That gets to my pet peeve. "Evolved to."

It's more subtle than that. Sickle cell anemia/trait is a great example. It's literally one nucleotide change that causes one amino acid change. Not hard to pop up spontaneously. And since sickle cell trait isn't as bad as sickle cell anemia, two people with sickle cell trait have a 25% chance of their offspring having normal hemoglobin, a 50% chance of passing on the trait, and a 25% chance of having a kid with sickle cell anemia. Assuming the anemia has a higher chance of dying before passing it on, that's a 2:1 ratio of trait to no trait.

Hardy-Weinberg aside (genes that affect survival don't always follow HW,) that alone can create a stable population of sickle cell trait carriers. The fact that the trait can cause a small resistance to malaria is almost an afterthought. It probably helps explain why the trait is more common in ethnic groups from malaria endemic areas, but it's not like it evolved as a response.

Sometimes, evolution means random changes occur that don't have a huge effect one way or the other, and it just happened to stick. More often than not, the answer to why we evolved a certain way can boil down to "well, we had to evolve some way, and this is the one that happened."