All but one of the Democratic candidates began their adulthoods in other parties

[Indy Texas]

Hillary Clinton was involved in Republican groups during her early college years.

Lincoln Chafee is the scion of a longtime Republican family and served for eight years as a Republican US Senator and as an independent governor before joining the Democratic Party two years ago.

Bernie Sanders was involved in Vermont's left-wing Liberty Union Party in the 1970s and has served in both houses of Congress as an independent. Even as he runs for the Democratic nomination, he has reaffirmed that he is not officially a Democrat.

Jim Webb was a Republican for many years and served in the Reagan Administration.

Even non-candidate Elizabeth Warren is a former Republican.

This is a striking contrast with the Republican field, where the only candidate with a history outside the GOP is Rick Perry.

Edit: I (like many voters) forgot about Martin O'Malley, who appears to have always been a Democrat.

[Indy Texas]

You left Martin O'Malley out of your post. Does he have a history with other parties?

Edited. Still, it's rather unusual when even the party frontrunner isn't a "dyed-in-the-wool" party member. Last time that happened would have been with Reagan the former New Deal Democrat.