it's long been known that there was some back migration to west africa, but when or how isn't remotely understood. theories range from pre-neolithic migrations to a something along the lines of greeks and romans getting lost in the jungle. another idea is liberia.

i think they have to be careful to ensure it's not just inter-breeding with dutch, english and german settlers.

http://phys.org/news/2014-02-western-eurasian-genes-southern-african.html