The Sun
“Oh my god! The Sun!”
“What is it?”
“It's a great big ball of fiery gas at the center of our solar system,
but that's not important right now.” (or is it?!)
Heat
Obviously the Sun is important. Without the Sun supplying the Earth
with heat, the Earth would be a cold, dark, lifeless, ball of
rock. Maybe down deep in the Earth’s warm crust, life could have
started, as some suggest, but it would be little more than organic
chemistry. Bacteria-like thingies struggling to gather enough
resources, perhaps dividing once every century.
Light
Photosynthesis, another keystone of our biosphere, requires a good
light source. Again, we need the Sun. It's true that there are other sources
of energy that fuels life, even on Earth. However, that life has not spread
over the globe and diversified the way life with photosynthesis as the energy source has.
In fact, I believe the complex critter living in the other ecosystems, clams, tubeworms, shrimp, etc. started
off as critters living in the photosynthic ecosystem and just moved to the others
after finding they could survive on the goo they found there.