FOR A GOOD four or five million years, or ever since the first stirrings of two-leggedness among our ancestors, we bipeds have wisely refused to bicker over which sex had the idea first.

Until now.

Anatomist C. Owen Lovejoy, who started it all, swears he never said, and does not believe, males were bipedal ahead of females. "There are no mammal species that I know about where male locomotion is substantially different from that of females," he notes. But Lovejoy did say that the male's emergence as a provider for his mate and off-spring, and the need to use his forelimbs for carrying food rather than locomotion, could have been the key factor in getting bipedalism started. He went on to say that a monogamous male keeping his brood well-nourished, while the female stayed at home in the trees tending to her young, would be acting in the way best-suited to the perpetuation of his genes.

Anthropologist Rebecca L. Cann, one of nine irate correspondents who tore into Lovejoy's thesis in the July 23 issue of Science, calls it a "heavily malecentered" scenario that examines early hominid evolution "in terms of Western cultural standards" and ignores "the fact that if you look at feeding patterns in hunter-gatherers today, you see that the female does most of the providing."

Besides, says Cann, monogamy is not a universal fact of human life, and in other sexual circumstances the female is far better poised to make genetic plans. She, after all, knows her children are hers, while the male can only speculate about such matters. This leads Cann and biochemist Allan C. Wilson to propose a "maternal model" for the appearance of bipedalism. "In Lovejoy's view, it's the male carrying food," says Cann. "In our view, it's the female carrying infants and food... It's more efficient it you think of females because by knowing their own offspring and directly aiding thir own offspring, the [natural] selection can act faster."

Whether the original stand-up guy was actually a guy is, perhaps, a small diversion in the great flow of current paleoanthropological thinking. But bipedalism itself -- how, why and when it happened -- has been looming larger and larger in the study of human origins. Although modern man may be prouder of his brains than his posture, the hard evidence of fossils has been arguing for some time now that uprightness, as Friedrich Engels presciently observed in 1876, was "the decisive step" in human evolution.

Since the 1920s, it has been known that the australopithecines -- preliminary candidates for the role of "missing link" between apes and humans -- walked on two feet and had brains scarcely a third the size of homo sapiens. Still, little was known about how they lived or when they lived (from about 2.5 million until 1.2 million years ago, it turns out), and for decades scientists could safely go on associating the advent of bipedalism with such advanced skills as tool use and organized hunting. Then along came the great hominid fossil finds of the 1970s -- "Lucy" and her kindred specimens of australopithecus afarensis, unearthed in Ethiopia by American Donald Johanson.

Named after the Beatles' song "Lucy in the Sky With Diamonds" (which happened to be playing on the camp tape recorder soon after she was dug up), Lucy showed, in Lovejoy's words, "an essentially complete adaptation to bipedal locomotion" as of 3.5 million years ago. According to Lovejoy's friend Johanson, Lucy "destroyed" the theory that bipedalism arose in gradual tandem with brain development, manual dexterity and higher culture.

Lucy has also encouraged the idea that bipedalism could have been a sudden development -- by evolutionary standards, that is. Berkeley's Allan Wilson and Vincent Sarich, developers of an evolutionary "clock" based on comparing amino acid sequences in related animals, believe apes and humans went their separate evolutionary ways no more than 6 million years ago. By Lucy's time, that split seems to have been completed, and the major result -- on our branch of the family tree -- was a new form of locomotion. Hence Harvard's Stephen Jay Gould calls bipedalism "the surprise, the difficult event... the great punctuation in human evolution."

To Johanson's associate Timothy White, it's a matter of common sense. "You don't gradually go from being a quadruped to being a biped," he says. "What would the intermediate stage be -- a triped? I've never seen one of those."

As the discoverer of Lucy, Johanson has emerged as the most celebrated man in the field, and he, White and Lovejoy have formed an influential alliance in the field. (Influential and also close: Developing his theories about bipedalism and monogamy, Lovejoy wrote that unlike other primates with defined mating seasons, "human females are continually sexually receptive," and he added a footnote crediting this insight to "D.C. Johanson, personal communication." This jest apparently eluded the editors of Science -- but not its readers. One female anthropologist retorted that "a personal communication from D.C. Johanson is unacceptable as evidence." Another declared that "no female is 'continually sexually receptive'" and "any male who entertains this illusion must be a very old man with a short memory or a very young man due for a bitter disappointment.")

The stunning announcement that Lucy was a complete biped and an ancestor to both humans and australopithecines has enjoyed wide acceptance -- as wide as acceptance gets in this fractious profession. But among their colleagues, Johanson, White and Lovejoy are not universally reversed. And in recent months, doubters have raised a troublesome question: Was Lucy really the last word in bipedalism, or was she a limited walker who still possessed the long, flexible toes of her ape-like ancestors and spent much of her time in the trees?

The second alternative is favored by Jack T. Stern and Randall I. Susman of the State University of New York at Stonybrook. "The best material -- that of Lucy herself -- indicates very substantially that although bipedalism was Lucy's predominant way of getting about, she spent a lot of time in the trees," says Stern. "Lucy's toes are much longer, proportionally, than modern humans', and very curved.... The curvature of the toe bones is virtually identical to the curvature of the toe bones of a pygmy chimpanzee. We know of no explanation for curved toe bones other than for grasping branches."

Stone and Susman see further hints of arboreality in an astonishing set of 3.7 million-year-old hominid footprints found in volcanic ground near Laetoli, Tanzania, by Mary Leakey -- the same footprints cited by Johanson, White and Loveyjoy as irrefutable proof of their thesis. Susman and Stern contend that these footprints lack the pronounced big toe of human footprints, while the impression of a human-like arch is deceptive. All in all, says Stern, "they did not look like the footprints of an entirely modern human."

Climbing, according to Stern and Susman, is a "predaptation" to bipedal walking, placing similar demands on the nervous and muscular systems. So a flexible, tree-dwelling primate that began paying regular visits to ground level might not need any special reason for choosing bipedalism oiver quadrupedalism; and a fairly crude brand of bipedalism might serve if the animal's aim were only to forage among nearby bushes and clumps of trees, falling back on its climbing talents as a defense against predators.

Michael Rose, an anatomist at the University of Medicine and Dentistry of New Jersey, theorizes that bipedalism began in this humble fashion over 5 million years ago, as the forests of the Miocene age gave way to a mixture of woodland and grassland. Then, over hundreds of thousands or millions of years, hominids could have learned skills that gave "positive feedback" to bipedalism, and "the whole thing could start snowballing," says Rose. "If you assume that it was this rather strungout process that started out sometime in the late Miocene and went right through to homo erectus times [1.5 million years ago], then the last phases of that process may well have been associated with increased brain size, tool use, language and so on."

Of course, this theory involves an altered picture of Lucy and, perhaps, some of her descendants as well. Homo habilis, a younger human ancestor who lived about 1.8 million years ago, after the human and australopithecine lines diverged, may also have been something less than a complete biped, in Rose's opinion -- to which he adds that "this is pretty contentious stuff."

In a paper submitted to the American Journal of Physical Anthropology, Stern and Susman go into "excruciatingly boring detail" to document their case, according to Stern. But Lovejoy, he says, "has refused to reconsider."

Lovejoy says this is because "I am in total disagreement with their conclusions." Lucy's non-opposable big toe is a bad climbing implement, he argues, and "the sum total of the evidence that we have from hip, knee, ankle, foot, shoulder and hands are indicative of an animal that was poorly adapted to climbing and well adapted to upright locomotion.... Their lower limb was restricted to certain positions at the knee and hip, which good climbers are not. They would have minimal leaping ability, which good climbers have."

Lovejoy's own scenario is a complex one, setting the male role as provider against the background of a broad "demographic dilemma" faced by the late-Miocene ages -- with their relatively low rates of reproduction, their long periods of child-nurturing, competition from more fertile monkeys, and the shrinking of the forests in which monkeys, apes and early hominids coexisted. Oddly, the complexity of his argument has drawn praise as well as criticism -- praise for emphasizing the intangibles of social structure rather than habits that leave a more obvious archeological record, and criticism for overreaching.

"I thought that the article was very interesting and very stimulating but based on no direct evidence," says Harvard anthropologist David Pilbeam.

Stern, less diplomatically, calls Lovejoy's analysis a "Just So Story," and notes that its least controversial element -- the value of bipedalism for carrying food -- is not new. Anthropologist Gordon W. Hewes made the same connection 20 years ago, and ran experiments with macaque monkeys to back it up. "Bipedal locomotion can be elicited instantaneously in macaques," wrote Hewes, "by presenting a sufficiently bulky food burden and subsequently threatening to take it away. Bipedalism will be maintained as long as the animal is pursued, provided he does not find a safe perch or is not so terrified by his pursuer that he abandons the food burden and reverts to the more rapid quadrupedal gait. If the pursuer halts, the animal may stand erect, attempting to stuff the food into his mouth, but warily poised for further bipedal evasion."

Berkeley's Glynn Isaac is another anthropologist who has written about food transport as a spur to bipedalism. But Isaac believes a primitive carrying device -- perhaps made of bark -- would be required to make this a worthwhile innovation. He also believes that meat -- "a portable class of food that's worth carrying around" -- would have to have been a significant part of the diet.

Although the jury is out on the exact diet of early hominids, little sympathy remains for the "Mighty Hunter hypothesis" popularized by Robert Ardrey and others in the 1950s -- the idea that a peace-loving herbivore, the ape, came down from the trees and became a club-wielding carnivore, man. As Lovejoy points out: "No form of locomotion could be less suited [than bipedalism] to the tracking and capture of large prey, prior to the evolution of relatively advanced material culture and social cooperative behavior."

But this is one of the few consensuses in paleoanthropology. The effort to tell the story of human evolution from fossils has been compared to reassembling the plot of "War and Peace" from 13 random pages. "Neither Lovejoy nor I nor anyone else has any direct evidence on the origin of bipedalism," says Michael Rose. "It's pure speculation." So controversy is inevitable, and so are wild ideas -- like the one advanced by anthropologists John Gribbin and Jeremy Cherfas in a book called "The Monkey Puzzle."

Gribbin and Cherfas dismiss Lovejoy's male provider as a mythical figure comparable to the "Mighty Hunter." Men have been telling tall talse about themselves for thousands of years, they say, and "nor is this braggard behavior entirely purged from the halls of academe." Then they offer their own "armchair speculation": that the australopithecines, instead of dying out a million years ago as postulated by Johanson, went back to the trees and evolved into today's gorillas and chimpanzees. Australopithecine teeth became larger as the teeth of their human cousins became smaller, Gribbin and Cherfas observe, and if that sort of reversal is possible, they suggest the australopithecines could have gone on growing more "apelike" until they turned into actual apes -- "giving up the difficult business of becoming human and settling down to a sybaritic life of ease beneath the trees."

Since a number of anthropologists believe that Lucy and Co. should have been classified as homo, or human, rather than as australopithecines, Gribbin and Cherfas are coming perilously close to suggesting that apes are descended from men. Obviously, nothing is sacred. Or as David Pilbeam says, "The actual data are not incongruent with any scenario."

Indeed. On Pilbeam's own flowchart of how monkeys, apes and hominids evolved from roughly 20 million years ago to the present, there is many a blank space and wavy line indicating uncertainties in the fossil record. And there is one long dotted line indicating gigantopithecus, an unusually large protoape whose relatives have all been extinct for 8 million years but who himself -- according to fossil teeth found in China -- mysteriously survived until a mere 500,000 years ago. Or if you listen to some people, theye may be a few gigantopitheci around to this day, otherwise known as yetis or abominable snowmen. If so, there is a place for them on Pilbeam's chart.

Johanson has spoken of a "black hole," a "pall of ignorance" in the fossil record after Lucy -- from 3 million to 2 million years ago -- and an even bigger gap precedes Lucy. "This is a situation where, in order to make significant advances, we would need a lot of new material," says Pilbeam. "In a sense it's not worth arguing about until we get a better fossil record." Lovejoy also acknowledges the problem, but expects the solution to turn up shortly. "I put in an order with both White and Johanson for a pelvis from 5 1/2 million years," he says jocularly, "and I hope UPS will deliver it sometime this fall."