Your downward-pointing nostrils mark you as an Old World monkey
Look at a spider monkey's nose and then at your own. Its nostrils open sideways; yours point down. That small difference splits the simians into two great branches, and it places humans, along with gorillas, gibbons and baboons, among the catarrhines, whose name comes from Greek words meaning down and nose.
The French naturalist Geoffroy grouped Old World monkeys and apes together in 1812 under the name Catarrhini. Their sister group is the platyrrhines, the flat-nosed monkeys of the Americas, whose ancestors seem to have crossed to South America from Afro-Arabia, probably over the ocean. Estimates of when the two lines parted vary: Schrago and Russo put it around 35 million years ago, while Begun and Harrison suggest 44 to 40 million.
Beyond noses, catarrhines never have grasping tails. They have flat nails on fingers and toes, a tube-shaped ear bone, and eight premolars instead of twelve, giving two incisors, one canine, two premolars and three molars in each quarter of the mouth. Most species live in groups, are active by day, and show marked differences between the sexes without forming pair bonds. All are native to Africa and Asia, and early catarrhines did not spread into Eurasia beyond Arabia until 18 to 17 million years ago.
After splitting from the American monkeys, the line lost an enzyme, alpha-galactosidase, that every other mammal group keeps. One idea is that an ancient pathogen carried the enzyme, so only individuals whose gene was switched off made antibodies against it and survived.
Apes branched off from the other Old World monkeys around 25 million years ago, and gibbons parted from the great apes roughly 15 to 19 million years ago. Strictly by family tree, that makes apes, humans included, a kind of monkey, a point Buffon grasped in the 18th century. English still resists the idea, though Russian uses one word for all simians, tailed or not.
Source: Catarrhini
