o.k. first of all, NA's are considered descendents of a group of Israelites that came over about 600 BCE, in the Mormon doctrine. They are not considered from the lost tribes of Israel although according to the genealogy provided in the Book of Mormon of this group they are descended from Joseph (one of the 12 sons of Israel). He was the one sold into slavery blah blah blah. Anyway, no, we don't believe that NA's came from space. You're confusing us with Scientology. There is some scriptural evidence (purely Mormon theology so don't get your panties in a wad) that there was a large population of people already in the Americas when this group of Israelites showed up on the scene (see Jaredites).
No Mormons theology isn't racist. Some racist ideology crept into the church which would not allow African American males to attain the priesthood (one of the reasons why I left the church for about 15 years that and being outed by my bishop at the time). Remember, this was the 1800's and slavery was still in full swing. Most churches wouldn't even allow blacks to walk in their churches let alone be part of the congregation. Why it took until 1978 to repeal that restriction is beyond me and placed a heavy burden on my faith. But to be fair, there was a law in Missouri under Governor Boggs during the Polk administration which made it legal to kill Mormons. That law was repealed in 1976.
No we aren't trying to populate the world with Israelites. 99.99% of Mormons are Gentiles. The whole idea of prolific families is the doctrine that family is forever and that we will still be family in the afterlife. That's one of the reasons why we have temples so we can have sacred ceremonies that seal families together for all eternity and you progress together as a family. It has nothing to do with skin color or genetics.
I'm not going to delve too deep into LDS theology. If you truly want to know the LDS version then just go to
http://www.lds.org It's all there. Well most of it. You won't see much on the priesthood ban for blacks because the church is still pretty embarrassed by it. No, we don't believe Cain was cursed with "black skin." That was a common Mormon urban myth that persists until this day. Just like that urban myth that Cain is Bigfoot and that's the mark God put on him (which caused many a sleepless night for a boy who was terrified of monsters). The so-called "skin of blackness" was in reference to the descendents of the Israelites that came to the Americas mixing with people that were already there. It merely meant that they fell into unbelief and violated the Laws of Moses by marrying outside of their own people. That reference to skin of blackness was taken out of later reprints to reflect spiritual darkness rather than a pigment issue.
As far as genetic evidence goes? There was a small band of them that came over. There were people already living in the Americas. This happened in (according to the BOM) 600 BCE. The Americas weren't "discovered" until 1492. That's well over a thousand years of living. Lot of genetic swapping going on during that time. So, very little evidence.
Anyway, I'm done. Ya'll have a great week.
