The evolutionary transformation of the australopithecines to Homo was accomplished by evolutionary change and reduction in the massive teeth and chewing muscles that typified our earliest hominid ancestors. The australopithecines had large teeth, large chewing muscles to move them, and consequently, large bony faces, to serve for anchoring the teeth and attaching the muscles. By the time the australopithecines yielded the evo-
The Homo erectus skull showing the areas of bony attachment of the two largest chewing muscles, masseter and temporalis. Note that the temporalis muscles attach well down the sides of the head from the sagittal keel. Their relative size therefore cannot explain the occurrence ofthis anatomical feature. The sagittal keel seen in Homo erectus is hypothesized here to be related to defensive strengthening of the skull.
lutionary stage to Homo, the size of the dentition had diminished. The face had changed significantly. The bone around the nose and extending back to enclose the molars became thinner, and the australopithecine "dish face" disappeared. The bony nose became more prominent, sticking out from the face, the maxillae of which sloped gently backward. The arch of bone holding the front teeth became a smooth parabola, no longer the straight ridge of bone extending from one canine socket to the other as in the australopithecines.
The strong ridges that the australopithecine chewing muscles left on the bones of their skulls decreased in size in the genus Homo. The temporalis muscles crept up the sides of the australopithecine skull, sometimes to meet themselves in the midline. When this happened, a vertical ridge of bone known as the sagittal crest formed. It is literally an upright plate of bone to which fibers of the temporalis muscles attach on both sides. Sagittal crests were most common in the robust australopithecines, and occasionally occurred in other, non-robust forms as well. But they are virtually nonexistent in Homo, whose small chewing muscles and enormously expanded skull vaults relegate the temporal lines to insignificant linear elevations far down on the sides of the head.
Decrease in the chewing apparatus explains some of the Homo erectus skull form—its relatively lightly constructed face, its curved dental arcade in the front, and its generally rounded skull lacking a sagittal crest. But there are still other parts of the Homo erectus skull that are not explained by
either dental changes or brain expansion. It is to these uniquely Homo erectus features that we now turn.
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