The sounds babies make - like ba, ba, ba, or ga, ga, ga, - are the first steps to language, say scientists.

Until now the significance of baby talk has been unclear.

Some argue babble is the start of speech but others say a baby is just making meaningless mouth movements.

Now researchers in the United States and Canada say they have evidence that the baby is learning to make real words.

When a baby smiles it uses the right side of the brain

They have found that when a baby babbles it engages the left side of its brain, the seat of language, which controls the right side of the face. This happens even as young as five months old.

"This discovery is the first to demonstrate left hemisphere cerebral specialisation for babies' production of language, just like we see in adults," says Professor Ann Petitto, from Dartmouth College, Hanover, New Hampshire.

"This suggests that language functions specialise in the brain at a very early age."

Pillow talk

She came to this conclusion by analysing video clips of 10 babies aged between five and 12 months.

The film showed the babies babbling, smiling or making other vocal sounds.

Dr Petitto and co-worker Siobhan Holowka of McGill University in Montreal slowed down the film and measured whether the babies' mouths opened more on the right or left side.

Writing in the journal Science, they say the babies' mouths opened more on the right side during babbling.

This was not the case for either smiling or non-babbling sounds.

The study suggests babbling is prompted by signals from a different part of the brain to non-babbling sounds or mouth motions.

The research could lead to ways to interpret mouth motion during babbling and, perhaps, make very early diagnosis of speech problems.