This is the base of the Homo lineage and the immediate ancestor to H. habilis. We see the emergence of a moderately developed frontal sulcus, temporal squama pneumatization is reduced, the postglenoid process is small and fused to the tympanic, the anterior zygomatic is inserted above the M1, and the dental complex and TMJ are closer in terms of overall distance. The nasal clivus is intermediate in length and the mandibular mental foramen hollow is absent. Following on from this, we see the reemergence of H. habilis, which is defined by a large number of apomorphic features. These include variability in the presence/absence of an asterionic notch, laterally developed compound temporal crests, laterally inflated mastoid, and the large auditory meatus is aligned to porion. An articular tubercle may or may not be present. The cranial base has become more narrow, the foramen magnum has drifted anteriorly, frontal sinus is intermediately developed, there is a decrease in subnasal height, the anterior palate is shallow, a nasal keel is present, anterior nasal pillars are variable, canine fossa is reduced in depth, and the incisive canal is undeveloped. The condition in H. habilis appears to be related to differential neck muscle development and an increase in lower facial prognathism.
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