"But until now, we didn't think any animal species, including monkeys, perceived it the way we do. Now we know that marmosets, and likely other primate ancestors, do," said Xiaoqin Wang, professor of biomedical engineering at Johns Hopkins University's school of medicine.

First, people are better at distinguishing pitch differences at low frequencies than high. Second, humans are able to pick up on subtle changes in the spread between pitches at low frequencies or hertz. And third, at high frequencies, peoples' ability to perceive pitch differences among tones played simultaneously is related to how sensitive they are to the rhythm.

Through a series of hearing tests, Wang's team determined that marmosets share all three features with humans, suggesting that human components of pitch perception evolved much earlier than previously thought.

The American continent, with its marmosets in place, broke away from the African land mass approximately 40 million years ago, before humans appeared in Africa. So it's possible that this human-like pitch perception evolved before that break and was maintained throughout primate evolution in Africa until it was inherited by modern humans.

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