Nitpicking, but I don't think we as a species have been here for 100 million years. I doubt we've been even as hairless as we are now for more than a few hundred thousand, so we may not have gotten as much sunlight due to that. But your point still stands.
I always wondered why the dogs always like to lay out in the sun and furred animals get their Vitamin D. Wikipedia: "In some animals, the presence of fur or feathers blocks the UV rays from reaching the skin. In birds and fur-bearing mammals, vitamin D is generated from the oily secretions of the skin deposited onto the feathers or fur and is obtained orally during grooming.[97]"
http://en.wikipedia.org/wiki/Vitamin_D
