Short answer?...its all genetic..
general answer?... while the mongols and europeans were taking their time farming and becomming desk jokeys the africans were still hunter-gathers' you cant tame lions and tigers to plow a feild. so they survived by hunting for a longer period than anglos or mongols did.
also during the times of slavery the biggest and strongest were the ones who got to live. while it was all a bussiness to the white man. they were forced to breed with bigger women for their offspring.
there is a term called eugenics. which is basically about selective breeding to create high quality off spring..essentially weeding out the weak by breeding weakness out..
try googleing "eugenics and slavery" if your still interested.
hitler was obsessed with the science of eugenics and thats what led to his belief of a "super race"
also the Spartains were eugencicits as well. if you have ever seen the movie in the beginin the narrorator talks about "if he was born weak or small he would have been discarded"... well thats actually how spartans did it. they actualy believed they were doing the weak a favor by sparing them a life of shame or embarassment.
anyways. there you have it.
This is absolutely correct. Between breeding the larger and dominant males with the strongest females (phenotypes), the race has been somewhat manipulated into producing those particular genes (genotypes). That and the fact that caucasians in general have taken a more scientific approach to laboring because of industry and commerce, therefore making a large portion of the populace "less" physical in daily demand. The bottom line comes down to the strongest survive and african americans have had a much longer run at this than most other races due to the difference in culture and societies. Those cultures without proper medicine died early on before bad genes were passed down to offspring, and those cultures with medicine prolonged the life of the "weak and sick" and their "bad" genes have been preserved.
Courses in genetics, evolution and Western Civ helped assertain most of the above info,...then again, you can't put 100% faith in everything you read.