Did humankind evolve because women became sexpots? That's what anthropologist Helen Fisher (Univ. of Colorado) proclaims loud and long in this popular treatment of how-it-all-began. Her thesis is that those women who first gave up ""heat"" and became sexually receptive throughout the monthly cycle were the most popular. They got protection; meat, when available. They produced more babies. To be sure, climatic changes leading to ground travel, food-carrying, and food-sharing were promoting a group cooperative life. Fisher follows through with the hypothesis that women who were able to give birth easily and earlier (bipedalism led to a narrowing birth canal) were burdened with more helpless infants, and needed and could demand the protection of males--if they were sexually receptive and impregnable not long after parturition. Thus: sexual dimorphism, sexual bonding, and the sexual contract--with the sexy females matched with the big clever males. Fisher clouds some hormonal issues (such as the ability to conceive if a woman is still nursing a child), and in general is given to excesses of enthusiasm and overstatement--not at all helped by her propensity to wax poetic from time to time. (She injects boldface paragraphs in which she imagines what Life was like for one of those pioneering femmes.) Away from that preoccupation, Fisher is well informed and even-handed in her treatment of paleontology--from the human precursors of 14 million years ago to the Leakey-Johanson controversy over whether Australopithecines died out or are ancestral to Homo sapiens. On the whole, then, a kind of feminist retort to Tiger and Fox and Ardrey and Morris. And like them, distortive.