For much of the last 400 million years, Antarctica has been a temperate place, covered with forests and animals. Due to continental drift, it has moved from straddling the equator to being centered on the South Pole, where it is today. Today, Antarctica is the coldest continent on the planet, almost completely covered in a layer of ice, and entirely lacking in animals aside from visiting penguins and a few small bugs in the coastal areas.
But it wasn't always that way. Antarctica was once part of the supercontinent Gondwana, which lasted up until about 160 million years ago, when it slowly began to break up. Gondwana included most of the continents in the southern hemisphere today, including South America, Africa, Arabia, India, Australia, and New Zealand. Gondwana straddled the equator and was one of the world's two supercontinents, along with Laurasia, which included present-day North America and Asia. Fossils of some of the earliest complex life have been found in the shallow seas surrounding it.
When life first came onto the land, Antarctica was one of the continental landmasses to be covered in forests and animals. Much of the fossil record of the Antarctic landmass is under the ice, but fossils, including those of dinosaurs, can be found in the Antarctic mountains, where rocks jut out from the continent's mile-deep ice cap.
In the times of the dinosaurs, Gondwana's north-south orientation blocked currents from circulating at a certain latitude, instead directing them north and south long distances. This prevented from the temperature differentials at any given latitude from forcing the waters into a permanent hot or cold temperature, as the poles are today.
As Antarctica began to break away from the supercontinent Gondwana 160 million years ago, cooling began. It moved south, still connected to Australia and South America but separated from Africa. At this point, Antarctica was still had a tropical or subtropical climate, but was located further south, near the latitude of present-day Australia. Like today's Australia, the continent had a marsupial fauna.
Around 40 million years ago, Antarctica separated from present-day Australia and began to cool down even more, its forests dying. Ice and glaciers began to cover the continent, but the final end of Antarctica's life came only about 23 million years ago, when Antarctica separated from South America and the Drake Passage opened. This permitted the existence of a Antarctic Circumpolar Current, a frigid current that continuously encircles the continent. It resulted in the continent being covered in a mile-deep layer of ice, as snow that fell never melted. Today, Antarctica's ice sheet contains about 70% of all fresh water on Earth.