200,000

Analysis of the DNA in the mitochondria of our cells has long suggested that we are all descended from a single woman. Mitochondrial DNA descends only through women and there isn't very much of it. This simplicity makes it a very useful tool for the tracing of ancestry, so there have been a lot of studies of its diversity and the distribution of various mutations. Now there is a new meta-study that sets the date of our common ancestor -- "mitochondrial Eve" -- about 200,000 years ago. This is actually a little before the first good evidence for fully modern humans, but that's ok; the evolution of Homo sapiens sapiens from ancestral groups must have taken tens of thousands of years.