3500 sounds about right, if we consider only Eurasia. Otherwise, it could be farther back, but possibly not that much further, according to the wikipedia entry

3500 sounds about right, if we consider only Eurasia. Otherwise, it could be farther back, but possibly not that much further, according to the wikipedia entry for Mitochondiral Eve (look for "most recent common ancestor"): "Monte Carlo simulations suggest the MRCA was born surprisingly recently, perhaps even within the last 5,000 years, even for people born on different continents."