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The present biogeography of the world's species strongly suggests common descent.

Evolution predicts that species arise from other species, and that therefore they will arise geographically close to those other species, and will share features of nearby species. If we look at the distribution of species today, we see that pattern.

If evolution (and common descent) were not true, we would expect to see similar species in geographically separated ecological niches all over the world. But there are no meerkats in Australia. Instead, we see that most of the native mammals in Australia are marsuipals, with a significantly different reproductive system from placental mammals.

Furthermore, evolution from common ancestors predicts that habitats exist where certain species would thrive, but are not found.

The few placental mammals in Australia today are introduced by humans, and they have flourished, because the habitat suits them so well. Some of them, such as rabbits, foxes and European rats, have become major pests and are outcompeting the native species. If the habitat is so well-suited to these species, why are they not native? Because they are descended from ancestors which evolved elsewhere.

The only reasonable conclusion to draw is that the Australian marsupials all descended from a common ancestor species which was isolated from the rest of the world's mammals and allowed to develop more or less on its own, parallelling the development of physically similar species in similar ecological niches on other continents. The geological evidence supports this theory of long isolation.

Furthermore, this pattern can be seen in the fossil record. The earliest marsupial fossils date from a time when palaeogeography shows that South America, Australia and Antarctica were all joined together as the supercontinent we call Gondwanaland. And where do these marsupial fossils occur? South America, Australia and Antartica (yes, we have found fossils in Antarctica). These fossils date from the same time as the placental mammal fossils found in Europe.