All the US presidents except one are related to each other

Sure - read: http://en.wikipedia.org/wiki/Most_royal_candidate_theory and then I guess you know better what this story is all about.

I'm almost certain if you go back a few more dozen generations they are all related to that black couple who lived in that small village near the Kilimanjaro ;-)

Even more impressive is often DNA analysis based ancestry research when it uses characteristics e.g. certain strands/strains that are only present / dominant with certain "races" / areas from where people came.

This research is so precise that you then get results like pharaoh Tutankhamun was 99.6% anglo-saxon or - my favorite one - research done with supporters / right wing neo-nazis in the UK - they are all related to one black roman slave who came to England with the Roman troops.