DEBBIE ELLIOTT, HOST:

Our understanding of one of the world's greatest historic sites recently got a bit of an update.

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UNIDENTIFIED ANNOUNCER: Near Cuzco, the ruins of one of the great Inca cities known as Machu Picchu sprawls across the mountain. The Incas managed to keep secret its location for centuries.

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ELLIOTT: Turns out, Machu Picchu is still holding out a few secrets.

NOEL KING, HOST:

Researchers from Yale found evidence that Peru's great hidden city was occupied earlier than previously thought.

Professor Richard Burger led that study of Machu Picchu.

RICHARD BURGER: One of the frustrations is that there are really no written documents from Inca times, except for the quipus, the knotted records. And those can't be read yet.

KING: Until now, historians could only guess the city's age based on documents from the Spanish conquest, which are not the most reliable sources.

BURGER: People were thinking that it dated to about 1450.

ELLIOTT: But Richard Burger's team now has evidence of human habitation dating a few decades earlier, to at least 1420. It involved skeletons discovered at the royal palace at Machu Picchu way back in 1912.

KING: Right. And technology has advanced to where carbon-14 can be dated using even the smallest quantities. And those ancient bones gave the Yale team what they needed.

ELLIOTT: Burger says, their discovery is only a beginning.

BURGER: Some people have often said, well, you know, the Inca empire is so huge and it left so many archaeological remains. Maybe we need a radical revision of the chronology and maybe even push it into, say, the 14th century. Maybe we're completely off base in terms of our estimates.

ELLIOTT: Richard Burger, professor of anthropology at Yale University, unlocking the history of Machu Picchu one secret at a time.

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