This post is part of a series on The Origin Of Species. It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.
Oh dear, the topic seems to have taken a distinct veer into creationism, around here, and I fear that will not change with chapter twelve of The Origin. This chapter deals with biogeography: the distribution of species and and groups of species across the Earth. Darwin discusses what he has learnt about mechanisms of migration and dispersal of species, and he describes the patterns of geographical occurrence of species.
We take it for granted that Australia has Kangaroos and Koalas, South Africa has lions and giraffes, and Uruguay has Capybaras and Armadillos. But it’s an observation that demonstrates the explanatory power of evolution as the overarching theory in biology. When you know that species arise by descent with modification, the geographic isolation of species, and of groups of species, makes sense. Marsupials evolved from a common ancestor on an isolated continent, and so they remain largely restricted to Australia. Similarly the Xenarthra (sloths, anteaters, armadillos) and the Cavies (Patagonia hares, Guinea pigs, and capybaras) are restricted to the Americas.
Other historical hypotheses for the generation of species did not have any such explanatory power. Spontaneous generation could not explain why three countries with relatively similar climate and conditions, but divided by continent, should have such vastly different flora and fauna. Nor would it explain why the marsupial mammals should all spontaneously generate in Australia, and none of the placental mammals do so. But there is one alternative hypothesis which epically fails when it comes to biogeography: the book of Genesis.
The book of Genesis describes a great flood sent by God in the 3rd millennium BC to destroy all life. Noah, a sprightly middle-aged six-hundred year old, thought God’s genocidal plans a great lark, and dutifully built a big boat for his family and a couple from any animal species which happened to be within seven days walking distance. After forty days of flooding, the water receded, presumably revealing great plains of dead animals and vegetation, killed by their drowning in the salty water. The boat then grounded in a mountainous region of eastern Turkey as the waters receded.
At this point, the Kangaroo bounced off back to Australia. The Capybara waddled home to South America, and the polar bear sweated his way up to the arctic. Apparently the rancid decaying victims of God’s temper were sufficient to sustain the individuals on their journey, as we can know from the fact that they survived to repopulate their species. And the geographical features which now prove insurmountable for the migration of species — such as the Atlantic Ocean — were no challenge for the three-toed sloth.
M’colleague put this to Answers In Genesis’ Paul Taylor at Skeptics in the Pub one time. I think Taylor’s response went something along the lines of, “oh, er,uhm, well, the water wouldn’t have been that salty.”