Far below my Garuda Indonesian Airways jetliner stretched a blue ribbon of water, the narrow strait separating the Indonesian islands of Bali and Lombok. Through the window I gazed down on a gorgeous landscape of tropical mountains, white sand beaches, coral reefs, and warm seas. To the tourists sitting around me, the sunlit scene promised sunbathing, snorkeling, spicy foods, and Balinese temples. That prospect appealed to me too, but I was thrilled by a different promise. I had read and dreamed of this strait for decades, and I was now at last taking it in with my own eyes. It constitutes part of Wallace’s line, which for a biologist is the sharpest and most famous boundary in the world, and the line whose crossing may have transformed our ancestors from glorified apes into real humans.
Wallace’s line is one of many biogeographic borders in the world, zones separating geographic regions with dissimilar groups of plants and animals. Most familiar to us are the divides between the native distributions of human populations--the lines that seem to divide us into races: the Atlantic Ocean, which separated Europeans from Native Americans; the Sahara, which lay between black sub-Saharan Africans and white North Africans; and the Indian subcontinent, between East Asians and the whites of West Asia and Europe. Not only humans are distributed geographically, of course. Any fanatical bird-watcher eager to spot as many of the world’s myriad bird species as possible knows that you have to visit tropical Central and South America to find toucans and antbirds, sub-Saharan Africa for turacos and mousebirds, tropical Southeast Asia for leafbirds and minivets, and so on.
By means of such localized distributions, biologists divide the world into six major realms termed biogeographic regions, areas that do not coincide with the seven continents. Among neighboring pairs of those six regions, the greatest contrast is found between the Oriental region (consisting of tropical Southeast Asia and neighboring islands) and the Australian region (Australia, New Guinea, and neighboring islands). One is a world of tigers, the other a world of kangaroos. Each region harbors entire suites of species absent from the other. Tropical Southeast Asia, for example, is home to a rich array of placental mammals--primates, squirrels, big cats, and hoofed mammals among them--that are not found in the Australian region. Among placental mammals, only bats and rats are native Australian species. Conversely, Asia completely lacks Australia’s many groups of marsupials (like the kangaroos) and its monotremes (egg- laying mammals like the platypus). Other animal and plant species are similarly localized: Asia is devoid of Australia’s wealth of cockatoos and eucalyptus, while Australia has none of Asia’s numerous woodpeckers and pheasants.
Why does such a contrast exist? Between Australia/New Guinea and the Asian mainland lies an almost continuous chain of islands--constituting Indonesia--most of which are separated by gaps of ocean less than 100 miles wide. With such apparently modest barriers to their migration, how did such a great contrast between Asian and Australian plants and animals arise?
