500 million years ago, during the Cambrian period, Antarctica was located on the equator, a hot climate surrounded by life in the shallow seas of its continental shelf. Over the next 140 million years, the continent drifted south, becoming centered on the South Pole, where it has remained since. Despite its location, for most of the time, Antarctica has been a relatively warm continent, even becoming a hot desert for tens of millions of years. As recently as 50 million years ago, Antarctica had a tropical or subtropical climate, complete with marsupial fauna, the descendants of which can be found today in Australia and some parts of South America.
About 40 million years ago, the supercontinent that Antarctica was a part of, Gondwana, started breaking up. This allowed cold water to build and circulate around the southern continent, displacing the warm north-south currents that previously made the area warm. Over tens of millions of years, glaciers began to form on the continent, mostly covering it by 15 million years ago. It was only 6 million years go that the ice caps reached their present extent. Today, 98% of the continent is covered by ice.
The contemporary Antarctic fauna are mostly sustained by the continent's meager flora, which only grows during the summer, and usually for just a few weeks at most. The majority of the plants there are the same plants that first evolved to live on the land — non-vascular plants like mosses and liverworts. Numerous microorganisms make up the majority of all photosynthetic organisms on the continent. In all, Antarctica contains about 200 species of lichens, 50 non-vascular plants, and just a couple flowering plants, Antarctic hair grass and Antarctic pearlwort. In recent years, due to global warming, germination rates among seeds have increased, resulting in a twenty-five fold increase the number of plants in some areas.
In the present, most Antarctic animals are tiny invertebrates, such as microscopic mites, lice, ticks, nematodes, tardigrades, rotifers, and springtails. The largest exclusively terrestrial member of the Antarctic animals is a flightless midge (very small fly), Belgica antarctica, just 12 millimeters (0.5 in) in size. The body fluids of many of these insects contain glycerol, an antifreeze which allows them to survive temperatures as low as −34 °C (−30 °F). These animals are most common on the Antarctic peninsula, which despite its extreme cold, dryness, and wind, is actually more habitable than the continent's vast interior.
Antarctic animals and their larvae have a number of other adaptations to survive in Antarctica, including the tendency to bunch together and the ability to survive without oxygen for weeks at a time. Some larvae of Antarctic animals are a dark blue-black color, thought to help with absorbing heat and possibly blocking out ultraviolet radiation caused by the ozone hole over Antarctica. They can tolerate wide swings in salinity and pH, caused by seasonal immersion in penguin guano, saltwater from the ocean, and freshwater from melting ice. Adult Antarctic animals are all wingless, to prevent them from being blown away.
Antarctica is one of the most inhabitable places on Earth, and may superficially resemble terrain (in terms of its hostility) created after the worst environmental stresses on the planet, such as supervolcano eruptions or large asteroid impact. This gives us a view of what life might be like today if the asteroid that wiped out the dinosaurs were several times larger than it was -- mostly invertebrate. The huge temperature swings and dryness resemble that harshest conditions of deserts throughout the planet's history, like the interior of the supercontinent Pangaea.
Antarctica has a small selection of freshwater fauna that dwell in small lakes and streams created by meltwater during the summer. These include small crustaceans called copepods, fairy shrimp (thought to be the ancestor of terrestrial arthropods), and the common nematodes. The longest river in Antarctica, the Onyx River, is just 30 km (18.6 mi) in length, so freshwater organisms are clearly not abundant here, but they can be found where they can survive.
More familiar Antarctic animals are the birds which dwell on the coasts, notably penguins such as the Emperor Penguin, Adélie Penguin, Rockhopper penguin, King Penguin, Chinstrap Penguin, and Gentoo Penguin. The beautiful white Snow Petrel is one of just three birds that breed exclusively in Antarctica, and the only bird to be sighted at the South Pole. All these birds survive because of their ability to fly to ice floes further north during the severe Antarctic winter. During the summer, the Antarctic coast is tolerable, reaching temperatures between 5°C and 15°C (41°F and 59°F). Large penguin colonies can be seen covering small coastal islands, basking in the sun.
The waters around Antarctica are surrounded by numerous animals, including squid, crabs, ice fish, krill, plunder fish, elephant and leopard seals, giant petrels and Antarctic terns, humpback and killer whales, and many more. Although penguins nest on land, they spent most of their lives, and get all their food, from the water. Some of the animals around the Antarctic coast display polar gigantism, a property whereby animals tend to get larger the further they are from the equator. Research teams have found starfish and crabs more than two feet across. This is a prime example of Bergmann's rule, a generality that states that animals get larger the closer you get to the poles.
The newest of the Antarctic animals is the familiar human, Homo sapiens, whose population numbers as great as 4,000 during the summer months, when researchers come to do field work and occasionally even bring their families along. About 70 research bases are maintained on the continent, producing substantial scientific returns for the large investment required to ship in supplies. Some of the biggest draws for researchers are unique fossils found on the slopes of Antarctic mountains, the McMurdo Dry Valleys, ghostlike gravel valleys in the Antarctic interior, the EM-interference and light-pollution free Antarctic high country, used as a site for telescopes and neutrino observatories, and Lake Vostok, a subglacial lake that has been sealed under the icecap for between 500,000 and more than a million years.