A Parisian engineer who has dedicated 3,000 hours to examining Leonardo Da Vinci's famous portrait now claims to have the answer.
Pascal Cotte says that ultra detailed digital scans of the 16th-century picture allow him to look into the past and see how the Florentine merchant's wife looked before years of restoration work.
Using the 240-megapixel scans, Mr Cotte, 49, says he can see traces of a left eyebrow long obscured from the naked eye by the efforts of the restorers.
His conclusion is that Mona Lisa once had both eyebrows and eyelashes, but that these have been gradually eroded to the point that they are no longer visible.
"If you look closely at Mona Lisa's eye you can clearly see that the cracks around the eye have slightly disappeared, and that may be explained that one day a curator or restorer cleaned the eye, and cleaning the eye removed, probably removed the eyelashes and eyebrow," he said.
Mr Cotte said his study of the painting also revealed that Da Vinci originally painted Mona Lisa with a wider face and a more expressive smile than in the final work.