It's kind of revolutionary in that sense. Previous experiments involved training the animals and used only short segments of speech. Brain mechanisms that allow us to perceive speech may not have evolved for that specific purpose, but as a general ability for detecting sounds, Hauser speculates. If so, it may go far back on the evolutionary road. Even crickets, which evolved billions of years ago, possess an ability to divide sounds into categories they avoid and categories they approach.
Birds also break up sounds into categories, particularly songbirds.
Looked at this way, it's not so surprising that monkeys perceive sound boundaries, and that these boundaries occur in the same place in humans, their evolutionary cousins. Nevertheless, the researchers did find subtle differences that may be important. The babies heard two different speakers each say 10 sentences in Dutch, then two others speakers voiced 20 sentences in Japanese. The investigators then tested them with synthesized speech from which they removed all speaker variations while precisely maintaining the rhythm of the two languages. Monkeys, on the other hand, hesitated more with synthesized than with natural speech.