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Once Upon a Raft

Biogeography, Common Descent & Seafaring Monkeys

by Casey Luskin

In Salvo 21, I noted that the theistic evolutionist Karl Giberson declared that "biologists today consider the common ancestry of all life a fact on par with the sphericity of the earth,"1 implying that those who doubt common ancestry are no better than flat-earthers. Such pressure tactics eventually filter down into popular discourse. One op-ed columnist in a local newspaper confidently boasted:

The evidence that all life, plants and animals, humans and fruit flies, evolved from a common ancestor . . . is a fact. Anyone who takes the time to read the evidence with an open mind will join scientists and the well-educated.2

Although proponents of common descent may bully dissenters with ridicule, that doesn't mean their theory is wrong. Despite the outlandish rhetoric, the evidence is worth considering carefully.

The case for common descent is often said to be "cumulative," that is, based upon multiple lines of evidence, including fossils, DNA, anatomy, embryology, and biogeography. Because these five lines are said to independently point to common descent, each one must be tested and assessed individually. So over the course of the next few issues of Salvo, we'll do just that, beginning in this issue with an examination of the evidence from biogeography, the study of the distribution of organisms in time and place.

The Puzzling Platyrrhines

Defenders of neo-Darwinism commonly contend that biogeography strongly supports their viewpoint. For example, the National Center for Science Education, a pro-Darwin advocacy group, has claimed that "consistency between biogeographic and evolutionary patterns provides important evidence about the continuity of the processes driving the evolution and diversification of all life," and that "this continuity is what would be expected of a pattern of common descent." However, such statements ignore the many biogeographical puzzles that have vexed evolutionary biologists.

According to the notion of common descent, for any two populations of organisms on earth, if we go back far enough, we will find their common ancestor. This is said to hold true not just for obviously related types like dogs and wolves, but for every living species on earth today. But sometimes it's virtually impossible to explain how two modern-day populations could have arrived at their current geographical locations from the place occupied by their common ancestral population. Thus, problems emerge when organisms are found in a location such as an island or isolated continent, but no standard migratory mechanism can account for their being located there.

One of the most severe biogeographical puzzles for Darwinians is that of South American monkeys, called "platyrrhines." Based upon molecular and morphological evidence, New World platyrrhine monkeys are thought to be descended from African "Old World" or "catarrhine" monkeys.

The fossil record shows that monkeys have lived in South America for about the past 30 million years.3 But plate tectonic history shows that Africa and South America split off from one another between 100 and 120 million years ago (mya), and that South America was an isolated island continent from about 80 to 3.5 mya.4 If South American monkeys split off from African monkeys around 30 mya, neo-Darwinism must explain how the monkeys crossed hundreds, if not thousands, of kilometers of open ocean to end up in South America.

This poses a problem for evolutionary biologists—one that has been recognized by multiple experts. Primatologists John Fleagle and Christopher Gilbert put it this way in a scientific volume on primate origins:

The most biogeographically challenging aspect of platyrrhine evolution concerns the origin of the entire clade. South America was an island continent throughout most of the Tertiary . . . and paleontologists have debated for much of this century how and where primates reached South America.5

Evolution à la Kon Tiki

How did Darwinians resolve this puzzle? After much debate and contemplation, they proposed, not that common descent might be wrong, but that monkeys must have rafted on floating vegetation across the Atlantic Ocean, from Africa to South America, to colonize the New World.

No, you don't need new glasses. A Harper Collins textbook explains this commonly accepted theory: "The 'rafting hypothesis' argues that monkeys evolved from prosimians once and only once in Africa, and . . . made the water-logged trip to South America."6

And of course, there couldn't have been just one seafaring monkey, because it would have soon died, leaving no offspring. A mated pair would technically be enough to get a South American population going, but the odds against their making the voyage successfully seem vanishingly small—after all, even the intrepid Thor Heyerdahl required a crew of six, and he understood navigation.

Fleagle and Gilbert admit that the hypothesis "raises a difficult biogeographical issue" because "South America is separated from Africa by a distance of at least 2600 km, making a phylogenetic and biogeographic link between the primate faunas of the two continents seem very unlikely." But, wedded as they are to an evolutionary paradigm, they nevertheless had to find such a "link," however unlikely. So, in light of "the absence of any anthropoids from North America, combined with the considerable morphological evidence of a South American–African connection with the rodent and primate faunas," they concluded that "the rafting hypothesis is the most likely scenario for the biogeographic origin of platyrrines."7

In other words, the "unlikely" monkey-rafting hypothesis is made "likely" only because we know common descent must be true.

Rafting by Sheer Chance

Needless to say, the rafting hypothesis faces serious problems. Given that mammals like monkeys have high metabolisms and require large amounts of food and water, Fleagle and Gilbert concede that "over-water dispersal during primate evolution seems truly amazing for a mammalian order," and acknowledge that "the reasons for the prevalence of rafting during the course of primate evolution remain to be explained."8 Or, as another expert puts it, "the mechanical aspect of platyrrhine dispersal [is] virtually irresolvable" because evolutionary models "must invoke a transoceanic crossing mechanism (rafting) that is implausible or suspect . . . at best."9

And there are even deeper problems. Monkeys apparently made the journey from Africa to South America, but many other African primates never colonized the New World. If it was so easy for monkeys to build a seaworthy raft, stock it with adequate provisions, and sail it safely across the proto-Atlantic ocean, why didn't these other primates also make the voyage?

The reason we're given by Fleagle and Gilbert is that there is no reason, and it all comes down to sheer chance. In their own words, rafting was "clearly a chance event," and "one can only speculate that by a stroke of good luck anthropoids were able to 'win' the sweepstakes while lorises and galagos did not."10

This is not the only case where evolutionary biologists have been forced to invoke a wildly speculative mechanism of "oceanic dispersal" to explain away a difficult problem. There are numerous other biogeographical conundrums, including the presence of lizards and large caviomorph rodents in South America,11 the arrival of bees, lemurs, and other mammals in Madagascar,12 the appearance of elephant fossils on various islands,13 and the appearance of freshwater frogs across isolated oceanic island chains.14

For Darwinians, Anything Can Happen

This problem exists for extinct species as well. A paper in the Annals of Geophysics notes the "still unresolved problem of disjointed distribution of fossils on the opposite coasts of the Pacific."15 But that paper doesn't propose some unlikely hypothesis like prehistoric rafting. No, indeed; it suggests instead that the species became separated due to an "expanding earth"—a geologically bizarre idea (different from well-accepted theories of plate tectonics) that could only be taken seriously by those desperate to save common descent from falsification.

A 2005 review in Trends in Ecology and Evolution explains the essence of the problem:

A classic problem in biogeography is to explain why particular terrestrial and freshwater taxa have geographical distributions that are broken up by oceans. Why are southern beeches (Nothofagus spp.) found in Australia, New Zealand, New Guinea and southern South America? Why are there iguanas on the Fiji Islands, whereas all their close relatives are in the New World?16

After considering a number of "unexpected" biogeographical examples that require oceanic dispersal, the review concludes: "these cases reinforce a general message of the great evolutionist [Darwin]: given enough time, many things that seem unlikely can happen."

Indeed, that does appear to be the message here. If you're going to stick with Darwin's theory, you're going to have to accept some extraordinary claims. When evolutionary scientists are forced to appeal to fantastical "expanding earth" hypotheses, or "unlikely" tales of monkeys rafting across oceans, you know that neo-Darwinism faces a challenge. Biogeography does not offer the neat and tidy congruency with common descent that we're often told it does. 

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