HER RECOVERED BONES would rattle around even in a small Tinker Toy canister. Her skull, if you could reconstruct it, would rest on your palm like a petrified grapefruit. Her V-shaped lower jaw suggests a weak chin and a sharp tongue; but although her theme song is a Beatles ballad celebrating a no-longer-chic hallucinogen, her leg bones and pelvic remains show her to have been a thoroughly upright, if not modern, lady.

Her name is Lucy (after "Lucy in the Sky with Diamonds," a tape of which played incessantly at the Hadar camp in Ethiopia on the evening of November 30, 1974, the day she was found); and at approximately 3.75 million years of age she is a stunning representative of a species of hominid, or "erect-walking primate," that appears to be an early human ancestor. In fact, throughly upright Lucy has throughly revamped the protean modern science of paleoanthropology by strutting her stuff to the foot of a brand-new family tree. Unless another fossil femme fatale comes along to bump her, she looks to have the stuff to stand her ground as an ancestor of us all.

Assisting Lucy to secure a place in the story of human evolution were Donald Johnson, the young American who found her, and Timothy White, a brash fellow countryman who had worked with Richad Leakey at Koobi Fora in Kenya and then with Richard's mother Mary at Laetoli in Tanzania. After much grueling if amiable infighting about what sort of creature Lucy might be, in 1978 Johanson and White (along with French geologist Yves Coppens) published a paper assigning her to a new species: Australopithecus afarenis. The naming of a new species, particularly when you are dealing with human ancestors, invites minute scrutiny of your data, the excitation of repressed rivalries, and attacks on both your credentials and your motives. These firestorms serve a purpose; they burn away flimsy designations and leave the strong one standing. For better than two years. A afarensis has taken the heat and withstood the flames. So have Johanson and White.

Tomorrow this could change.

In Lucy. The Beginnings of Humankind, Johanson and science writer Maitland Edey snopsize the entire tangled and fascinating history of the search of human origins. For the most part, they avoid retracing the terrain so carefully mapped in Richard Leakey and Roger Lewin's excellent popular study Origina (1977). Where Leakey and Lewin engage in some provocative behavioral and cultural speculations about our hominid forebears and ourselves, Lucy' s authors perfer to emphasize the discoveries and the conclusions of such renowned 20th-century field workers as Robert Broom, Raymond Dart, and the founders of the Leakey dynasty, Louis and Mary -- in whose direct line of descent, though not by birth a Leakey, Johanson altogether legitimately considers himself.

Johanson's premier find is Lucy. Her importance lies in the fact that she establishes the advent of bipedalism approximately 1.75 million years earlier than does Richard Leakey's important 1972 discovery of a representative of Homo habilis, an early hominid that a combination of dating techniques have recently declared to be not quite 2 million years old. Further, the small size of Lucy's brain, as deduced from her cranial capacity, does not permit Johanson to classify her as human. These two points -- upright walking and a decidedly ape-like brain -- strongly suggest that she belonged to a species ancestral to both the earliest humans and our extinct austrolopithecine cousins. The dispute over this interpretation, however, continues to rate, for the Leakeys believe that recognizably man-like hominids coexisted with Lucy's kind.

Unlike Origins, Lucy has a distinctly autobiographical flavor. And, despite the author's consious, even conciliatory, effort to keep the needle of self-deflation at hand, the book does not completely escape the toils of puffery and smugness. I do not see how it could. The "I" in the narrative is Donald Johanson (coauthor Edey selfessly extinguishes his identity in this pronoun), and at the heart of the book lies the gritty, absorbing chronicle of his four fateful seasons at Hadar, a stiflingly hot desert of ridges and wadis in Ethiopia's Afar Triangle. Here, between 1973 and 1977, Johanson and a team of highly trained French and American specialists made the discoveries -- the first hominid knee joint, Lucy, an entire hominid band nicknamed the First Family -- that elevate Johanson to a status comparable to that of "paleoanthroplogy's certified supernova, Richard Leakey." Although by no means a grandstander, Johanson clearly enjoys keeping such company, and he has earned the right. This book explains how and why.

Today, Johanson and the Leakeys hold mutually exclusive opinions about Lucy's significance. By lumping his Hadar finds and several of Mary Leakeys Laetoli fossils together as representatives of the same ancient australopithecine family, Johanson challenged two of the Leakey's entrenched beliefs about human evolution: first, that human beings are descended from an earlier variety of the genus Homo rather than from the australopithecines (extinct bipedal primates whose name means "southern apes"); and, second, that the genus Homo has bona fide representatives much older than 2 million years, whose remains have not yet been discovered. The nomenclature A. afarensis -- the southern ape from Afar -- effectively undermines both these notions. Moreover, it receives formidable scientific support from the accuracy with which the Hadar fossils were dated and from the rigor with which Johanson and White scrutinzed the dental structures of their finds.

One of the peculiar, oddly engaging aspects of Lucy is Johanson's seeming tendency to talk past the reader at his famous rivals in Kenya. These passages, although ostensibly aimed at a general audience, read like snippets of a passionate private letter alternately reasoning, cajoling, exhorting, and teasing the Leakeys out of the hidebound errors of their ways. Let me cite a fairly subtle example. Late in the book Johanson reports that a journalist investigating his and White's collaborative work on Lucy and the First Family had been put in mind of the partnership of the Noble laureates James Watson and Francis Crick in riddling out the double-helix structure of DNA.

"In fact," Johanson smoothly comments, "there was a geneticist, Rosalind Franklin, working at the same time they were, who all of her own nearly broke the DNA problem several times. She came so close, but she had nobody to talk to. She needed that last push -- another mind. She never got it. Tim and I, in having each other, had been extraordinarily lucky."

These sentences contains a puckish, or maybe infuriating, implication. Richard and Mary Leakey may be two people, but they cleave to the Louis S.B. Leakey line. Like Rosalind Franklin, they require a perspective other than their own. Indeed, they might have made the taxonomic breakthrough engineered by Johanson and White if only they had had another mind to prod them out of the bunker of their own preconceptions, hang-ups, and biases. Johanson wants to be that mind.

How to censure, or resist such winning chutzpah? Easily, perhaps, if your name is Leakey -- but I cannot help feeling that Johanson will not desist until his Kenya counterparts have raised their tight hands and repeatedly solemnly after him, "I love Lucy. . . " In any event, this is an exciting, informative, mind-rocking, and important book.