We have presented a hypothetical model that explains the mechanisms of the evolution and eventual extinction of Homo erectus. We believe the model is consistent with fossil and genetic evidence. But the evolutionary changes in anatomy, function, and overall adaptation from Homo erectus to Homo sapiens remain to be explained. Many of the changes that we know of have to do with the head.
Theoretically, a species could have both a commodious skull to house an enlarged brain and a heavily armored and thick skull for protection. Reality steps in when the weight of such a structure has to be supported and balanced atop the spine. Many large-bodied species, whose skulls must be correspondingly large, have bone that is honeycombed inside to reduce its weight. A section through the skull of an elephant or a giraffe is surprising because so much of the inside of it is air, surrounded by paper-thin bone organized into structures called "diploe." Evolving Homo erectus had a similar problem with the weight of the skull. If the brain size was increasing, the enclosing bone would also have to increase, but skull weight would have to be minimized. As Homo erectus evolved to Homo heidelbergensis, skull weight was decreased by lessening the thickness of cranial bone. And, as was discussed earlier, there may have been a "fourth function" that helped account for the decrease in skull thickness—cooling of the brain via more efficient venous blood flow between the skin and the cranial vault.
The defensive functions of the pachyostotic Homo erectus skull were lost as Homo sapiens evolved a larger, more globular, and thin-walled skull. Human intraspecific violence by no means ended, but other means to protect themselves from trauma or to avoid attack, or both, were evolved by the descendants of Homo erectus. Almost certainly, these adaptations were cultural, the hallmark of Homo sapiens, and no longer biological.
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