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When a baby babbles and their parents respond, these back-and-forth exchanges are more than adorable-if-incoherent chatter—they help to build a baby's emerging language skills.

But it turns out this learning strategy makes humans an oddity within the animal kingdom.

Only a handful of other species—including a few songbirds such as cowbirds and zebra finches—learn to "talk" by noting their parents' reactions to their initial coos and gurgles.

How did humans become adept at learning language this way? A new study across multiple members of the primate family tree suggests the answer may lie, in part, in newborn babies' fast-growing brains.

Published August 19 in the journal Proceedings of the National Academy of Sciences, come from research on a squirrel-size monkey called the marmoset.

Babbling beyond humans

In the wild, marmosets use their high-pitched calls to stay in touch when they're out of sight of one another in the thick, dense forests of northeastern Brazil.

Just over a decade ago, while studying marmoset vocalizations, Princeton professor of neuroscience and psychology Asif Ghazanfar and colleagues noticed that baby marmosets go through a , just like humans do.

As newborn marmosets grow, their first sputtering cries transform into the more whistle-like calls of adults. The researchers also found that baby marmosets who received more frequent adult feedback during their babbling bouts were quicker to catch on. They learned to produce adult-like calls significantly faster than the controls.

"That was a pretty big 'aha!' moment," Ghazanfar said.

These studies, published in 2015 and 2017 by Ghazanfar with his then-postdoc Daniel Takahashi, now at the Federal University of Rio Grande do Norte in Brazil, were some of the first evidence of what appeared to be vocal learning in another primate.

But humans and marmosets last shared a common ancestor some 40 million years ago. Even our closest living relatives, chimpanzees, need very little tutoring to make the sounds of their kin.

"So that kind of presents a puzzle," Ghazanfar said.

Since then, the researchers have been trying to figure out why humans and marmosets arrived at such similar learning strategies despite being so distantly related.

Neural growth spurt supports learning

In the new study, led by Princeton Ph.D. student Renata Biazzi, the researchers collected and analyzed previously published data on the brain development of four primate species including humans, marmosets, chimpanzees and rhesus macaques, from conception to adolescence.

The results suggest that, in early infancy, the brains of humans and marmosets are growing faster than those of other primates. Importantly, most of that growth happens not in the confines of the womb, as is the case for chimpanzees and macaques, but right around the time they are born and first experience the outside world.

In marmosets, as in humans, this also happens to be an incredibly social time, Ghazanfar said. That's because marmoset moms, like human mothers, don't raise their offspring without help. Babies interact with multiple caregivers who respond to every cry.

"They are a handful," Ghazanfar said.

And because their brains are still developing, "that means that the social environment an infant is born into has a tremendous influence" on their learning, he added.

Using a mathematical model, the researchers were able to show how these early interactions, when coupled with rapid brain growth, set the stage for vocal skills to develop later on.

Baby talk

Next, the team plans to look into whether adult marmosets use specific sounds when interacting with infants, much like human adults use "baby talk" to communicate with our babies.

By looking at the only other primate whose infants are capable of using feedback to learn sounds, scientists hope to better understand how a child goes from cooing and babbling to, say, negotiating their way out of chores or joining the debate team.

This doesn't mean that other primates can't change their calls later in life.

"We're only talking about vocal learning during infancy," Ghazanfar said. "This is the period when their brains are especially malleable."