GEOLOGICALLY, AUSTRALIA IS a continent comprising mainland Australia, Tasmania, New Guinea, and neighboring islands. Australia, the nation state of the mainland and Tasmania (plus some small islands), has a surface area of around 7.7 million square kilometers (roughly 84 percent that of the United States). Biologically, Australia is a megadiverse nation continent, replete with an abundance of unique species, ecosystems, and human cultures. Since the Australian continent broke free from Antarctica around 60 million years ago, much of Australia's terrestrial biota has been evolving largely in geographical isolation, with the exception of a few rodent species who migrated during the Pliocene (between 2 and 5 million years ago) and the dingo (Canis lupus dingo, a top predator)-a wild dog that turned up about four thousand years ago. Humans arrived some fifty thousand years ago; and European colonization (and with it the modern era), in 1778.