The sounds of marmoset monkeys chattering may hint at the mysterious origins of human language.

A new study shows that marmosets exchange calls in a precisely timed, back-and-forth fashion typical of human conversation, but not found in other primates. The monkeys don't appear to have a language, but the timing suggests the foundations of our own.

"That could be the foundation of more sophisticated things, like syntax," said psychologist Asif Ghazanfar of Princeton University, co-author of the study, which was published today in Current Biology. "You can't have any of those other really cool aspects of language without first having this."

How language, so complex and information-rich, evolved in Homo sapiens and, as far as we know, no other species, is one of anthropology's outstanding questions. The traditional, seemingly intuitive answer is that it arose from the vocalizations of ancestors who were capable of a few rudimentary noises and wanted to say more.

Confounding that narrative, though, is the comparatively less-vocal nature of many other primates, including our closest living relatives, chimpanzees and bonobos. They do vocalize, of course, and even say some interesting things, but not with the same flow expected of some proto-human linguistic capability.

That conundrum has led researchers to propose another possible origin of language, one rooted not in our voices but rather our bodies, and in particular our hands. According to this narrative, gesture would have been as important to our ancestors as sound. Indeed, neurological processes underlying speech and language are also intimately linked with motor skills, raising the possibility that language formed on the cognitive scaffold of gesture — and chimpanzees do have a large repertoire of hand movements.

But many scientists, including Ghazanfar and the study's lead author, fellow Princeton psychologist Daniel Takahashi, aren't convinced. If human language did follow on gesture, they wonder, why don't chimps talk more? There's also no evidence in chimpanzees for vocal turn-taking, or waiting for another person to finish speaking before replying, which is universal in human languages. "If we don't take turns, if we're overlapping, it's very difficult to understand each other," said Ghazanfar. "Turn-taking is foundational."

Yet even if chimps don't take turns, Ghazanfar and Takahashi found that marmosets do. In the new study, they placed pairs of marmosets in the opposite corners of a room, separated by a curtain that allowed them to hear but not see each other, and recorded the ensuing chatter.

These proved to follow turn-taking patterns, with a pause of several seconds between the completion of one monkey's whistles and the other's beginning. And unlike the duets of birds, which are often highly synchronized, the exchanges had nothing to do with mating or territoriality. The monkeys were conversing.

As for what they said, marmoset whistles are thought to encode information about a caller's identity, age, gender and location. Ghazanfar thinks the conversations are a sort of "vocal grooming," a way of easing stress or conveying affection, but delivered at a distance. It only works when monkeys know they're being addressed individually, which is conveyed by the turn-taking form.

"It could be a pre-adaptation for language," said evolutionary biologist Thore Bergman of the University of Michigan, who was not involved in the study. Bergman's own research involves human-sounding lip smacks made by monkeys called geladas.

As for why marmosets and humans take turns, but not chimpanzees, Ghazanfar suspects it's a function of our social systems. Marmosets are cooperative breeders: Group members take care of offspring unrelated to them, creating community-oriented dynamics of behavior and communication. Ancestral humans may have lived the same way.

Without a time machine, of course, questions about the origin of human language won't ever be settled. As Bergmann noted, the findings don't exclude the possible importance of gesture. It's possible that human language arose from the fortuitous interactions of gesture, vocalization and social structure with evolutionary pressure.

Indeterminacy aside, though, it's fun to speculate, and also to wonder whether the seeds of complex language now exist in animals other than ourselves. Many whales and dolphins, along with syntax-using monkeys and even prairie dogs, communicate in very sophisticated ways.

"If you went back 10 million years, you'd be hard-pressed to predict an ape would end up with the planet's most complex vocal communication system," said Thore Bergman. "Why that happened is a really big puzzle."

Citation: "Coupled Oscillator Dynamics of Vocal Turn-Taking in Monkeys." By Daniel Takahashi, Darshana Narayanan and Asif Ghazanfar. Current Biology, 17 October 2013.