The last idea that I want to mention is
the snowball Earth theory. This theory
says that the Earth has been covered by
global ice sheets at various times in its
history. These global ice ages lasted
hundreds of millions of years.
You can read more about this idea
here.
The last of these global ice ages could have
happened 500 million years ago; others may
have happened previously.
Note that these are
different than the "ice ages" you have probably
heard about, in which glaciers covered much of
North America but there was still a tropical region.
A global ice age has the entire planet covered
in glaciers.
Among the
evidence
for these global ice ages is the presence of
glacially deposited sedimentary rocks at the
equator.
More on the Earth's evolving climate in a minute.