WASHINGTON - How did we learn to talk?

"Essentially a continuum existed from the verbal communications of our great-ape ancestors to fully modern language," Stanford University biologist Paul Ehrlich writes in another new book, Human Natures. This new light on the origin of speech comes from recent discoveries in archaeology, biology, genetics, brain science and child development. Here's how scholars think the process worked:

According to evolutionary theorists, a random genetic mutation that offered a small gain in communications skills - such as the ability to pronounce more distinct sounds - would help prehistoric groups perform any activity requiring cooperation. This includes vital skills such as hunting, fruit and nut gathering, and child rearing. This competitive advantage tucked into a gene was passed on to offspring, biologists say. Similar tiny advances followed until, after thousands of generations, a crude "proto-language" emerged.

Using fossil evidence, researchers now can trace this transformation back more than 3 million years. It required both physical and mental changes unique to our human ancestors. By contrast, a chimpanzee cannot raise its tongue to the roof of its mouth to cut off the passage of air and make sounds like "t" or "k." Nor can it form the various vowels necessary for human language.

"The ape lacks the motor control of the tongue and the diaphragm to make speech production, as we perform it, possible," said Sue Savage-Rumbaugh, an expert on chimpanzee communication at Georgia State University in Decatur, Ga.

This anatomical handicap persisted through the age of the earliest hominids - animals that walk erect, have opposable thumbs and relatively large brains. (The hominid family includes modern humans as well as an advanced class of apes known as australopithecines.)

Bones collected in Africa show that "Lucy," a famous australopithecine who lived in Africa 3.6 million years ago, had an ape-like vocal tract and therefore could not speak. Ehrlich suggests that Lucy and her contemporaries communicated by "a combination of gestures and grunts, like the systems used by medieval monks under vows of silence and by modern hunters and soldiers."

But as millennia passed, the shape of hominids' mouths and throats slowly altered. Their brains expanded, and new nerve connections enabled them to control their tongues more precisely. By 2 million years ago, an archaic species of humans known as Homo erectus had developed the physical organs and mental capacity to produce a rough form of speech.

Derek Bickerton, a linguist at the University of Hawaii, said this proto-language would have enabled Homo erectus to string together three to five words at a time. Their utterances probably would have consisted mostly of simple nouns - sounds for "tree," say, or "danger" - not verbs or adjectives, which are more abstract, Bickerton surmised. Nevertheless, it was a significant advance in communication. By 300,000 years ago, the Neanderthals, an extinct species of early humans, could do much better, but fossils indicate they could not pronounce sounds such as "ee" and "oo."

A Neanderthal would have sounded "like a village idiot," Lieberman said. "Their speech capabilities were intermediate between those of still earlier hominids and modern humans." The first modern vocal tract appears in fossils of Homo sapiens, our own species, about 100,000 years old.

While these anatomical changes were going on, our ancestors' brains were tripling in size and complexity. Lucy's gray matter occupied about 30 cubic inches in her skull, Homo erectus about 45 cubic inches and modern humans' brains about 90 cubic inches. This expanding brain power reflected developments in hominid society, such as the formation of larger social groups, more sophisticated collaborations, the beginning of specialization, and the ability to make tools and draw pictures.

Larger brains were both the cause and the result of these social advances. Those equipped with higher skills were more likely to survive and pass their genes on to their children. Those who were able to speak fared better than their less gifted contemporaries and came to dominate the species.

"The superior brain of our ancestors, not their brawn, enabled them to displace the archaic human beings, the Neanderthals and Homo erectus populations," said Lieberman. "Eve and Adam and their progeny prevailed because they talked."