Bonobos use the same sound--a high pitched squeal--- in a variety of contexts, independent of their emotional state. This is strikingly similar to human homonyms, such as the bark on a tree or the bark of a dog. We have to use context to figure out which "bark" meaning is intended; so, too, do bonobos.
This is important because scientists used to think that non-human animals only communicated using sounds that were tied to emotions, such as a scream for fright or a bark/yell for anger. Instead, the evidence of bonobo homonyms indicates that bonobos have "functional flexibility," or the ability to use the same sound in multiple contexts (independent of emotion) to convey different meanings. Without using differences in acoustic structure, bonobos have to differentiate between different meanings of the same call. This "functional flexibility" is viewed to be a necessary precursor of language.
Thus, this finding pushes the evolution of language far back along our shared family tree with bonobos- about 6-10 million years! Alternatively, language and its precursors have evolved convergently at least twice among ape lineages.
Many thanks to Elliott Bannan for sending along this article!
For a long time, it was assumed that non-human primates, including great apes like chimpanzees and bonobos, could only communicate using calls that were tied to specific emotional states - such as screaming in alarm, or barking for aggression. Using a single vocal signal in multiple contexts - referred to as "functional flexibility" - was thought to be a human ability. And it is something we develop very early. Babies as young as 3-4 months, for example, have been shown to use squeals and growls across a wide range of situations, whether they are happy, distressed or neutral. These sit alongside other noises that are obviously tied to particular emotions, such as crying and laughing.