Scientists used to think that apes were not intelligent enough to speak; however, it is now thought that an ape’s vocal cords are not “built” for speech. After many years of observation, it is now known that apes do use vocal communication, but it is usually in the form of hoots and grunts, with accompanying gestures.

But apes are not the only primates, and researchers have discovered that most primates lack the vocal anatomy necessary to make more “sophisticated” sounds. But they still wanted to know how humans could have developed speech. In 2012, researchers began to look closer at the sounds made by other primates. One study suggested that human speech may have begun similar to a behavior many primates have called lipsmacking, in which the animals move their jaws, tongues, and lips in much the same way humans do when they talk. In addition, in 2013, researchers studying the gelada, a monkey found in the highlands of Ethiopia and closely related to the baboon, noticed that the animal made a gurgling noise. They believe that the noises made by the monkeys have some speechlike properties—a sound that they describe as a cross between a yodel and a baby’s gurgle.