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Fossil Friday: Did Monkeys Raft Four Times Across the Atlantic?

Photo: Ashaninkacebus molar, after Fig. S4 Marivaux et al. 2023, fair use.

A few years ago, I published an article at Evolution News titled “Rafting Stormy Waters” (Bechly 2018), which discussed the various highly implausible events of oceanic dispersal with vegetation rafts to explain the biogeographic patterns of living animals. This includes the dispersal of monkeys from Africa to South America that implied a 60-day voyage of 1,400 km across the Eocene early Atlantic Ocean (Gabbatiss 2016).

Now, a new fossil find has made this problem much worse. Marivaux et al. (2023) describe in the journal PNAS a new primate genus from the Paleogene of Western Amazon and provide a new phylogenetic analysis of the earliest fossil Neotropical monkeys. This phylogenetic study shows that three genera of fossil monkeys, all known by fossil teeth from the Early Oligocene of South America, are not related to living New World monkeys (Platyrrhini), but are nested in three distinct African clades respectively: Ashaninkacebus is nested within the Eosimiidae clade (Marivaux et al. 2023), Perupithecus is nested within the Oligopithecidae clade (Bond et al. 2015), and Ucayalipithecus is nested within the Parapithecidae clade (Seiffert et al. 2020). This implies three independent Eocene colonization events of South America by rafting from Africa, additional to the dispersal of platyrrhine monkeys, and additional to the dispersal of caviomorph rodents.

Beyond Ridiculous

Did viable populations of monkeys really raft successfully four times across the Atlantic Ocean from Africa to South America? Seriously? One such event is already a stretch, but four times is beyond ridiculous. If such events happened so often with unlikely passengers like monkeys, why don’t we find many more cases of similar Neotropic-Afrotropic relationships in much more likely candidates such as reptiles or insects, which could far more easily survive long transoceanic rafts? Not to mention the simple fact that in the whole history of human seafaring we have never observed rafting vertebrates in the middle of the ocean and only observed rafting dispersal events in cases of relatively close islands and even there only with reptiles. Longrich (2021) called this an “incredible ocean crossing” which “beat odds that make Powerball lotteries seem like a safe bet.” Something is clearly wrong here, and I mean way off. But evolutionary biology has a cheap cop out that was made explicit by Nobel laureate George Wald (1954):

Given so much time, the [nearly] “impossible” becomes possible, the “possible” becomes probable, and the “probable” becomes virtually “certain.”

It does not require that one be a Darwin doubter to recognize that this hardly qualifies as good science, as shown, for example, in this highly recommended article by Lu (2021) from an AI perspective. Time is not the hero of the plot when actual improbabilities and probabilistic resources are ignored or glossed over with fancy storytelling according to the unspoken dogma of evolutionary biology: It must have been possible because it happened. After all, God forbid if we were to consider explanations beyond blind naturalistic mechanisms.