We have to consider that we, the Homo sapiens, being classified under the Mammalian classification of Primates, have ancestors that were very similar to us back in the day, about 200.000 years ago, quite before our existence as we know it today. These proto Homo sapiens have a body full of fur, like today's extant other primates, but their migration from cold habitats to warmers caused the loss of it. At that time, primitive humans with no fur for them to be safe from UV rays arising from the sun developed a dark skin (full of melanine) so those rays to not harm their skin and provoke diseases in them.