The Evolution of Primates:
Primate evolution has been a long process of natural selection that has produced the species of monkey, ape, and even human beings that we have today. Within the evolutionary pressures that arose around sixty-five million years ago, a species of arboreal mammal developed a series of typical physiological and psychological traits that scientists associate categorically with modern primates. These traits of grasping hands, depth perception, single offspring birth, larger brains, and high sociality would ultimately define many of the old and new world primates of prosimians, monkeys and apes that survive today.
Through examination of the fossil record scientists determine that around sixty-five million years ago an arboreal mammal similar to a modern rodent underwent far-reaching evolutionary changes in response to a relatively unknown environmental pressure. Termed a "speciation event," this phenomenon encompasses any occurrence that results in a species of animal branching out into multiple lineages of independently identifiable species. Speciation events are the result of four major variables: reproduction isolation, differential reproductive success, environmental changes, and natural selection. Reproductive isolation is when groups of the same species are cut off from each other, through geography, distance, etc., and are unable to cross reproduce and share their genetic information. Differential reproductive success is when one member of a species will successfully raise more offspring to a reproductive age when compared to another member of that same species because of selective traits that their genetics possess. Environmental changes are any natural or unnatural shift in the environment that could either push a species to adapt or offer new niches for them to expand into. Natural selection is, of course, the naturally occurring process of evolution through which members of a particular species survive to pass on adaptive traits that are eventually exhibited in all members of that species. When all of these variables combine they create a relatively sudden evolutionary boom of different species originating from an earlier original species; a speciation event.
The speciation event that caused the initial selection pressures that then caused the branching of multiple early primates is still a mystery, but modern theory is trying to fill the void. Although what had initially occurred is still only speculation, Matt Cartmill's "Visual Predation" hypothesis seems to best explain the development of the traits that we now associate with primates. For whatever reason, small arboreal mammals living around sixty-five million years ago began filling predatory niches that had otherwise been unavailable to them through competition with, what can only be assumed to be, larger more aggressive predators. These primitive primate's physiology would evolve to better hunt the insects of the lower canopy that would make up the majority of their diet, and ultimately become the traits that would become characteristic of modern primates.
The traits that these mammals would evolve between sixty-five and fifty-five million years ago would be: grasping hands, depth perception, giving birth to single offspring at a time, larger more complex brains, and high sociality. The grasping hands of the primate, though not unlike many of the other grasping hands of other arboreal species, was still original in its independently movable digits; many primates today can articulately pick a piece of fruit and peel it whereas many other species lack the dexterity to do so. Their hands also evolved nails instead of claws, a physical trait exhibited exclusively by primates. Depth perception was also a very necessary adaptation; forward facing eyes produce the overlapping fields of vision giving primates the three dimensional perception that they needed to be successful hunters. Another trait of primates would be having a single offspring at a time that would then stay dependant on the parent for many years after birth. This dependency would far outlast the average duration of parental investment found in other mammals their size that typically have litters. One of the most important physiological features of a primate is the larger, more complex brain; the proportion of the primate's brain is almost twice the mass of the average mammal of the same size. This larger brain would also make it possible for the final primate feature: high sociality. By having a brain structure that is adapted to social life and learning socially from their peers, primates had a lot of flexibility to learn a wide range of skills they would need to survive. Primates, especially humans, would be defined by their complex and reciprocatory exchanges of information, skills, and companionship.
These five primate-typical traits would prove the basis for primate taxonomy, physiology, and behavior throughout the next fifty-five million years. Even in the primates we have today, prosimians, monkeys, and apes, these traits are still generally observed across the board. Prosimians, the first of the primates to evolve around sixty-five to fifty-five million years ago, survive today as lemurs, tarsiers and bushbabies. They are characterized by their large nocturnally adapted eyes, highly developed sense of smell, a dental cone in their lower jaw for grooming, and the retaining of a single claw in their grasping hands instead of a primate typical nail. Prosimians, compared to other primates, have smaller brains and are considered the least intelligent will a more primitive form of sociality. It's important to note that though the Lemurs of Madagascar are highly social, most other prosimians are actually solitary nocturnal creatures.
Anthropoids, the next to evolve after prosimians, are made up of monkeys and apes. Monkeys, because of their success throughout the world, save North America, have a very broad range of characteristics spread out over two-hundred and fifty species. Generally, monkeys are diurnal, group living animals with fully developed primate-typical characteristics: nails, large brain, etc. These include tamarins, macaques, and baboons. It's important to note that monkeys are classified into "Old World" and "New World" monkeys; the old world monkeys living in Africa and Asia wile the new world monkeys live in South America. This classification is relevant to primate evolution because it is the old world monkeys of Africa that are closely related to, and would have evolved into, apes.
Apes, surviving exclusively in the old world, are defined by a few ape-typical characteristics; larger tailless bodies with longer arms and a more powerful upper body, ideal for locomotion through climbing and swinging. These primates still share the typical traits of other primates save for a larger, more complex brain and far more complex social groups. These apes include Asia's Gibbon and Orangutan, and Africa's Guerillas, Bonobos and Chimpanzees. It is these great apes of Africa are the closest relatives to humans; somewhere between ten and five million years ago a branch of ape would have broken off and created hominids, the ancestors of today's humans.
Over sixty-five million years ago a speciation event allowed for a small arboreal mammal to expand into several different predatory niches. The ancestors of this tiny creature would progress through time as prosimians, monkeys, apes, and then, through an old world ape, into human beings. The primate-typical traits of grasping hands, depth perception, single offspring, larger brains, and sociality are as strong today in humans as they were in prosimians sixty-five million years ago. This common ground is enough to make one wonder how human beings could push primates, so closely related evolutionarily to themselves, so near the brink of extinction.