if the occipital lobe is affected, or inability to walk, stand, or move in a coordinated fashion if the cerebellum is injured. The torus angularis overlies and protects the sigmoid venous sinus inside the cranial cavity as it conducts blood into the internal jugular vein exiting the base of the skull. It also helps to protect the ear region from behind.
Blunt trauma to the back of the head is a common cause of death today. The pattern of injury demonstrates exactly what we believe Homo erectus anatomy evolved to prevent.
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