The marsupial faunas of South America and Australia are at least as ecologically diverse as placental mammals worldwide (with some exceptions, see the discussion of developmental constraints in our response to chapter 8). The convergent evolution of Australian mammals and placentals found in comparable habitats elsewhere shows the power of evolution to adapt species to similar conditions. That they have similar adaptations to those found in placentals, but achieve such adaptations by different means, indicates how flexible evolutionary processes can be. Because of the ecological diversity of South American and Australian marsupials, and the biogeographic history which made such diversity possible, marsupials could serve as a useful exploration of the interplay of evolution and biogeography.
Unfortunately, the discussion of marsupial biogeography in Explore Evolution is laughably bad: too brief to education, and so inaccurate as to be utterly useless. It begins with a mischaracterization of the evolution of marsupials:
The first mammals with the marsupial's distinctive mode of reproduction arose on the ancient southern super-continent of Gondwanaland. Later, after this great land mass broke up into separate continents, the ancestors of marsupials were separated from other mammals and evolved in isolation on the new continents of Australia and South America.Explore Evolution, p. 75.
This is a straw man. Textbooks and researchers in the field do not claim that marsupials originated in Australia or South America. The best evidence is that marsupials originated Asia, migrated to North America via a land bridge, and that the co-existed with placental mammals in the northern hemisphere for some time. Marsupials colonized first South America, and from there moved on to Antarctica and then Australia. The marsupial populations in Asia and North America went extinct, possibly as a result of competition with placental mammals among other factors, and the populations on southern continents remained in those safe havens.
Explore Evolution's perfunctory and inaccurate coverage of this basic biogeography does students a disservice. Students cannot be guaranteed to have a background to know what Gondwanaland was, nor to appreciate that the supercontinent was already largely broken up by the time marsupials were crossing landbridges between the continents. If students do not have the background to appreciate the interplay of diversification and continental drift, the book's explanation will not help.
Having created the straw man, Explore Evolution proceeds to knock it down:Critics of the marsupial argument insist that it, too, fails to establish Universal Common Descent or even the descent of all marsupials. At best, it shows that various groups of marsupials first originated in the same general area in the Southern Hemisphere and were then distributed more widely as the Southern continents separated from one another. But even this is questionable, some critics say. They point out that marsupials are not restricted to the southern continents of Australia and South America. Marsupials such as the opossum live in the northern hemisphere. And, in a recent development, paleontologists have unearthed the oldest marsupial fossil of all in China.Explore Evolution, p. 77-78.
Marsupials exist in North America because of migration, a process described in this very chapter, but ignored now when the authors find it inconvenient. Around 3 million years ago, a combination of continental drift and the rising Andes brought South America into contact with North America. For the first time in 50 million years, North American species and South American species came into contact. Some marsupials (like the opossum) spread north. Some placentals from North American spread south. For reasons that scientists continue to investigate and discuss, the ability of North American placentals to persist and diversify in South America was much greater than the ability of South American marsupials to diversify in the north, or to outcompete the northern invaders (Stehli and Webb, eds. 1985. The Great American Biotic Interchange. Plenum Press: New York). Today, half of South American mammals are descended from North American ancestors. South American descendants represent no more than 20 percent of modern North American species, and most of those are in Central America, near the point of initial contact (Marshall, et al. 1982. "Mammalian evolution and the Great American Interchange," Science 215:1351-1357). This helps explain the confusion of the apparently biogeographically illiterate authors of Explore Evolution about why "marsupials such as the opossum live in the northern hemisphere." It is because of migration, a process they described as uncontroversial only a page earlier. It is not a mystery, and it is unclear why the authors would regard it as such.
The claim that North American opossums are evidence against the common ancestry of marsupials bears striking similarities to a critique of biogeography by young earth creationist Kurt Wise:There are very few examples of macrobiogeographical evidences for macroevolution, and none of them is very strong. The best-known claim is the concentration of marsupials in Australia. But there are several reasons that marsupials in Australia are actually a poor example. First, all marsupials are not in Australia. The Virginia opossum of North America, for example, is a marsupial. It is thought to have come from South America, not Australia. Thus not all similar organisms are known from every continent. Third, marsupials are the oldest fossil mammals know from Africa, Antarctica and Australia in that order. The fossil record seems to show a migration of marsupials from somewhere around the intersection of the Eurasian and African continents and then a survival in only the continents farthest from their point of origin (South America and Australia).source Wise, Kurt (1994) "The Origins of Life's Major Groups," ch. 6 in J. P. Moreland (ed.) The Creation Hypothesis: Scientific Evidence for an Intelligent Designer, Intervarsity Press:Downers Grove, IL, p. 223).
Wise's confusion over the status of the Virginia opossum perhaps reflects his confusion of the New World order Didelphimorphia commonly called opossums with the Australian order Diprotodontia some of which are commonly called possums (without the first "o"). The groups are morphologically and molecularly distinct, with well-established paleontological histories. If the opossum truly had roots in Australia, it would indeed be a biogeographic conundrum. In fact, the only close link between opossums and Australia is Wise's typo.
Similar misunderstandings plague the discussion of the marsupial fossil record in Explore Evolution and its creationist source material. It is not surprising, the breathless tone of Explore Evolution notwithstanding, that "paleontologists have unearthed the oldest marsupial fossil of all in China." The authors wonder "If the ancestors of the marsupials originated in the Southern Hemisphere, why has the oldest known member of the group been discovered in the Norther Hemisphere?" The answer is simple: paleontologists do not claim that marsupials originated in the Southern hemisphere, only that they migrated there.
Marsupials and placental mammals separated roughly 125 million years ago, according to the most recent fossil data. Several lines of evidence indicate that the marsupials originated in Asia, spread to North America over a land bridge, which can be seen in the figure. The ages and characteristics of fossils found in Europe, South America, Africa, Australia and Antarctica suggest that marsupials spread to Europe and South America from North America. South America, Africa, Australia and Antarctica still had linkages at that time, allowing species on the Southern continents to spread easily.
Despite the feigned confusion of Explore Evolution's authors, the fossil record gives a very clear picture of the biogeographic history of marsupials, though there are many questions scientists continue to investigate. The earliest marsupial fossils (and the earliest placental fossils) are found in Asia. Fossilized marsupials are found in North America in rocks that are only a few million years younger than the Chinese fossils. During that period in geological history, plate tectonics had brought North America and Asia close enough together to forge a land bridge, allowing many species to migrate between those continents.
At that time, the southern continents were all connected, North America and Europe were still very close, and South America had not drifted far from North America, allowing dispersal during periods when ocean levels dropped. Marsupials related to North American species colonized Europe briefly, through a northern land bridge, and others colonized South America. Africa was in the process of separating from the supercontinent which also included Antarctica, Australia and South America, so the presence of marsupial fossils in Africa gives a good measure of how quickly they entered South America and dispersed across the supercontinent Gondwana. Fossilized marsupials in Antarctica also allow us to track their dispersal to Australia. This pattern is consistent with the fossil record of placental mammals, and with other lines of evidence (John P. Hunter and Christine M. Janis. 2006. "'Garden of Eden' or 'Fool s Paradise'? Phylogeny, dispersal, and the southern continent hypothesis of placental mammal origins," Paleobiology, 32(3):339 344).
As the southern continents drifted apart as well, the marsupial faunas in each isolated continent followed a different path. As Antarctica drifted south towards its current polar position, it became colder and colder, ultimately driving its resident marsupials and palm trees extinct. South American and Australian marsupials produced diverse radiations which filled many of the same ecological niches occupied by placental mammals elsewhere. In the northern continents, which were periodically linked by land bridges, biotic interchanges resulted in periods of intense competition, which seem to have driven the native marsupials extinct.
When South America drifted north again and connected with North America around 3 million years ago, the Great American Biotic Interchange had the same devastating effect that biotic interchanges had on other marsupial faunas.
The same pattern of diversification and migration seen in marsupials can also be seen in other groups of plants and animals. That consistency between biogeographic and evolutionary patterns provides important evidence about the continuity of the processes driving the evolution and diversification of all life. This continuity is what would be expected of a pattern of common descent. The creationist orchard scheme gives us no reason to predict this pattern.