Spanish art curators have discovered a secret the "Mona Lisa" kept behind that enigmatic smile: a long-lost twin.

-- Madrid's renowned Prado Museum unveiled on Wednesday what its curators believe is the oldest copy of Leonardo da Vinci's "Mona Lisa," painted around the same time and possibly in the same room as the original masterpiece.




"It is as if we were in the same studio, standing next to the easel," Gabriele Finaldi, the Prado's deputy director of collections, told reporters.




The so-called "Mona Lisa of the Prado" has long been in the museum's collection, tucked away in its vaults and displayed only occasionally, its significance not fully understood. Not until restorers lifted off an 18th-century coat of black paint obscuring the background did curators realize the painting was much older than that -- with a backdrop of Tuscan hills similar to the one in the original, which hangs in the Louvre in Paris.




"This is very, very close to how the "Mona Lisa" looked in 1505," when Leonardo finished his masterpiece, Finaldi said. There are dozens of other copies, he said, but none has been dated as close to the original.




X-ray tests also revealed that smudges and changes made in the Prado version correspond with changes Leonardo made on his canvas. Museum officials said the copy is probably the work of Francesco Melzi, an apprentice of Leonardo's, who may literally have been standing next to his master while replicating his every brush stroke.




The Prado plans to display its find this month before sending it to Paris to hang side by side with the original, at a Leonardo exhibit in March.




"Our colleagues at the Louvre now have a whole lot more information they can use in their research on their own painting," Finaldi said.




The "Mona Lisa" is believed to be a portrait of Lisa Gherardini, the wife of a wealthy cloth merchant, Francesco del Giocondo, who lived in Florence around the start of the 16th century.




The Prado's Mona Lisa looks fresh-faced and younger than the original, an effect Finaldi attributed to the fact that it has not been continuously displayed, and it lacks a graying varnish. The other major difference between the Spanish Mona Lisa and the one in Paris is eyebrows: The original figure has none.




Perhaps some mysteries still remain behind that enigmatic smile.