<Li> Deserts in North and South America have native cacti, but deserts in Africa, Asia, and Australia have succulent (apart from Rhipsalis baccifera) which are native euphorbs that resemble cacti but are very different . </Li> <P> Even greater differences can be found if Australia is taken into consideration, though it occupies the same latitude as much of South America and Africa . Marsupials like kangaroos, bandicoots, and quolls make up about half of Australia's indigenous mammal species . By contrast, marsupials are today totally absent from Africa and form a smaller portion of the mammalian fauna of South America, where opossums, shrew opossums, and the monito del monte occur . The only living representatives of primitive egg - laying mammals (monotremes) are the echidnas and the platypus . The short - beaked echidna (Tachyglossus aculeatus) and its subspecies populate Australia, Tasmania, New Guinea, and Kangaroo Island while the long - beaked echidna (Zaglossus bruijni) lives only in New Guinea . The platypus lives in the waters of eastern Australia . They have been introduced to Tasmania, King Island, and Kangaroo Island . These Monotremes are totally absent in the rest of the world . On the other hand, Australia is missing many groups of placental mammals that are common on other continents (carnivorans, artiodactyls, shrews, squirrels, lagomorphs), although it does have indigenous bats and murine rodents; many other placentals, such as rabbits and foxes, have been introduced there by humans . </P> <P> Other animal distribution examples include bears, located on all continents excluding Africa, Australia and Antarctica, and the polar bear solely in the Arctic Circle and adjacent land masses . Penguins are found only around the South Pole despite similar weather conditions at the North Pole . Families of sirenians are distributed around the earth's waters, where manatees are located in western Africa waters, northern South American waters, and West Indian waters only while the related family, the dugongs, are located only in Oceanic waters north of Australia, and the coasts surrounding the Indian Ocean . The now extinct Steller's sea cow resided in the Bering Sea . </P> <P> The same kinds of fossils are found from areas known to be adjacent to one another in the past but that, through the process of continental drift, are now in widely divergent geographic locations . For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica, which can be dated to the Paleozoic Era, when these regions were united as a single landmass called Gondwana . Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates . </P>

Which organisms represent the common ancestor of all