system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it always said that all living species need water and food to survive? Can't there be a super-evolved alien species that can survive with just sunlight?

I've always wondered this. Thanks!
gpt
Well, one thing that you learn about the universe when you start looking is that it can be incomprehensibly weird, which is why instead of saying "absolutely not," I'll say "pretty bloody unlikely."

Let's say you have an alien life form who's skin is made up of photovoltaic cells, which in theory could provide all of the power needed to run the alien's body. 

But wait! How did the alien's body make that skin? How was the body made? Sunlight alone does not provide the raw materials necessary to make structures. The alien needs to ingest those raw materials somehow for later conversion into the solar-cell-skin... This raw material the alien ingests is also known as food ;)

(edit) Or, to bring it closer to home, we already have species who's "skin" (not really skin) is made up of solar collectors - they're called plants. The leaves harvest sunlight, but then have to combine the solar energy with raw materials to actually make more plant (which is why you can't just stick a seed in a clean jar and expect to get a tomato out of it - it needs nutrients delivered through the soil, etc, to make the actual plant).