Regarding the basal position of Ateles marginatus in our inferred topology and the separation of A. paniscus from other species of Ateles by the lower Amazon, it is worth noting that, in both of the biogeographic scenarios we propose, A. paniscus likely diverged from other Ateles somewhere 3.5–4.5 Ma, again before the full development of the current Amazon system. At that time, still and even as early as 6.7 Ma when we estimate the A. marginatus lineage originated the Amazon drainage to the Atlantic probably would have constituted a significant barrier to northward gene flow from the Brazilian to the Guiana Shield in the eastern part of the continent. Thus, if A. paniscus were indeed the next lineage to diverge after A. marginatus (Fig. 5B) then a south-to-north vicariance event across a long-established lower Amazon drainage would have been required. In the other possible scenario we suggest (Fig. 5A), gene flow from the south-central Amazon to the west and north (into what is now the northwestern Amazon of Colombia, Ecuador, and Peru) as well as the origins of Ateles belze- buth, chamek, and hybridus would have occurred somewhere around 4.5–4.0 mya, in the Pliocene, and would have coincided with a period of drying of the western Amazonian Pebas lake for- mation. As Aleixo and Rossetti (2007) note, a number of avian species also expanded from the Brazilian Shield into the western Amazon lowlands at this time.