If you’ve ever cooed at a baby, you have participated in a very special experience. Indeed, it’s an all but unique one: Whereas humans constantly chatter to their infants, other apes hardly ever do so, a new study reveals.
“It’s a new feature that has evolved and massively expanded in our species,” said Johanna Schick, a linguist at the University of Zurich and an author of the study. And that expansion, Dr. Schick and her colleagues argue, may have been crucial to the evolution of language.
Other mammals can bark, meow, roar and hoot. But no other species can use a set of sounds to produce words, nor build sentences with those words to convey an infinite variety of meaning. To trace the origin of our gift of language, researchers often study apes, our closest living relatives.
These studies hint that some of the ingredients of language had already evolved in the ancestors we share with living apes, which lived millions of years ago. Chimpanzees can make dozens of distinct calls, for example, which they can join into pairs to communicate new things. Building meaning from smaller units is what lets us create sentences from words.
Humans and apes are similar in another way: Their babies need time to learn how to make sounds like adults. Scientists have done much more research into how human infants develop language than into how wild baby apes learn to make calls. One striking feature of humans is the way that adults speak to young children. Baby talk — known to scientists as infant-directed speech — often features repeated words, an exaggerated stress on syllables and a high, singsong tone.
This distinctive pattern is very effective at grabbing the attention of young children — even when they’re too young to understand the meaning of the words that adults are saying. It’s possible that children pay attention to infant-directed speech because it helps them learn some of the basic features of language.
But the point in time at which infant-directed speech evolved has long been a mystery, without any in-depth studies of wild apes. “There was a huge lack of data,” said Franziska Wegdell of the University of Zurich.
To gather that data, Dr. Wegdell traveled to the Democratic Republic of Congo to observe bonobos, a species closely related to chimpanzees. Each day, she found an infant bonobo to follow, and observed every time that an adult communicated with it or with another adult in the infant’s presence.
At the same time, Caroline Fryns, a behavioral biologist at the University of Neuchâtel in Switzerland and an author on the study, went to Uganda. There, she and her colleagues observed chimpanzees, using the same methods employed by Dr. Wegdell on bonobos.
To round out their data on apes, the researchers used observations from earlier expeditions. Dr. Fryns had made observations of orangutans in Indonesia, while Lara Nellissen, a primatologist at the National Museum of Natural History in Paris, had watched gorillas in the Central African Republic.
The scientists gathered data not only on apes but also on children from cultures across the world. Dr. Schick went to the Amazon rainforest in Peru to do field work with an Indigenous group known as the Shipibo-Conibo. She filmed a child each day, noting each time an adult spoke to him or her. The researchers also analyzed similar observations made by other scientists in New Guinea, Nepal and the Swiss Alps.
The researchers discovered a stark difference between humans and apes: Young apes hardly ever heard infant-directed communication from the adult apes around them. Even among chimpanzees, which chatter to one another on a regular basis, the adults might call just once to an infant over the course of an entire day. On other days, the young chimps received no communication at all, not even from their mothers.
Human children have a profoundly different experience with language, the researchers found. In every culture, children were spoken to by adults many times a day — every few minutes, in some cases. The rate that children heard infant-directed communication was 69 times as high as what Dr. Fryns observed among chimpanzees, and 399 times as high as what Dr. Wegdell observed among bonobos.
“We can’t help ourselves, basically,” said Simon Townsend, a comparative psychologist at the University of Zurich and an author of the study.
The researchers speculated that young apes learned how to make calls by listening to adults call to one another. That was enough training to instill a relatively simple system of sounds. But when early humans began gaining a complex language, children needed more help. Talking to them a lot even before they could speak may have enabled them to master the spoken word.
“Children had an easier way into this more and more complex system,” Dr. Schick said.
Asif Ghazanfar, a neuroscientist at Princeton University who was not involved in the study, speculated that human babies gained the opportunity to learn from infant-directed speech. Human babies undergo a much longer period of brain development than baby apes do, leaving them helpless for longer.
“Not only are human brains therefore more plastic early in postnatal life, but they require much more caregiver attention for a much longer period of time,” Dr. Ghazanfar said. That close contact with caregivers could provide more time for infant-directed speech.
Marina Kalashnikova, a linguist at the Basque Center on Cognition, Brain and Language in Spain, praised the study as a comprehensive analysis. But she also noted that the youngest child in the study was 11 months old, while most were between the ages of 2 and 4. “I think that the age of the children may have influenced both how adults speak to them and also how much,” Dr. Kalashnikova said.
It’s possible that the patterns the scientists observed in humans are different for babies in the first few months of life, when they are just hearing human speech for the first time. But Dr. Kalashnikova noted that observing newborns would be especially hard. “While challenging, these data would be truly valuable for achieving the main aim of this work,” she said.
Dr. Fryn said that she and her colleagues were especially intrigued to find that apes communicated directly to their infants, even if they only did so rarely. The earliest roots of infant-directed speech might be hiding in those calls. But it will take more research to pin down what adult apes are saying to their infants.
“It’s not random noises, for sure,” Dr. Fryn said. “Clearly something is happening.”
Carl Zimmer covers news about science for The Times and writes the Origins column.
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