I read another post somewhere that said all you need is 12 minutes of direct sunlight to maximize your Vitamin D intake. It's because your body can only absorb so much vitamin D at one time, from the Sun.

As for food products, there aren't that many natural sources of vitamin D. Milk is fortified with vitamin D but doesn't come with it naturally.

Given how few sources of vitamin D there are: http://www.whfoods.com/genpage.php?tname=nutrient&dbid=110 I don't see how our species could have evolved to depend on vitamin D (other than being outdoors).