Other than a slightly enlarged brain and the use of relatively simple stone tools, there was little to suggest that later members of the genus Homo would one day dominate the earth. But dominate it they eventually did, once their ancestors achieved a series of herculean tasks: a marked

increase in brain size (encephalization), intelligence, and technological sophistication; the rise of complex cultural behavior built on an unprecedented reliance on learned behavior and the use of technology as a dominant mode of adaptation; a demographic and geographic expansion that would take their descendants to the ends of the earth (and beyond); and a fundamental realignment in the relationship of these hominins to the natural world. As always, there is much debate about the origins, taxonomy,

and relationships of various hominin species. The hominin evolutionary tree is much bushier selleck screening library than once believed (see Leakey et al., 2012), but what follows is a simplified summary of broad patterns in human biological, technological, and cultural evolution. Genetic data suggest that hominins only diverged from the chimpanzee lineage, our closest living relatives, between about 8 and 5 million years ago (Klein, 2009, p. 130). Almost certainly, the first of our kind were australopithecines (i.e., Australopithecus anamensis, Australopithecus afarensis, Australopithecus garhi, Australopithecus Guanylate cyclase 2C africanus), bipedal and small-brained apes who roamed African landscapes from roughly 4 to 1 million years ago. Since modern chimpanzees check details use simple tools, have rudimentary language skills, and develop distinctive cultural traditions ( Whiten et al., 1999), it seems likely the australopithecines had similar capabilities. Chimpanzees may dominate the earth in Hollywood movies, but there is no evidence that australopithecines had significant effects on even local African ecosystems, much less

those of the larger planet. The first signs of a more dominant future may be found in the appearance of Homo habilis in Africa about 2.4 million years ago. It is probably no coincidence that the first recognizable stone tools appear in African archeological sites around the same time: flaked cobbles, hammerstones, and simple flake tools known as the Oldowan complex ( Ambrose, 2001 and Klein, 2009). H. habilis shows the first signs of hominin encephalization, with average brain size (∼630 cm3) 40–50% larger than the australopithecines, even when body size is controlled for ( Klein, 2009, p. 728). Probably a generalized forager and scavenger, H. habilis was tethered to well-watered landscapes of eastern and southern Africa. For over 2 million years, the geographic theater of human evolution appears to have been limited to Africa.