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And yet in inorganic objects, e.g. solar panels, we find a dark surface increases UV absorbtion, while a light surface reflects UV (the albedo effect).
It may not be best to think that the adaptation was "white skin;" the adaptation was decreasing the concentration of melanin in the epidermis.
The melanin absorbs the UV radiation before it can reach other cells (keratinocytes?) that synthesize vitamin D. In an environment with a high sunlight intensity, enough UV gets through to the cells to synthesize vitamin D. In an environment with a low intensity of sunlight, not enough UV gets to the keratinocytes to produce a sufficient amount of vitamin D. So melanin production was selected against (hence lighter skin) to varying degrees, depending on latitude.
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