If our hypothesis on the meaning of Homo erectus skull anatomy is correct, and Homo erectus as a species was ancestral to later Homo sapiens, why did thickened cranial bones evolve out of our biology? If a thickened skull was adaptive for Homo erectus when these hominids got hit on the head, why did evolution discard it for us? If modern children had thicker skulls, significantly smaller numbers of them would suffer serious head injuries when they crashed on their skateboards, bicycles, and snowboards, for example.
As we saw in hypotheses on the appearance of pachyostosis in Homo erectus, the first two functions of cranial evolution—increased size of the brain and decreased size of the chewing apparatus—do not help explain its origins. Neither do they help explain its disappearance. A counteracting selection force, or a combination of such forces, is important to identify in attempting to understand how and why cranial pachyostosis disappeared in more advanced hominids.
Anthropologist Dean Falk has hypothesized that the heat generated by the extremely enlarged human brain became a significant physiological factor in evolution.16 In her "radiator brain hypothesis" she proposed that the pattern of venous blood drainage in the head became reorganized to cool the brain. Many small holes known as emissary foramina pierce the skull and serve for the passage of veins from the surface skin to the large venous sinuses inside the skull. Blood cooled by heat exchange from evaporating sweat on the scalp moves into the venous sinuses. Falk discovered that emissary foramina are much more common in large-brained Homo species than in small-brained australopithecines. The deduction then is that cool scalp blood flows back through the skull bone, where it cools the brain and keeps it at optimum temperature. Falk's hypothesis is still under debate, but it does explain some important aspects of hominid cranial anatomy in the evolution of a large brain.
We suggest that the radiator brain hypothesis may also explain why skull thickness in Homo erectus decreased as this species evolved. A thick skull would have been substantially more difficult for low-pressure and delicate emissary veins to pierce, thus making it more difficult for the enlarging brain to be cooled adequately. Natural selection may well have favored a thinner skull for this reason as the brain increased in size and metabolic heat output.
Our explanation for the thick skull of Homo erectus is a hypothesis of exclusion—it simply makes the most sense of any possible reason we can think of. But the behavioral implications of the hypothesis will be disturbing to many who may want to believe than humanity has a basic adaptation for cooperativity and sociality. We agree, but our behavioral evolution was more complex than can be summarized in one or two words. Homo erectus still has much to teach us about the evolution of our behavior, and research continues.
In the next chapter we turn to the Longgushan Cave's primary evidence of behavioral complexity in Homo erectus—the use of stone tools and, above all, fire—whose effective use may have been the driving force behind the brain's remarkable evolution.
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