Since the centerpiece story in my book is the case of monkeys apparently crossing the Atlantic Ocean from Africa to South America, I now have a special (and vested) interest in studies about this example. So, when I heard about a new paper called “Eocene primates of South America and the African origins of New World monkeys,” I quickly devoured it, even skimming the supplementary material.

The paper, by Bond et al. in the journal Nature, reports fossil teeth of monkeys from Amazonian Peru, from what the authors believe to be Late Eocene deposits. Three findings from this study seem especially important: (1) If the estimated age of the specimens is correct, they greatly extend the fossil record of monkeys in the New World, from about 26 million years ago (the age of the previous oldest known New World monkey) to some 36 million years ago. (The older age, by the way, is still fully consistent with molecular clock estimates for the separation of New World and Old World monkeys.) (2) The teeth represent three different species, indicating a surprising diversity of early South American monkeys. (3) The new species, Perupithecus ucayaliensis, described from the best-preserved tooth, shows an apparent evolutionary connection to a specific extinct African monkey, Talahpithecus.

That last result is particularly relevant for validating the African origin of New World monkeys (although previous evidence from fossils also pointed to that scenario). However, the grouping of the South American Perupithecus with the African Talahpithecus, rather than with all other New World monkeys, also implies a more complex ocean-crossing scenario than previously imagined. Specifically, that relationship requires either two separate crossings from Africa to South America or a single such dispersal followed by another from South America back to Africa (see figure).

These more complex dispersal scenarios can’t be discounted; if monkeys crossed the Atlantic once, I suppose they could have done it twice (although a scenario involving a “back-dispersal” to Africa has, as far as I know, never been supported for a land vertebrate). But, if I had to bet on it, I would choose a different explanation: the grouping of Perupithecus with Talahpithecus is wrong. That wouldn’t be a shock considering that Perupithecus is known from just a single tooth and Talahpithecus from three teeth, two of them broken. Also, in the analysis by Bond et al., the pairing of these two taxa is very tenuous. (A tree that does not group these two taxa is just one step longer than the most parsimonious tree.) I wouldn’t be surprised if it turns out that Perupithecus groups with other New World monkeys, with Talahpithecus outside of this group. That result would bring us back to the simpler scenario of a single crossing of the Atlantic.

In any case, Bond et al.’s study is a big step forward in deciphering the history of monkeys in the New World. I’m hoping that they or others will eventually find other critical fossils that will further flesh out the story of how and when monkeys reached the Americas. Maybe it’s too much to ask, but it would be fantastic to have a skull and some other bones from these early New World monkeys!

Thanks to Darren Lettinga for letting me know about this paper.