Among popular yet earnest attempts to solve the Sphinx's riddle, we've had Desmond Morris' Naked Ape, Ardrey's Territorial Imperative, Tiger's Male Bond--all provocative, all ultimately unconvincing. Edward M. Keating--lawyer, anti-war activist, author of Free Huey! and a founder of Ramparts--now comes up with the Arboreal/Terrestrial Conflict Thesis, which he has the audacity to believe is ""the explanation of man's suffering."" And such is his logic, his organization, his thoroughness, his somewhat humorless intelligence, that he may manage to convince quite a few readers, even some professionals, that he's really got something. The thesis is that man's ancestor, a tree-dwelling primate--who lived a relatively paradisal life of leisurely browsing, harmless and guiltless pollution, and sexual promiscuity--was forced down from the trees by drought too quickly to modify his fundamental nature. Driven for the sake of survival to ""unnatural"" acts like tool use, killing, sleeping in a camp, and dividing labor between the sexes, he began to develop ""mortar""--what most people call ""culture""--a set of habits that fill the gaps in his genetic nature, plus the cerebral cortex and language needed to make ""mortar."" As a result, our original nature has persisted in a repressed state instead of evolving to a civilized carnivore's. Thus the instincts against soiling the nest, overpredation, and killing one's own kind are imperfect. Keating interprets everything from child socialization to the Oedipus complex, female inequality, and drinking and smoking quite adroitly in terms of the A/T conflict; his one big blooper is the failure to label our culture-making as itself a true evolutionary adaptation, a secondary nature, however rudimentary. But the reader will recognize, in him- or herself, the tree-swinging goof-off behind the compulsive worker or behind the trained soldier.