FIGURE 2.2 Geographic distribution of major exploration sites for hominins older than 1.8 Ma (i.e., prior to the first dispersal of H. erectus out of Africa). East Africa Rift System shown in purple. SOURCE: Digital elevation model image courtesy National Oceanic and Atmospheric Administration National Geophysical Data Center.

ancestors had split from the chimpanzee lineage and become adapted to bipedal locomotion, which is the major difference that separates us from great apes.

The fossil record of hominins between 6 and 3 Ma is patchy, but samples from Ethiopia, Kenya, Tanzania, and Chad record several bipedal hominins that have been placed in the genera Ardipithecus and Australopithecus. The early part of this hominin record (5.8 to 4.4 Ma) is represented by Ardipithecus, which had acquired some features seen in later Australopithecus, but which still exhibited primitive traits seen in African great apes (White et al., 1994, 1995, 2009 and associated articles).

The species in the genus Australopithecus all have larger molar and premolar teeth and thicker enamel than their predecessors (Ward et al., 1999, 2001;