Forget bipedalism. Forget language. Forget pair-bonding and cooperation. What distinguishes Homo sapiens from all other species is the need to explain. And what better explainer than paleoanthropologist Johanson, here joined by his filmmaker/scientist spouse and science writer and editor Edgar (Pacific Discovery magazine) in the text that's to accompany a three-part PBS series celebrating Nova's 25th anniversary. Much of the book reprises earlier accounts by Johanson, Leakey, and other popularizers, absent the acrimony that accompanied Johanson's landmark discovery of Lucy in the Afar region of Ethiopia. Uninformed readers would receive no idea here of the contention that surrounded the naming of Lucy as Australopithecus afarensis. Not that the authors bury the debates. They are forthright and fair in discussing the pros and cons of current theories: out-of-Africa origins vs. the ""Multiregional Model""; man the hunter vs. man the scavenger, Neanderthals as ancestors vs. Neanderthals as an interesting but failed experiment. New to the history are recent findings in Ethiopia and a fascinating account of the 40,000-year history of the aborigines of south-central Australia. The latter sparks a theory that the human ""revolution"" (that's ""evolution"" plus an ""r"") is due to the birth of art: the symbol-making and/or sacred activity that bespeaks a mind that can dream -- and explain. The 175 visuals should be splendid.