Washington ? Every parent is familiar with “motherese,” that slow, high-pitched, singsong tone that mothers all over the world use to talk to their babies.

You might suppose that this manner of speaking is just a simplified, degenerate form of grown-up language.

To the contrary, some scientists think motherese may lie at the root of human language — long, long ago when our primitive ancestors came down from trees and started to walk on two legs.

Dean Falk, the head of the anthropology department at Florida State University in Tallahassee, calls it the “putting the baby down hypothesis.”

As they began to walk erect, instead of on their knuckles like chimps, early members of the human family developed taller, thinner, less hairy bodies than their apelike predecessors. The female pelvis narrowed even as infants’ brains were getting bigger. Babies had to be born early to squeeze through the birth canal.

These helpless creatures couldn’t cling to their mothers’ fur or ride piggyback the way 2-month-old chimp youngsters do. They had to be held in their mothers’ arms.

But mothers foraging for food needed to park their babies on the ground so they could use both hands. While they were busy, they emitted soft, cooing sounds to keep the infants calm and quiet, so as not to attract predators.

“To feed yourself, you have to put the baby down right there next to you,” Falk said in a telephone interview. “You have to keep the baby with you or it’ll be eaten by a lion. You don’t have touching contact, but you can reassure the baby with melodic tones. It’s a substitute for cradling arms.”

Early human mothers who behaved this way raised more children successfully than those who didn’t, Falk theorized. Their genes were passed on to their descendants and became a universal human trait.

“The urge to go ‘ga-ga, goo-goo’ to small babies is universal,” she said. “This became a genetic pattern. If mothers didn’t vigilantly attend to their infants, their infants were lost, and the mothers’ genes were lost to the future.”