Many people take an understandably human-centered view of primate evolution, focusing on the bipedal, large-brained hominids that populated the jungles of Africa a few million years ago. But the fact is that primates as a whole - a category of megafauna mammals that includes not only humans and hominids, but monkeys, apes, lemurs, baboons, and tarsiers - have a deep evolutionary history that stretches as far back as the age of dinosaurs. (See a gallery of prehistoric primate pictures and profiles.)
The first mammal that paleontologists have identified as possessing primate-like characteristics was Purgatorius, a tiny, mouse-sized creature of the late Cretaceous period (just before the K/T Impact Event that rendered the dinosaurs extinct). Although it looked more like a tree shrew than a monkey or ape, Purgatorius had a very primate-like set of teeth, and it (or a close relative) may have spawned the more familiar primates of the Cenozoic Era. (Genetic sequencing studies suggest that the earliest primate ancestor may have lived a whopping 20 million years before Purgatorius, but as yet there's no fossil evidence for this mysterious beast.)
Scientists have touted the equally mouse-like Archicebus, which lived 10 million years after Purgatorius, as the first true primate, and the anatomic evidence in support of this hypothesis is even stronger. What's confusing about this is that the Asian Archicebus seems to have lived around the same time as the North American and Eurasian Plesiadapis, a much bigger, two-foot-long, tree-dwelling, lemur-like primate with a rodent-like head. The teeth of Plesiadapis displayed the early adaptations necessary for an omnivorous diet - a key trait that allowed its descendants tens of millions of years down the line to diversify away from trees and toward the open grasslands.
Primate Evolution During the Eocene Epoch
During the Eocene epoch-from about 55 million to 35 million years ago-small, lemur-like primates haunted woodlands the world over, though the fossil evidence is frustratingly sparse. The most important of these creatures was Notharctus, which had a telling mix of simian traits: a flat face with forward-facing eyes, flexible hands that could grasp branches, a sinuous backbone, and (perhaps most important) a bigger brain, proportionate to its size, than can be seen in any previous vertebrate. Interestingly, Notharctus was the last primate ever to be indigenous to North America; it probably descended from ancestors that crossed the land bridge from Asia at the end of the Paleocene. Similar to Notharctus was the western European Darwinius, the subject of a big public relations blitz a few years back touting it as the earliest human ancestor; not many experts are convinced.
Another important Eocene primate was the Asian Eosimias ("dawn monkey"), which was considerably smaller than both Notharctus and Darwinius, only a few inches from head to tail and weighing one or two ounces, max. The nocturnal, tree-dwelling Eosimias - which was about the size of your average Mesozoic mammal - has been posited by some experts as proof that monkeys originated in Asia rather than Africa, though this is far from a widely accepted conclusion. The Eocene also witnessed the North American Smilodectes and the amusingly named Necrolemur from western Europe, early, pint-sized monkey ancestors that were distantly related to modern lemurs and tarsiers.
A Brief Digression - The Lemurs of Madagascar
Speaking of lemurs, no account of primate evolution would be complete without a description of the rich variety of prehistoric lemurs that once inhabited the Indian Ocean island of Madagascar, off the east African coast. The fourth-largest island in the world, after Greenland, New Guinea, and Borneo, Madagascar split off from the African mainland about 160 million years ago, during the late Jurassic period, and then from the Indian subcontinent anywhere from 100 to 80 million years ago, during the middle to late Cretaceous period. What this means, of course, is that it's virtually impossible for any Mesozoic primates to have evolved on Madagascar before these big splits-so where did all those lemurs come from?
The answer, as far as paleontologists can tell, is that some lucky Paleocene or Eocene primates managed to float to Madagascar from the African coast on tangled thatches of driftwood, a 200-mile journey that could conceivably have been accomplished in a matter of days. Crucially, the only primates to successfully make this trip happened to be lemurs and not other types of monkeys - and once ensconced on their enormous island, these tiny progenitors were free to evolve into a wide variety of ecological niches over the ensuing tens of millions of years (even today, the only place on earth you can find lemurs is Madagascar; these primates perished millions of years ago in North America, Eurasia, and even Africa).
Given their relative isolation, and the lack of effective predators, the prehistoric lemurs of Madagascar were free to evolve in some weird directions. The Pleistocene epoch witnessed plus-sized lemurs like Archaeoindris, which was about the size of a modern gorilla, and the smaller Megaladapis, which "only" weighed 100 pounds or so. Entirely different (but of course closely related) were the so-called "sloth" lemurs, primates like Babakotia and Palaeopropithecus that looked and behaved like sloths, lazily climbing trees and sleeping upside-down from branches. Sadly, most of these slow, trusting, dim-witted lemurs were doomed to extinction when the first human settlers arrived on Madagascar about 2,000 years ago.
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.