DISCLAIMER: This question is not racially motivated or intended to denigrate any racial groups, I am genuinely curious and want to know what the most accepted scientific and specifically non-racist answers are to this question.

It appears according to this study (1994 so maybe not relevant scientifically anymore) that they think that African races are the most distinct genetically:

http://www.andamans.org/the-genetic-relationships-of-modern-human-races/

My question is this: Why are African genetics so distinct given that the leading theory is that we migrated out of Africa and Africa is so centrally located to the rest of the ancient world? This seems to imply that when the non-African races migrated out of Africa they left and never returned but there is no obvious geographical reason that needed to be so. Wouldn't you expect the European and Asian genetics to be much more closely tied to African genetics. Is maybe the Mediterranean in the north a possible justification? Could the red sea and Mediterranean being connected be a possible reason? Were African populations totally cut off at one point? Does this imply that the Suez connection could have existed intermittently and non African groups got through in the interim?

https://www.sciencedirect.com/science/article/abs/pii/S1040618215008459