Holidays are defined by traditions –  those we keep and those we break. And nothing could be more traditional than a roast turkey on Thanksgiving. This year, nearly 250 million turkeys will be raised to satisfy American demand. But how traditional is the turkey? The fact is, we don't know. The first Thanksgiving included venison, the Wampanoag people brought plenty. And the colonists shot some wild fowl, but there probably weren't any sweet potatoes, and definitely no cranberry sauce.  Thanksgiving didn't become a national holiday until Abraham Lincoln's time, what was traditional when the tradition was new? Jessica Harris knows. She's a food historian and the author of "High on the Hog: A Culinary Journey from Africa to America."