After analyzing decades-old videos of captive chimpanzees, scientists concluded that the animals could say a human word: “mom.”
The dialogue isn’t as expansive as the one in 2018’s “Reign of the Planet of the Apes.” But the discovery, published Thursday in the journal Scientific Reports, could offer important clues about how speech evolved. Researchers say our common ancestors with chimpanzees had brains already equipped with some of the building blocks needed for speech.
Adriano Lameira, an evolutionary psychologist at the University of Warwick in Britain and one of the study’s authors, said the ability to speak is perhaps the most important feature that sets us apart from other animals. Being able to talk to each other allowed early humans to cooperate and accumulate knowledge over generations.
“It’s the only feature that explains why we’ve been able to change the face of the Earth,” Dr. Lameira said. “Without it, we’d just be an ordinary ape.”
Scientists have long wondered why we can talk and other apes can’t. This curiosity led to a series of strange and cruel experiments in the early 1900s. A few researchers tried raising monkeys in their own homes to see if living with humans could get the young animals to talk.
In 1947, for example, psychologist Keith Hayes and his wife Catherine adopted a baby chimpanzee. They named her Viki, and when she was five months old, they began teaching her words. After two years of training, the couple later claimed that Viki could say “daddy,” “mommy,” “stand up,” and “cup.”
In the 1980s, many scientists dismissed the experiments of Viki and other adopted monkeys. For one thing, separating babies from their mothers was probably traumatic. “It’s not the kind of thing you could fund today, and for good reason,” said Axel Ekstrom, a speech scientist at KTH Royal Institute of Technology in Stockholm.
Beyond ethical considerations, the adoption experiments did not result in the animals becoming fluent speakers. The animals had difficulty producing even simple sounds.
This gap between the abilities of humans and apes has fueled a debate: Were chimpanzees unable to speak because of their vocal anatomy or because of their brains?
For decades, Philip Lieberman, an anthropologist at Brown University, was a vocal advocate for the existence of the vocal tract. In 1969, he observed that the human larynx and tongue were lower in the throat than in other primates. Dr. Lieberman, who died in 2022, argued that this anatomical change allowed humans to produce the wide range of sounds needed for complex speech.
But in 2016, a team of scientists took X-rays of vocalizing monkeys and discovered that the primates’ vocal tract was actually “ready to speak.” That led some researchers to wonder whether monkey brains were missing something essential for speech.
Some studies suggest that the human brain is unique in that it can send coordinated commands to the jaw and throat. This evolutionary step may have allowed our ancestors to combine consonants and vowels into syllables, which could then be combined into words.
And humans, unlike the vast majority of other animals, can learn new sounds from others.
But the authors of the new study suspect that the apes have been underestimated. In his own research on orangutans, Dr. Lameiro is convinced that apes are capable of learning to vocalize. Wild orangutans from neighboring groups, for example, emit different calls. In zoos, they have learned to imitate the whistle of a janitor.
Mr. Ekstrom also wondered whether scientists had been too quick to dismiss the adoption experiments as failures. No one had ever analyzed the sounds made by Viki and the other chimps.
He began searching for recordings. Eventually, Mr. Ekstrom discovered that Viki appeared in a 1959 documentary. In the film, the young chimp appears to say “daddy” three times and “cup” once.
Mr. Ekstrom recorded himself saying “daddy” and “cup,” then compared his voice to Viki’s. Every time she said “daddy,” she made the same sound, suggesting that she had indeed learned to say something new.
But as Mr. Ekstrom reported last year, Viki’s version of “daddy” was fundamentally different from his. She pronounced only two loud “puh” sounds, with no vowels. She pronounced “cup” the same way, with only a “c” sound followed by a “p.”
“It’s not very convincing if you try to say, ‘Look, this chimpanzee has learned to talk,'” Ekstrom said.
Mr. Ekstrom and Dr. Lameira teamed up to search for other recordings. They found a short video on YouTube of a chimpanzee named Johnny who had lived at the Suncoast Primate Sanctuary in Palm Harbor, Fla. In the video, posted anonymously in 2007, Johnny appears to say “mom” in response to a woman’s encouragement.
Nancy Nagel, a 30-year board member at the sanctuary, confirmed that the chimp in the video was Johnny. “He was saying in a very raspy voice, ‘Mom,’” she said in an interview. “That’s all he was saying.”
Mr. Ekstrom and Dr. Lameira then found a 1962 newsreel of an Italian chimpanzee named Renata. She also said something that sounded like “mama.”
In the new study, the scientists analyzed how Johnny and Renata pronounced the word. Their sounds more closely resembled Mr. Ekstrom’s version of “mama.” Unlike Viki, both Johnny and Renata could add a vowel after a consonant.
“It basically sounds like a word,” Mr. Ekstrom said. “It’s a very particular, very unique acoustic profile. You can’t mistake it for anything else.”
Mr. Ekstrom then played the recordings to 61 volunteers, who then noted the sounds they heard. Most of them agreed that Renata and Johnny were saying “mom.”
“This study is a good example of the fierce struggle in the field of monkey language,” Julia Fischer, a cognitive scientist at the German Primate Center in Göttingen, said by email. She was not convinced by the study. “What monkeys do vocally has nothing to do with human language,” she added.
According to Dr Fischer, Renata and Johnny may instead be making a typical chimpanzee sound known as a “gasp-grunt.” “The simplest explanation is that they simply added two of these gasp-grunts together and were rewarded for it,” she explained.
But Michel Belyk, a psychologist at Edge Hill University in Britain, said the findings actually suggested that ancient apes may have already possessed some of the mental requirements for speech.
“Humanity, for all its peculiarities, was not born from a rock, but was shaped by evolution from the clay of our primate ancestors,” he said.
Mr. Ekstrom does not believe that Johnny and Renata alone can solve the question of the origins of speech. “It clearly shows that some theories are too simple to account for all the facts,” he said.
To find new clues, he is now studying fossils of our ancestors after they split from other apes.
“This black box spans about six million years, so it’s very spacious,” he said.