This is a really good question, and one I have been pondering for almost half a day now...

I think my answer, without reading anyone else's answer, except yours @bengy, would be...

Nitrogen fertilizer! This stuff has made it possible to feed the billions of people we currently do, and in doing so destroy the pessimistic Malthusian predictions at every turn. Every time we thought we would not be able to feed more people, we've learned to nitrogen better, from bat poop to using natural gas.

I currently, along with many others around the world, am working on the next generation understanding of nitrogen, fixing nitrogen from the air directly into the soil. But however you slice it, nitrogen is what has allowed our civilization to grow to this level. (Water was a pretty good one too ;p)