Black Squirrels: Melanization in Squirrels vs. Humans Question

A general internet search suggests a predominant theory that black squirrels, typically a melanized morph of gray squirrels, are darker because it helps the squirrel to retain heat.

A general internet search also suggests a predominant theory that humans have darker skin because it protects them from damage from sunlight. Some sites suggest that darker human skin absorbs heat more easily than lighter skin.

It appears established that black squirrels tended to originate in more northern climates while people with darker skin tended to originate in warmer climates.

Why is it believed that darker fur retains heat but darker skin protects from sun damage? 
Why did squirrel genetics change to promote heat-retention in the colder climates, but human genetics changed in a way that may promote heat retention in the hotter climates?

The free internet is quiet on the most intriguing questions and I’m not important enough to start cold emailing researchers.