There was a time when all of Earth's continents were sitting together in a single great continent, according to some scientists.
(remember that this is a theory, and not necessarily my view.)
According to these scientists, the continents basically covered most of one of Earth's hemispheres.
That is, they covered either the Eastern or the Western Hemispheres. The other hemisphere, and all of both polar regions, was covered by one great ocean.
The great ocean, apparently the forerunner of today's Pacific, was a much warmer ocean than any ocean of today's world, and warm currents moved unobstructed to the poles, keeping them free of ice.
Without cold air from the poles to cool the Earth, the planet enjoyed a much warmer climate than today's, and the supercontinent was quite the steamy place.
What we call Antarctica today occupied the southernmost part of this supercontinent, and even then it was close to the pole, being in latitude of today's Sub-Antarctic.
Warm winds from the ocean constantly flowed across this landmass, keeping it warm even after the sun set for the last time below the Antarctic Circle.
During the long dark of winter, occasional cold snaps, with freezes, would still occur. They always ended as warm air pushed back in from the warm ocean.
Subtropical vegetation, such as ferns, were found throughout the continent, and when the sun set for the winter these plants went dormant, and were not subject to damage from the freezes.
After the supercontinent broke up, Antarctica drifted over the pole.
In itself, this fact would not have caused Antarctica to freeze up, for there was too much warm ocean water.
However, Antarctica, in occupying the polar region, cut off the flow of warm water to that area - the area where Earth loses the most heat, the area where the loss of the warmth from the water would make the greatest difference.
Thus, Antarctica grew cold because the ocean could not convey warmth to the pole. As Antarctica grew cold, it chilled the water around it, which made the continent turn colder.
Eventually, the glaciers crept down from the mountains and started to form an icecap. The icecap produced it's own climate, making Antarctica even colder and further cooling the surrounding oceans, further chilling the yet unfrozen parts of the continent, allowing the Icecap to advance.
The water temperature at the bottom of the ocean fell from 70 degrees Fahrenheit to below freezing as cold water around Antarctica sank to the bottom, and all the water around Antarctica for some distance out became deathly cold - cutting Antarctica off from any heat the seas might have transferred to it, isolating and protecting it's cold environment.
Cold currents from the now glacial continent flowed northward, chilling the subtropical and tropical oceans, creating a feedback mechanism that cooled the entire planet.
This feedback ultimately caused the Arctic to cool, and eventually freeze. Of course, the freeze-up of the Arctic cooled the whole planet even further.
If you believe the scientists, we live in the second coldest epoch the Earth has ever been through (although, it must be noted we are in an Interglacial, a very brief spike of warmth amidst the endless cold.)
The coldest epoch in Earth's history allegedly took place 700 million years ago, and was known as Snowball Earth. The entire planet froze over several times. Each freeze lasted 10 million years. Each freeze was followed by a thaw and warm up which sent the Earth into it's hottest known periods, but these hothouse-Earth epochs lasted only a few thousand years. The planetary temperature fluctuated from an average of I believe somewhere around minus 50 Fahrenheit to somewhere between 100 and 120 Fahrenheit.
