Until recently, scientists did think that all
life on Earth depends on the Sun to survive. Now,
I would say that almost all life on Earth depends
on the Sun to survive. First, let's talk about
life that does depend on the Sun to survive. Let's
start by looking at the bottom of the food
Plants do not eat other living things to grow
and survive: they are autotrophs because
they make their own food. Instead, they use
nutrients in the soil and energy from the Sun to
make their food (mostly sugar). We named this
process photosynthesis (which roughly means
"making things [food] from light"). Without
sunlight, plants could not survive.
The next level of the food chain is made up of
herbivores, animals that eat plants. Since
herbivores depend on plants to survive, herbivores
could not survive without sunlight.
Most of the rest of the food chain is made up
of carnivores, animals that eat animals.
Some of these animals eat herbivores to survive:
they could not survive without the plants that
feed the herbivores. Other carnivores eat
carnivores to survive. But in the end, these
animals depend on plants as a food source, either
for themselves, for their food to eat, for their
food's food to eat, or so on.
So what about life that does not depend on
the Sun to survive? Deep in the ocean,
there are some autotrophs that use energy from the
Earth's hot core to make their food. The only
examples I know about are bacteria that live near
hydrothermal vents (jets of hot water
coming from deeper in the Earth). These bacteria
use chemicals like methane and hydrogen sulfide
and the energy from hydrothermal vents to make
their food. We call this process
chemosynthesis (which roughly means "making
things [food] from chemicals"). Other life forms
can eat these bacteria and also survive without
sunlight. The only examples I know about are
called giant tube worms, riftia pachyptila.
Scientists only discovered these strange
chemosynthetic bacteria in 1977. Now we know that
life can exist without sunlight, but there are
still a lot of things that we don't know about how
it works. I would definitely say that humans need
the Sun to survive, and we probably always will.