hat a hairy back!" was Lily Tomlin's candidate for the first human sentence. But whatever the content of that original remark, if Michael C. Corballis is correct, it was expressed in gestures, not words.

Mr. Corballis, a psychologist at the University of Auckland, in New Zealand, is the latest proponent of a controversial idea known among language experts as the "gestural theory." In essence, gestural theorists contend that long before early humans spoke they jabbered away with their hands.

Where language comes from remains one of human evolution's enduring puzzles. But in a new book, "From Hand to Mouth: The Origins of Language" (Princeton Univerity Press), Mr. Corballis pluckily takes a stand, arguing that speech was an ingenious innovation but not quite the freakish marvel that linguists have often made it out to be. Proposing that human ancestors made the switch from gestures to speech quite recently — he puts the date at around 50,000 years ago, a mere yesterday in evolutionary terms — Mr. Corballis believes that language itself, and the sophisticated mental capacities necessary to produce it, are far older.

"The common ancestor of five or six million years ago would have been utterly incapable of a telephone conversation but would have been able to make voluntary movements of the hands and face that could at least serve as a platform upon which to build a language," he writes. "Grammatical language may well have begun to emerge around two million years ago but would at first have been primarily gestural, though no doubt punctuated with grunts and other vocal cries that were at first largely involuntary and emotional."

It sounds plausible enough. All you have to do is look around to see how much hand-waving still accompanies human communication today — even people on cell phones do it. But Mr. Corballis has yet to convince many linguists of the theory's merits.

"He's not a linguist, and I think he doesn't appreciate the sophistication of grammatical organization," said Ray Jackendoff, a professor of linguistics at Brandeis University. "I never saw any reason one way or the other to say that language started gesturally rather than vocally. If it started in the gestural modality, you still have to explain how in switching to that vocal modality there's this terrific adaptation."

Here, of course, the fossil record is of little help. As Mr. Jackendoff put it: "The problem of talking about the evolution of language in any detail is that there is no evidence. It's pure speculation."

But that hasn't stopped scholars from pursuing all manner of theories — or engaging in charged debate. In 1866, the Linguistic Society of Paris banned all discussion of the evolution of language — presumably in order to keep tempers in check. A few years later, Charles Darwin ventured that human speech may have evolved from animal cries, a notion that was famously derided by his opponents as the "bow-wow" theory. The French philosopher Abbé Étienne de Condillac, whom Mr. Corballis credits with being the first gestural theorist, took a more strategic tack: when he presented his theory in 1746, he delivered it in the form of a fable so as not to arouse the ire of the Catholic Church. (In those days, the official wisdom was that language came from God.)

In recent decades, resistance to the origins question has come less from clerics than from cutting-edge linguists and biologists. For example, Noam Chomsky, the M.I.T. professor whose ideas have dominated the field for more than 40 years, has often been accused of depicting language as a trait so remarkable that natural selection is virtually helpless to explain it.

Mr. Chomsky's celebrated theory of Universal Grammar supposes that human languages share an underlying set of rules that are innate rather than learned. But some readers of his work have taken him to mean that the capacity for language arose all at once rather than incrementally, the product of what one critic derisively termed "the cognitive equivalent to the Big Bang."

Lately, however, Darwinian accounts of language have begun to proliferate, buoyed by new research on primate communication and human sign language as well as the more general scholarly vogue for evolutionary theory. In his 1994 best seller, "The Language Instinct," the M.I.T. professor Steven Pinker eloquently defended the idea that language evolved by natural selection, though he conceded that "the first steps toward language are a mystery." (If forced to speculate, he added, he would be inclined to bet on primate calls rather than gestures as a likely precurser to speech.)

Mr. Pinker's book seems to have opened a floodgate of possibilities. In 1996, the British anthropologist Robin Dunbar proposed that language had evolved from primate grooming behavior. Among apes and monkeys, physical contact — tickling, scratching and picking at each other's lice — functions as social glue, establishing hierarchies and alliances and communicating empathy or remorse. But back-scratching a whole band of baboons takes time. As early hominid populations expanded, Mr. Dunbar theorizes, speech simply became the more efficient option — think of it as a kind of group massage.

More recently, Peter MacNeilage and Barbara Davis, researchers at the University of Texas at Austin, have developed an "ingestive theory," which links the evolution of speech to the movements the mouth makes while chewing. "The mouth closes and opens in chewing just as it closes for consonants and opens for vowels," Mr. MacNeilage explained in a telephone interview.

Meanwhile, Michael Arbib, a computational neuroscientist at the University of Southern California, is working on a variant of the gestural theory based on the discovery of similarities in the way human brains recognize language and monkey brains recognize gestures.

"The whole field is not settled," said William Calvin, a neurobiologist at the University of Washington in Seattle, whose own theory is that language evolved from the rapid mental reflexes required to, say, throw a spear at a running mammoth. "Everybody's got a theory."

Mr. Corballis says evidence to support the gestural theory is growing. Researchers now know, for example, that sign languages are as grammatically sophisticated as spoken ones. Moreover, both speech and signing depend on the left side of the brain — the same side that happens to control most people's dominant hand, the right one.

From an evolutionary standpoint, Mr. Corballis argues, the gestural theory has several advantages. For one thing, it would help explain why chimpanzees — mankind's close cousins — are adept at learning forms of sign language and notorious failures when it comes to imitating human speech or even controlling their own cries.

Moreover, he suggests, the upright posture adopted by early hominids — humans' apelike ancestors — as long as two million years ago would have facilitated hand-based communication. "Bipedalism encouraged manual gesturing," Mr. Corballis said in a talk at a recent Harvard University conference on language and evolution.

He hinted that gestural theory could clear up another mystery about this period as well: why the stone tools of these early hominids show little evolution for almost two million years, despite increases in brain size. What if these bipedal creatures were so caught up in five-fingered chit-chat that it got in the way of their tool making? In the 1970's, one anthropologist went so far as to suggest that the reason humans evolved unpigmented palms — unlike other primates — is so their hand signals would show up better around the campfire at night.

That leaves the sticky questions of why and when these hypothetical skillful signers bothered to switch to speech. For a while, Mr. Corballis speculates, they probably used a mix of both. Then, about 50,000 years ago, there was a momentous change: an explosion of technology, cave art, textiles and even musical instruments. Mr. Corballis's interpretation? Freed from the task of communicating, hominid hands were finally able to get down to the real toil of creating civilization.

But his most provocative idea is that human ancestors stopped gesturing and started talking not because their brains underwent a sudden mutation — a cognitive Big Bang — but rather because it seemed to some Homo sapiens at the time like a good idea. He called the advent of autonomous speech a "cultural invention," like writing, and one that "may have occurred long after it became possible."

And once speech caught on, he argues, it gave Homo sapiens a decisive advantage over less verbal rivals, including Homo erectus and the Neanderthals, whose lines eventually died out. "We talked them out of existence," Mr. Corballis said with a satisfied grin.

The gestural theory makes for a captivating story. Yet like so many other theories, it may turn out to be little more than that. The question of where language comes from may simply be unanswerable, said Richard Lewontin, a professor of biology at Harvard. "If you don't have a closely related species with a similar trait you have the problem of novelty," he said. "And what you and I are doing right now, no bonobo or chimpanzee will ever do.'`

Mr. Chomsky agreed. "This task intrigues people because it's about us," he said. "But that doesn't make it a scientific question. It may be important for us to know where we came from, but if we can't answer that question scientifically, we can't answer it. If you want to tell stories, well then, tell stories."

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