Weird Skull and How It Got That

Homo erectus skull bone can be technically described as "pachyostotic" (literally "thick-boned"). In understanding how pachyostosis evolved, we can look to comparative anatomy. A few other vertebrate species have or had thick bones, and attempting to understand their adaptations can give us some idea as to the reason for the massiveness of their bones. When comparing animal species that have evolved similar anatomy, we are not looking at traits inherited in common from ancestors, but instead traits that have evolved in parallel for similar reasons. Some species, such as sirenians (marine mammals such as the dugong and Florida manatee) have dense bones throughout their bodies to give them negative buoyancy in water. The ribs of sirenians are essentially ballast. The postcranial bones of Homo erectus, however, like ours today are not massively thickened for an aquatic existence.

Among terrestrial animals, extremely thick skull bones are seen in species as diverse as modern bighorn sheep (Ovis canadensis) and the Cretaceous dinosaur Pachycephalosaurus. The adaptive significance of pachyostosis in these species, based particularly on behavioral observations of male bighorn sheep, is protection of the brain and sense organs during intraspecific competition. Bighorn sheep (and presumably Pachycephalosaurus in the c

The pronounced cranial thickness of the skull of Homo erectus. This is a photograph taken by Davidson Black in the process of preparing and reconstructing Zhoukoudian Homo erectus Skull III. In this view the two parietal bones have been removed and a natural endocast of limestone that has a canine of a cave bear imbedded in it just behind the frontal bone can be seen.

past) use the head as an organ of offense, butting heads so forcefully that it sounds like an explosion. Two males run at each other and collide at 30 to 45 miles/hour, generating impact forces of up to 2,700 pounds. The sound can reportedly be heard a mile and a quarter away. What causes such intense conflict among male bighorn sheep? Females. Charles Darwin long ago explained such behavior in species as a result of sexual selection, a type of natural selection that works primarily inside social species and results in same-sex competition for access to opposite-sex mates.

A comparison of the skull bone thickness of male bighorn sheep and the skull bone thickness of Homo erectus might at first seem strained. Who would seriously postulate that early hominids might have charged at one another and banged their heads together like rutting sheep? If you asked people on the street (and managed to get them actually to ponder it for a moment), they might suggest that the only modern humans who might be so rash, illogical, and violent as to engage in such behavior for sex would be young adult males. And they would be right. Ample statistics show that 15- to 24-year-old men in the United States die at four times the rate of females from fights, aggressive acts, or accidents that their risky behavior originally made likely.1 Regardless of their ethnic or socioeconomic backgrounds most disputes between young human males directly or indirectly involve competition for the attention, affections, and affiliation of young females—not all that different from rutting sheep after all. So it is well within the general behavioral capabilities of at least one age group and gender of modern people to fight in a way quite analogous to that of bighorn sheep. We can surmise that it was also possible behavior for Homo erectus. Using the head as an offensive weapon may be a less attractive evolutionary option for hominids than for sheep, however. The easily injured human brain is much larger and less well covered by protective bone than is a sheep's brain. We suggest that thick cranial bones in Homo erectus might be adapted less for offense than for defense.

Unlike bighorn sheep, human beings have always tended to fight with their hands, and (leaving out gunshots) almost all cases of violent trauma inflicted during nonsexual assaults are to the head. The general pattern of interpersonal attack in humans is to hit the face and head with the hands. Anatomy also suggests that head butting was not a primary adaptation of our ancestors. Pachyostotic species that use the head as a weapon also have cranial outgrowths of bone that evolved for that purpose. Sheep have sharp horns rising out of their thick skull and the Pachycephalosaurus had nasty-looking bony knobs projecting around its head like a crown. Homo erectus had none of these offensive adaptations. Thus, just from general principles, we interpret the cranial pachyostosis of Homo erectus as protective of the brain and sense organs, but defensive in nature. In functional terms we might think of the Homo erectus skull as also similar to the defensive carapace of a turtle.

The human head has more small structures, spaces, holes, canals, vessels, muscles, and wiring in its relatively small compass than has any other region of the body. Physical anthropologists and anatomists have been obsessed with describing, explaining, and understanding this complexity for centuries. We now know enough of the human skull's evolution, em-bryological development, comparative anatomy, and function to feel confident that the major forces affecting human skull form are understood. Thus, when we consider one aspect of the unique skull form of Homo erectus— pachyostosis—we must also look at what else this species's skull did for its owners, and how this knowledge helps us understand how it lived.

We interpret evolving hominid skull form as resulting from three major functional imperatives: housing a rapidly increasing brain, serving as the bony anchor for the teeth and the muscles that move them, and, in the case of Homo erectus, defending against blunt trauma. All three functions are important to understanding the unusual cranium of Homo erectus.

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