Mona Lisa: an Earlier Version?

And finally tonight, could a priceless painting actually be a copy? Mona lisa, da vinci's mysterious masterpiece is now being challenged by a younger rival. Her owners, of course, have millions of... See More

And finally tonight, could a priceless painting actually be a copy? Mona lisa, da vinci's mysterious masterpiece is now being challenged by a younger rival. Her owners, of course, have millions of reasons to believe their painting is the original. So, abc's jeffrey kofman takes a close look so you can decide for yourself. Reporter: Could it be? The most famous painting in the world now has a twin? Mona lisa -- leonardo's earlier version. Reporter: That's what the owners of the so-called isleworth mona lisa told the world today. Hidden in a swiss bank vault for almost half a century, it emerged accompanied by a stack of "evidence" meant to prove that this was day v vinci's first version of this. A younger mona lisa. Note the perkier smile. There are a lot of reasons to be suspect of this wannabe da vinci. A second mona lisa? There are a lot of reasons to be suspect. One of them? It only surfaced here in 1914. There is no record of its existence before then. Does it add up? Well, it adds up to being an interesting copy. Reporter: An interesting copy? No more than that. Reporter: There are many reasons to believe this is a copy by a lesser artist. First, the younger painting is on canvas. Leonardo painted on wood. The background on the younger mona lisa, is, well, muddy, not like the masterpiece. But the real give away? Translucence. In the original, layers of light. They're just not there in the other one. When you look, you just go, wow. You look at that mona lisa and you go -- ah. Reporter: A new leonardo would be worth at least $100 million. Imagine, with only about 20 of his works in existence, how exciting it would be to discover one more. Jeffrey kofman, abc news, oxford.