H. erectus arose in Africa more than 2 million years ago, and soon thereafter some populations of this early human migrated out of the continent. Descendants of the migrants reached eastern Asia at least 1.9 million years ago. The stone tools they manufactured have been discovered at various sites, but the earliest fossils in eastern Asia have been found only in Java and China. Java man comprises both the earliest and the most recent specimens of H. erectus; the fossils span a period that lasted from 1.8 million years ago until just 50,000 years ago. Peking man dates more narrowly, from between 670,000 and 410,000 years ago.
The skullcaps discovered in eastern Asia tend to be more robust than the ones in Africa. Hence some paleoanthropologists have regarded the African fossils as a distinct species, which they call H. ergaster. But one African skullcap just as robust as any Asian specimen was discovered by Louis Leakey in Olduvai Gorge, Tanzania. It dates from about 1.4 million years ago. And even the strapping youth known as Turkana boy, the most complete H. erectus skeleton discovered so far, probably would have had a thick skull when fully grown. In any case, there is little doubt that H. erectus was on the line that ultimately led to the first modern humans. Whether that further evolution took place in Africa or was a more widespread phenomenon is a matter of debate, but one way or another we got bigger brains and thinner skulls.
Many differences in hominid skulls can be accounted for by the evolution of the brain and the chewing apparatus. Large skulls are needed to contain large brains, and large jaws and teeth for processing tough foods need heavy-duty skull bones to anchor massive chewing muscles. Unfortunately, neither of these tried-and-true explanations can entirely account for the unique attributes of the H. erectus skull. What seems more likely is that the species badly needed some protective headgear. Functionally, the H. erectus skullcap is similar to the defensive carapace of a turtleindeed, some excavators have mistaken cranial fragments of H. erectus for fossil turtle shell. But what special sources of traumatic injury did hominids face that might have encouraged the evolution of such a robust skull? We dont think it was exposure to predators (which can readily attack other, more vulnerable parts of the body), or a habit of venturing into slippery or precarious territory where the hazards of falling were increased. In examining the protection afforded by the H. erectus skull, we think the evidence points to some kind of violence perpetrated within the species itself.
|Large skulls are needed to contain large brains, and large jaws and teeth for processing tough foods need heavy-duty skull bones to anchor massive chewing muscles. Unfortunately, neither of these tried-and-true explanations can entirely account for the unique attributes of the H. erectus skull. What seems more likely is that the species badly needed some protective headgear.
When a person is injured in the head today, whether or not the skull is fractured often makes the difference between life and death. What might seem like a relatively minor break in the skull can tear blood vessels that adhere tightly to its inside surface. The buildup of blood under the skull, known as a hematoma, pushes on the brain. Coma and, eventually, death can result.
In modern skulls one of the commonest kinds of fracture is the so-called eggshell. The concussion caused by a fall or by a blow from a blunt object can crack and push a section of cranial vault inward without disjointing the bone. The bone may remain depressed, but in an eggshell fracture, the bonepulled by skin, muscle, and other tissues attached to the scalpsprings back to nearly its original shape. In either case, though, the damage is done. Branches of the blood vessels serving the meninges, or fibrous coverings of the brain, begin to bleed. As the hematoma expands and begins to compress the brain, sometimes hours after the injury, neurological symptoms become progressively severe.
In the days before emergency rooms, X rays, and intracranial surgery, people suffering from intracranial bleeding managed the best they could. Usually that meant not very well. Even if a person regained consciousness and survived the hematoma, profound neurological deficits often continued. Partial paralysis, gait problems, lack of hand-eye coordination, difficulties in speaking, or any number of disruptions in cognitive functioning were the result. For active early humans, it is hard to imagine a more debilitating condition. Any traits that reduced the chances of cranial fracture would have given a substantial evolutionary advantage to the individuals who possessed them.
As one might expect, the thicker the bone, the less likely it is to break on impact. In our most recent work we have been experimentally testing and quantifying the advantages of thick bones. With a nine-foot-high, guillotinelike bone-testing apparatus, we administer calibrated impacts on one-inch circular pieces of bone from human cadavers. Certainly thick bone does confer a competitive advantage. But minimizing weight and optimizing protective value at the same time is a problem that we continue to study.
In the 1920s an American surgeon named E.R. LeCount classified skull fractures by type. A heavy blow falling directly on top of the head tends to cave in the bone overlying the so-called superior sagittal sinus, a channel within the meninges for venous blood draining along the midline of the brain. LeCount hypothesized that the strongly constructed midline of the skull is an adaptation that protects against such damage. In most H. erectus skulls the same adaptation appears in exaggerated form as the so-called sagittal keel.
Blows delivered in a fight, however, are more likely to land at eye level than to rain down on top of the head. LeCount regarded the eye-level armor of the modern skull as the main protection against blunt trauma to the head. Again, the H. erectus skull, with its even thicker ring of bone, would have afforded even more protection. The bony ridge above the eyes protected the orbits, or eye sockets. The bony bulge on either side of the skull overlay the sinus that conducts blood into the internal jugular vein, and helped protect the ear region from blows to the side of the head. And the bony ridge on the rear of the cranium shielded the confluence of sinuses carrying venous blood inside the back of the skull, the rearmost lobe of the cerebrum, and the cerebellum.
In addition to the skullcap, other features of the H. erectus skull and jaws seem well adapted for defense against trauma. René Le Fort, a French surgeon working at the turn of the twentieth century, studied and classified the pattern of facial fractures in modern people.
The H. erectus skull has a number of unique bony structures. Three of them, namely a beetling brow ridge and bony thickenings on the sides and rear of the cranium, form a bony ring starting above the eyes, extending back around the head above the ears, and meeting on the back of the head.
Le Fort type II and type III fractures are highly debilitating breaks in which the facial skeleton is separated from the brain case. In H. erectus the face evolved to become tucked under the protecting brow ridges, making it less likely that a fracture would cause such a separation. A strong blow to the face would have resulted in soft-tissue damage, and perhaps in a fracture of the forward part of the upper jaw and damage to the incisors. But more serious fractures, such as breaks of the cheek bones, would have been reduced. H. erectus incisors were also reinforced by a thick layer of enamel on their inner sides and, often, by their shovel shape.
An old boxing adage warns would-be fighters to avoid the ring if they have a glass jaw. A broken mandible makes chewing painful and difficult, if not impossible, and even today the injury requires that the broken sections be surgically wired together. For H. erectus, such a fracture would have been life-threatening. Weidenreich was the first to point out, in a monograph on Peking man, that the H. erectus jawbone thickens on the inside of each mandible, just behind the chin. That is exactly where the jaw most commonly breaks from trauma in modern people. The thickening makes the most anatomical sense as a defense against trauma to the lower face.
Another vulnerable part of the anatomy protected by the skull is the middle meningeal artery. In modern humans the main branch of the vessel runs beneath the temple, in a region of intersecting bone sutures known as the pterion. The bone here is particularly thin, though the overlying chewing muscle provides some protection. But a good blow to the temple is still likely to break the bone and tear the arterya dangerous injury because arterial blood can bleed out so rapidly. Damage at the pterion usually results in a large hematoma and rapid loss of consciousness or coma. The little flange on baseball batting helmets, extending down the side of the helmet that faces the pitcher, is specifically designed to protect the batter against this injury from a hard-pitched ball.
Contrary to what one might predict, the H. erectus skull is not particularly thick at the pterion. But other details of its anatomy are just as revealing. In modern humans the middle meningeal artery divides into two branches, a large branch that runs forward on the inside of the skull, under the pterion, and a smaller branch that runs toward the back of the skull. In H. erectus the middle meningeal artery divides as well, but the forward branch is minuscule compared to the large rearward branch. Weidenreich, who discovered the anomaly, devoted a paper to it, considering it a reflection of the primitive quality of the H. erectus skull and brain.
We think this flow network, like the thick cranium, evolved in response to interpersonal violence. Moving the main blood supply to the meningeal coverings of the brain away from this vulnerable area of the skull helped mitigate the effects of arterial breakage. But why rearrange arteries when evolutionary change had so readily thickened other parts of the skull? Perhaps skull sutures created developmental or structural problems for such thickening, particularly while the cranial vault was expanding through evolution to house a larger brain.
Franz Weidenreich was trained as a medical doctor, and worked most of his career in medical institutions in Germany. He even served briefly as a medic in the German army during the First World War. Doubtless, then, he had more than a passing familiarity with the devastating effects of head traumaa familiarity that became invaluable when he began to analyze the skulls of Peking man.
In those fossil specimens he identified a number of depressed fractures that had subsequently healed. In other words, half a million years or so after these hominids had sustained massive blows to the head, Weidenreich had suddenly stumbled on evidence that could still reveal not only the kind of trauma that resulted, but also, because the trauma victims had survived, the protective value of their skulls. Tragically, the original fossils of Peking man were lost during the Japanese invasion of China in the Second World War. Fortunately, careful casts of the excavated remains had been made before the war, and so we were still able to re-examine the head trauma systematically.
Some of the damage Weidenreich first attributed to hominids he later ascribed to carnivores. Other damage was clearly geological: some bones have been crushed by overlying sediment, others bear the impressions of rocks pushed into them as they themselves turned to stone. But in the end, Weidenreich classified some ten depressions or defects in the skulls as having been caused by blows from other hominids. We agree. The damage closely matches in size, form, and even location the healed depressed fractures seen in human skulls today [see photo below].
Females. Darwin long ago explained that such behavior is the result of sexual selection. Among many species, the males ability to procreate depends to a substantial degree on attracting members of the opposite sex, or to winning access to females through competitions with other males. In the evolution of bighorn sheep, for instance, males that defeated their rivals in butting contests got to mate more often. The mating passed on whatever attributes had given the victor an edgeand one of those attributes was apparently a reinforced skull. Paleontologists speculate, by analogy, that Pachycephalosaurus engaged in similar behavior, but no one knows for sure.
We arent suggesting that early hominids charged at one another and banged their heads together like rutting sheep (although if you think about football and some martial arts, the idea is not as bizarre as it might seem at first). Pachyostotic species that use the head as a weapon also have bony cranial outgrowths that evolved along with their behavior. Sheep have sharp horns rising out of their thick skulls, and Pachycephalosaurus had nasty-looking knobs projecting from the back of its domed head. H. erectus had none of these offensive adaptations. Modern human beings tend to fight with their hands, and (leaving out gunshots) almost all cases of serious or lethal trauma inflicted during nonsexual assaults are to the face and head.
When animals compete over mates or rank (which often amounts to the same thing), their combat tends to fall within certain instinctively understood limits. Nowadays, of course, violence among modern humans can be unrestrained, both in intergroup conflict (warfare) and between members of the same group. But some forms of human violence remain culturally circumscribed. In Western society, duels were historically carried out with matched weapons, and boxers (even fistfighters) follow the dictates of clean fighting. Among the Yanomami of Venezuela and Brazil, violence is traditionally limited, despite their reputation as the fierce people. As the anthropologist Kenneth Good and the writer David Chanoff report in their book Into the Heart: One Mans Pursuit of Love and Knowledge Among the Yanomami:
When a situation really got heated, the men of two lineages or two villages might get involved in chest-pounding matches, where individuals took turns giving and receiving punches, either open-handed or with closed fistsdepending on the level of anger. . . . A step up the scale were club fights, where antagonists traded blows to the top of the head with eight-foot-long staves. Often they carried the scars of these duels for life. But this too was ritualized violence, a substitute for deadly bloodshed.
A particularly instructive example comes from nineteenth-century ethnographic reports of Australian Aboriginal groups, particularly for central and southeastern Australia. Men or women who had a bone to pick with another group member followed a code for resolving the conflict. They challenged their adversary to a duel with a combination club and throwing stick called a nulla-nulla. Once the bout began, it continued until one of the combatants won by knock-out or TKOthat is, until the adversary was disabled and could not continue.
Peter Brown, a paleoanthropologist at the University of New England in Armidale, Australia, has investigated skull thickness in modern and historical Australian Aboriginal populations, whose cranial bones are the thickest of any living H. sapiens. In a sample of 430 Aboriginal crania, Brown found evidence of healed depressed fractures on the frontal or parietal bones in 59 percent of the female crania and in 37 percent of male crania. Depressed fractures occurred in these people and they survived; undoubtedly, many others did not. His findings led Brown to hypothesize that the thick skull vaults of the Aboriginals may have evolved as a consequence of the traditional method for settling conflicts.
A similar explanation may account for the evolution of pachyostosis and other unique features that strengthened the H. erectus skull. We are reasonably confident that the distinct anatomical features, as well as the healed fractures that have been preserved in the fossil record, are primarily a response to violence within the species. We can only speculate about whether the violence involved ritualized fights with clubs or rocks among hot-headed young males competing over females, or instead revolved around other kinds of conflict. But we would lay bets that, as in many other species, we are detecting the results of sexual selection.
If H. sapiens evolved from H. erectus, why dont we, too, have thickened cranial bones? If modern children had thicker skulls, for instance, significantly smaller numbers of them would suffer serious head injuries when they crash on bicycles, skateboards, and snowboards. Theoretically, a species could have both a commodious skull to house an enlarged brain and a thick, heavily armored skull for protection. But reality steps in when the weight of such a structure has to be supported and balanced atop the spine. Cranial bone may have become thinner in modern humans simply to reduce skull weight.
|Theoretically, a species could have both a commodious skull to house an enlarged brain and a thick, heavily armored skull for protection. But reality steps in when the weight of such a structure has to be supported and balanced atop the spine. Cranial bone may have become thinner in modern humans simply to reduce skull weight.
Another possible explanation comes out of the work of Dean Falk, an anthropologist at Florida State University, in Tallahassee. Falk argues that the heat generated by the enlarged human brain became an important factor in evolution. She hypothesizes that the pattern of venous blood drainage in the head became reorganized in order to cool the brain. Many small holes known as emissary foramina pierce the skull, enabling veins to transport blood from the scalp into the venous sinuses. This blood, cooled at the surface by sweat evaporating from the scalp, then enters the skull to help cool the brain.
Falk discovered that emissary foramina are much more common in large-brained Homo species than they are in the earlier, small-brained hominids known as australopithecines [See A Good Brain Is Hard to Cool, by Dean Falk, Natural History, August 1993]. Falks hypothesis is still debated, but we think it could explain why skulls became thinner as modern humans evolved: a thick skull would have presented a far greater obstacle to penetration by the delicate, low-pressure emissary veins.
Thus was defensive armor reduced, as H. sapiens evolved a larger, more globular, thin-walled skull. Human violence by no means ended, but other means of protection from trauma or avoidance of attack, or both, were evolved by the descendants of H. erectus. Almost certainly, those adaptations were no longer primarily biological but cultural. Culture was to become the hallmark of our species.
Noel T. Boaz and Russell L. Ciochon met thirty years ago, when they were both graduate students in paleoanthropology at the University of California, Berkeley. Their interests in Homo erectus fossils from the Chinese site of Longgushan have brought them back together. Their new book, Dragon Bone Hill: An Ice-Age Saga of Homo erectus, is being published this month by Oxford University Press. Boaz is a professor of anatomy at the Ross University School of Medicine, in the Commonwealth of Dominica. Ciochon is a professor of anthropology at the University of Iowa in Iowa City.
Copyright © Natural History Magazine, Inc., 2004