NEW YORK -- Listening to a newborn's plaintive squall, parents hear the hungry promise of a life to come.

In the babbling that precedes an infant's first halting attempts to talk, however, scientists are discovering crucial clues to how children master the languages that connect humans to one another.

A remarkable picture of how infants learn to speak is emerging from clinical child studies, language surveys and brain imaging, all colored by the hard-won crib-and-nursery knowledge gathered by generations of parents.

The newest insight offers clear evidence that the brain has become specialized for language by the time an infant is 5 months old, according to a report published Friday in the journal Science. Dartmouth College scientists found the clue in the lopsided way an infant's mouth opens and closes when he or she babbles or smiles.

Revealing expressions

Researchers Laura-Ann Petitto and Siobhan Holowka analyzed videotapes of 10 babbling babies from French- or English-speaking families. They collected 150 examples of facial expressions from each infant.

When the babies babbled, their mouths always opened more on the right side. When they made random cooing noises, their mouths always opened equally on both sides of the mouth. When they smiled, the mouth opened more on the left.

The expressions revealed the inner workings of the brain animating the baby's face.

The two sides of the face are controlled by a different half of the brain. The brain's language-related machinery is located on the left side of the brain, which controls the right side of the face. The distinctive right-side movement of the babies' mouths when they babbled showed that the brain's left hemisphere was in control, Petitto said.

"The findings were quite compelling," Petitto said, because they show that the brain has organized itself around language years earlier than previously believed.

"Babbling is the first step on the road to learning a language," she said.

So important is human language that the communication centers of the brain are among the first neural structures to mature, the new research shows. The compulsion to communicate can spur dramatic rearrangements in the way a baby's neural circuits function.

In a child's fledgling efforts to talk, scientists can detect an echo of primitive language skills that humans share with a surprising range of other creatures, from crickets to chinchillas.

Babies and Tamarin monkeys, for example, share the ability to discriminate between random noise and human speech in different languages, even though neither can understand the words it hears, French and American researchers found.

But no other creature babbles.

Underestimated babies

Elizabeth Bates at the University of California, San Diego, and other language anatomy experts argue that infants' brains -- perhaps the world's most powerful learning devices -- are quite capable of deducing the nature and structure of their native language from the conversations around them.

"I believe we have underestimated what babies bring to the world in terms of being able to learn," said neuroscientist Mark S. Seidenberg at the University of Wisconsin in Madison.

On average, an infant will hear some 16,000 examples of his or her parents' speech in the first few months of life.

To sort the sense of words and grammar from nonsense, infants' brains use surprisingly sophisticated statistical computations. They can quickly deduce rules and patterns about words and sentence structure well before they know what the words mean or even how to say them.