>>2540131 lb
Not so crazy of a theory actually - there could be something to this.
Own pet theory is that has something with particular blood types & genetics (haplogroups) and the relative purity of the genes to Africans (very little intermingling). And when there is, there are very unique combinations of types.
Funny you should mention Australian Aborigines -- they are of Haplogroup Q.
https://en.wikipedia.org/wiki/Haplogroup_Q_(mtDNA)
>>2540354 lb
These two need not be mutually exclusive. Perhaps it's a matter of having pure African DNA so close in proximity to the U.S., along with the relative ease of procuring children.