It happened because it happened, and nothing to do with evolutionary resistance. The genome is mostly stable but slowly altered via copying errors. Natural selection is a search for “must be the reason”, and IMHO just 19th century secular superstition.
OTOH, it’s long been known that a single gene (one from one parent) for sickle cell protects against malaria, while two genes (one from each parent) is no freakin’ good, and actually makes carriers more susceptible to the effects of malaria.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3499995/
sidebars:
https://dnalc.cshl.edu/view/15404-Chromosome-9-gene-for-blood-group-Matt-Ridley.html
https://freerepublic.com/focus/chat/3993910/posts?page=5#5
My curiosity was how could it be type O as 99% of South America, yet very diluted in the so called origin of man?
I am sure they have had Mosquitoes and Malaria since the beginning of time in the old world and the far east.
Logic would dictate it should be the other way around right? If man came from Africa, Asia, and then east to the new world?
Logic is type O originated from South America and then spread west... But that opens another door...