She can click. She can warble vowel sounds. She’s Tilda, the ape that learned to communicate like a human. (Courtesy of Cologne Zoo)

The murky origins of human speech have long baffled researchers. One of the main sources of confusion stemmed from the curiosity of monkey speech. Monkeys are capable of producing any number of human-like sounds, so called “precursors” to human speech. They can chortle with a speech-like rhythm. They can smack their lips together. And they can produce harmonic tones that almost sound like words. So, the thinking went, monkeys must represent an evolutionary stepping stone on the path to human speech.

But then there was the problem of the great ape. The great ape, which includes gorillas and orangutans and has closer evolutionary ties to humans, could not make like the monkey. As far as researchers knew, it couldn’t make any sounds that sounded like human speech. Its sounds seemed to be emotion-based or involuntary: the “grumphs, gorkums and grumbles,” as told in a new study. That left an unexplained gap of some 25 million years between the monkey speech-rhythms and humans.

So did the capacity for speech leapfrog the great ape? What were scientists missing?

They were perhaps missing an orange, 50-year old orangutan named Tilda. Tilda is a very special ape, for she can do something that has never before been witnessed in a great ape. She can make noises that sound just like talking. The animal, housed at Cologne Zoo in Germany, can click her tongue and smack her lips to make the letters “t,” “p” and “k.” She can also murmur vowel sounds like some sort of invented language out of a science-fiction movie.

The realization that Tilda can make these sounds has some scientists thinking she might fill in that nettlesome evolutionary gap. “That was indeed the situation before our study with Tilda,” Adriano Lameira, lead author of a new study on Tilda published this week in Plos One, told The Washington Post. “Primates signals exhibiting very quick open-close cycles of the lips were only known in a couple of old world monkeys, not great apes, raising questions of continuity between these signals and human speech.”

A female orangutan called Tilda uses unusual, human-like sounds when she wants to draw the attention of nearby zookeepers. (PLOS)

Tilda was born in the wild on the island of Borneo, long before timber barons robbed Asia’s largest island of 90 percent of its primary forest. When Tilda was captured in 1965 around the age of 2, the forest was still lush, but she never saw it again. The next eight years in her life — between the time she was acquired by a Swiss owner and 1975, when she was brought to a Swiss zoo — are a mystery.  Because she exhibits human-like behavior unseen in the wild, such as hand-clapping and arm-waving, researchers believe she was probably “trained for human entertainment, possibly in Belgium.”

Tilda, while the first wild-born ape to produce these human-like sounds, isn’t the first ape on record to whistle. That mantle belongs to the National Zoo’s Bonnie, who was found to spontaneously whistle in 2008. That too was big news for researchers. “It counters long-held assumptions that non-human primates have fairly fixed sound repertoires that are not under voluntary control,” Serge Wich of the Great Ape Trust said then. “Being able to learn new sounds and use these voluntarily are also two important aspects of human speech.”

There are several key, exciting differences between Tilda and Bonnie. For one, Tilda’s sounds aren’t spontaneous. They’re concerted and direct. She makes human-like sounds when she wants to draw the attention of nearby zookeepers and cajole them into giving her food. And two, Tilda can make sounds substantially more complex and subtle than Bonnie.

Lameira and his team studied Tilda’s significance by recording her. They compared the recordings with those produced by 110 other orangutans that had already been observed for a total of 6,000 observation hours. Two of those sounds — the click and the vowel warble — were distinct to Tilda.

“These calls were produced by quickly opening-and-closing the lips, much alike humans do when talking,” Lameira said in a statement. “One of these calls presented similarities with human consonants, and the other with human vowels, the two basic building blocks of human speech.”