This study introduces a large-scale, detailed computer model of recent human history which suggests that the common ancestor of everyone alive today very likely lived between 2,000 and 5,000 years ago. Furthermore, the model indicates that nearly everyone living a few thousand years prior to that time is either the ancestor of no one or of all living humans.
Me thinks your doctor is a friend of Al Gore. He casts his hypothesis in extremes much like the Global Warming crowd. Really, a common ancestor of everyone alive could have lived as recently as 2,000 years ago - from the Americas, Europe, Africa, Asia, and Australia.
Noah would appear to be the most recent common ancestor, no?