South America's monkeys probably arrived from Africa on floating rafts of vegetation
About 40 million years ago the ancestors of marmosets, capuchins and howlers were in Africa. The leading theory holds that some drifted across a narrower Atlantic on mats of plants, perhaps hopping between islands now underwater. Rodents made the same improbable voyage, and fossils hint primates did it more than once.
New World monkeys are the five primate families living in the tropics of Mexico and Central and South America: the marmosets and tamarins, the capuchins and squirrel monkeys, the night or owl monkeys, the titis, sakis and uakaris, and the howler, spider and woolly monkeys. Together they form the Platyrrhini, Greek for broad-nosed, a nod to their flat noses with sideways-facing nostrils, unlike the narrow noses of their closest relatives, the Old World monkeys and apes.
Simians split into these two branches somewhere in Africa roughly 40 million years ago. When platyrrhines crossed during the Eocene, there was no Isthmus of Panama, ocean currents favoured westward drift and the Atlantic was perhaps a third narrower than its present 2,800 kilometres, since the seafloor spreads about 25 millimetres a year. Ucayalipithecus from Amazonian Peru, which may have rafted over between about 35 and 32 million years ago, belongs to a different African group, implying at least two primate crossings, and a fossil molar named Ashaninkacebus hints at a third with South Asian links.
Genetics adds detail. The ancestral species appears to have had 54 chromosomes, whereas today's titi monkeys have just 16 and woolly monkeys 62. A statistical Bayesian analysis puts the last common ancestor of today's species between 27 and 31 million years ago. How to group the families has shifted repeatedly among specialists, from two families in one scheme to four or five in others.
Sizes span a wide range, from the pygmy marmoset, the smallest monkey at 14 to 16 centimetres and under 200 grams, to the southern muriqui at up to 70 centimetres and 15 kilograms. Members of the howler and spider monkey family stand out as the only primates with prehensile tails that can grip branches.
Source: New World monkey
