It sounds like the gentle whisper of someone proposing marriage.

But the recording below is actually the sound of how a monkey could speak, if it had the brain power for language.

Scientists at Princeton have discovered that monkeys have a vocal anatomy which is capable of 'clearly intelligible' speech, just like humans.

So the fact that they don't speak is down to differences in brain structure which suggests that human speech stems from the unique evolution and is not linked to vocalization-related anatomical differences between humans and primates.

"Now nobody can say that it's something about the vocal anatomy that keeps monkeys from being able to speak -- it has to be something in the brain," said Dr Asif Ghazanfar, a Princeton University.

"Even if this finding only applies to macaque monkeys, it would still debunk the idea that it's the anatomy that limits speech in non-humans. Now, the interesting question is, what is it in the human brain that makes it special?"

Previous examinations of primate vocal anatomy conducted on cadavers had concluded that monkeys and apes have a very limited range of sounds they could produce relative to humans.

In the new research, the team used x-ray videos to capture the macaque's vocal anatomy, such as tongue, lips and larynx.