On what single energy source do most energy alternatives depend, either directly or indirectly?

Directly most of alternative energy is gained from water. But eventually almost all energy on earth comes from sun. Without sun we wouldn't have rivers (no rains), we wouldn't have wind (let alone oil or coal) etc. We would only have geothermal and seismic power.

Direct sun radiation conversion (via collectors or photovoltaic) is not very popular yet. But indirectly sun is the main source of earths energy.