Old World Monkeys, New World Monkeys and the First Apes
Often used interchangeably with "primate" and "monkey," the word "simian" derives from Simiiformes, the infraorder of mammals that includes both old world (i.e., African and Eurasian) monkeys and apes and new world (i.e., central and South American) monkeys; the small primates and lemurs described on page 1 of this article are usually referred to as "prosimians." If all this sounds confusing, the important thing to remember is that new world monkeys split off from the main branch of simian evolution about 40 million years ago, during the Eocene epoch, while the split between old world monkeys and apes occurred about 25 million years later.
The fossil evidence for new world monkeys is surprisingly slim; to date, the earliest genus yet identified is Branisella, which lived in South America between 30 and 25 million years ago. Typically for a new world monkey, Branisella was relatively small, with a flat nose and a prehensile tail (oddly enough, old world monkeys never managed to evolve these grasping, flexible appendages). How did Branisella and its fellow new world monkeys make it all the way from Africa to South America? Well, the stretch of Atlantic Ocean separating these two continents was about one-third shorter 40 million years ago than it is today, so it's conceivable that some small old world monkeys made the trip accidentally, on floating thatches of driftwood.
Fairly or unfairly, old world monkeys are often considered significant only insofar as they eventually spawned apes, and then hominids, and then humans. A good candidate for an intermediate form between old-world monkeys and old-world apes was Mesopithecus, a macaque-like primate that, like apes, foraged for leaves and fruits during the day. Another possible transitional form was Oreopithecus (called the "cookie monster" by paleontologists), an island-dwelling European primate that possessed a strange mix of monkey-like and ape-like characteristics but (according to most classification schemes) stopped short of being a true hominid.
The Evolution of Apes and Hominids During the Miocene Epoch
Here's where the story gets a bit confusing. During the Miocene epoch, from 23 to 5 million years ago, a bewildering assortment of apes and hominids inhabited the jungles of Africa and Eurasia (apes are distinguished from monkeys mostly by their lack of tails and stronger arms and shoulders, and hominids are distinguished from apes mostly by their upright postures and bigger brains). The most important non-hominid African ape was Pliopithecus, which may have been ancestral to modern gibbons; an even earlier primate, Propliopithecus, seems to have been ancestral to Pliopithecus. As their non-hominid status implies, Pliopithecus and related apes (such as Proconsul) weren't directly ancestral to humans; for example, none of these primates walked on two feet.
Ape (but not hominid) evolution really hit its stride during the later Miocene, with the tree-dwelling Dryopithecus, the enormous Gigantopithecus (which was about twice the size of a modern gorilla), and the nimble Sivapithecus, which is now considered to be the same genus as Ramapithecus (it turns out that smaller Ramapithecus fossils were probably Sivapithecus females!) Sivapithecus is especially important because this was one of the first apes to venture down from the trees and out onto the African grasslands, a crucial evolutionary transition that may have been spurred by climate change.
Paleontologists disagree about the details, but the first true hominid appears to have been Ardipithecus, which walked (if only clumsily and occasionally) on two feet but only had a chimp-sized brain; even more tantalizingly, there doesn't seem to have been much sexual differentiation between Ardipithecus males and females, which makes this genus unnervingly similar to humans. A few million years after Ardipithecus came the first indisputable hominids: Australopithecus (represented by the famous fossil "Lucy"), which was only about four or five feet tall but walked on two legs and had an unusually large brain, and Paranthropus, which was once considered to be a species of Australopithecus but has since earned its own genus thanks to its unusually large, muscular head and correspondingly larger brain.
Both Australopithecus and Paranthropus lived in Africa until the start of the Pleistocene epoch; paleontologists believe that a population of Australopithecus was the immediate progenitor of genus Homo, the line that eventually evolved (by the end of the Pleistocene) into our own species, Homo sapiens.
The following is a list of the most important prehistoric primates; just click on the links for more information.
Afropithecus This African primate was discovered by Richard and Mary Leakey.
Archaeoindris A gorilla-sized lemur from Madagascar.
Archaeolemur This lemur only went extinct a thousand years ago.
Archicebus Was this small, mouse-like mammal the first true primate?
Ardipithecus This early hominid was unusually mild-tempered.
Australopithecus The immediate predecessor of genus Homo.
Babakotia A pint-sized "sloth lemur" from Madagascar.
Branisella The oldest "new world" monkey yet identified.
Darwinius Did this primate lay at the root of human evolution?
Dryopithecus Guess where this "tree ape" spent most of its time.
Eosimias This "dawn monkey" was only a few inches long.
Ganlea Does it imply an Asian origin for anthropoids?
Gigantopithecus This "giant ape" was twice the size of modern gorillas.
Hadropithecus Yet another ancient lemur from Madagascar.
Megaladapis This prehistoric lemur weighed over a hundred pounds.
Mesopithecus This Pliocene monkey resembled a modern macaque.
Necrolemur A big-eyed tarsier of the Eocene epoch.
Notharctus This lemur-like primate lived 50 million years ago.
Oreopithecus Paleontologists call it the "cookie monster."
Ouranopithecus (Graecopithecus) This prehistoric ape was discovered in Greece.
Palaeopropithecus This "sloth lemur" looked like a cross between...
Paranthropus A bigger, smarter version of Australopithecus.
Pierolapithecus Was this the common ancestor of great apes and lesser apes?
Plesiadapis One of the earliest primates in the fossil record.
Pliopithecus One of the first ancient primates ever to be identified.
Proconsul Could this have been the first true ape?
Propliopithecus (Aegyptopithecus) This early ape resembled a small gibbon.
Purgatorius Could this have been the ultimate ancestor of human beings?
Saadanius This primate "missing link" was discovered in Saudi Arabia.
Sivapithecus (Ramapithecus) This primate may have been ancestral to modern orangutans.
Smilodectes A lemur-like primate from North America.