An orangutan called Rocky could hold the secret to the origin of language, after displaying an ability to emulate human speech.

The ginger ape has astonished experts by producing sounds similar to words in a “conversational context”.

Researchers conducted a game in which Rocky mimicked the pitch and tone of human sounds and made vowel-like calls.

Comparing Rocky's sounds against a large database of recordings of wild and captiveorangutans showed they were markedly different.

Rocky was able to learn new sounds and control the action of his voice in the way humans do when they conduct a conversation, the scientists concluded.

They believe Rocky could be the key to understanding how human speech evolved.

Lead researcher Dr Adriano Lameria, from the University of Durham, said: “It's not clear how spoken language evolved from the communication systems of the ancestral great apes.

”Instead of learning new sounds, it has been presumed that sounds made by great apes are driven by arousal over which they have no control, but our research proves thatorangutans have the potential capacity to control the action of their voices.

“This indicates that the voice control shown by humans could derive from an evolutionary ancestor with similar voice control capacities as those found in orangutans and in all great apes more generally.

”This opens up the potential for us to learn more about the vocal capacities of early hominids that lived before the split between the orangutan and human lineages to see how the vocal system evolved towards full-blown speech in humans.“

Eight-year-old Rocky was studied at Indianapolis Zoo in the US, where he still lives, between April and May 2012.

In the ”do-as-I-do“ game he attempted to copy random sounds made by the experimenter which included variations in tone and pitch.

His calls were compared with sounds collected from more than 12,000 hours of observations of more than 120 orangutans from 15 wild and captive populations.

The findings are published in the latest issue of the journal Scientific Reports.

A previous study led by Dr Lameira when he was based at the University of Amsterdam found that a female orangutan called Tilda was able to make sounds that had the same rhythm and pace as human speech.