why didn't natives of south america evolve darker skin?
Could not fit in a comment....

Northern Africans and Indonesians have pretty much the same skin color (to my first impression). Sub-Saharan African (that you don't talk about) are darker. And Northern Africa in not an equatorial region (not more than Los Angeles) as it is right under the tropic of cancer.

Also I am a little confused about who you call "we" and who call "they". I think that by "we", you mean any non-sub-Saharan African human but I am not sure. If this is the case just naming sounds more appropriate that to make a big category that you call "we" to exclude some ethnic groups :)

I think that when you are talking about Brazilians (and other people in South America living close to the equator such as Ecuadorians), you are mostly talking about people from Spanish and Portuguese descent. So let's talk about the great migrations

The Indonesians arrived in their lands about 70,000 years ago. The indigenous South-Americans arrived in South America about 10,000 years ago. The Spanish and Portuguese arrived in South America about 500 years ago.

I think you might want to read a bit more about the geography of those big migrations.