io9 writes about photomedicine, but here's what I've been wondering about lately: why have no animals ever evolved that directly convert sunlight into cellular energy?
In other words, why don't we all have large flat heads lined with natural solar cells? Why go through this ugly process of having to eat the plants (or, worse, eat the animals that eat the plants) that gather solar energy for us?
Are we just too small? I don't think so. Humans need about 3000 kcal/day to survive. (What nutritionists call a "calorie" is actually, in the language of physics, a kilocalorie.)
The Sun shines on the top of our atmosphere with about 1360 W/m2, but averaged over the planet that is, at most, about 300 W/m2.
If I've done the math correctly, that means you'd only need a "solar-cell" area of about 0.5 m2 atop you to power the human body, or about something the size of a chessboard.
That's not unimaginable, is it? So where are these animals?