Early Primate Evolution and Adaptations
The Order Primates
All of us are members of the order primates, which include most tree–loving placental mammals. These are two suborders:anthropoids (apes, humans, and monkeys) and prosimians (lemurs, tarsiers, and other so–called pre monkeys). The many similarities in behavior and physical characteristics between the hominids (primates of the family Hominidae, which includes modern humans, earlier human subspecies, and their direct ancestors) and the pongids (our closest living nonhuman primate relatives) can be explained by identical characteristics that each group inherited million of years ago from a common ancestor. In other words, humans and our closest non human primate relatives evolved along parallel lines from a common ancestor.
When did humankind separate from the nonhuman primates? Experts disagree violently about the answer. It was in Africa that humans and apes diverged from monkeys, but no one knows when the divergence took place. Several species of apes were flourishing in Africa at the beginning of the Miocene epoch, some 24 million years ago. The basic anatomical pattern of the large hominids appears in the Middle Miocene, 18 to 12 million years ago. A second radiation began in the Late Miocene, between 8 and 5 million years ago. This radiation eventually produces four lineage's, at least one of which, human beings, is known to have considerably modified. It is interesting to note that a similar evolutionary pattern occurs among herbivores such as elephants. In both cases, the patterns reflect changing climates and habitats—from warmer, less seasonal, more forested regimens to colder, more seasonal, and less forested conditions. They also mirror changes in the configuration of continents, mountain systems and Antarctic ice.
For the hominids, the critical period was between 10 and 5 million years ago, when the segment of the African hominoid lineage radiated to produce gorillas, chimpanzees, and hominids. Unfortunately, this 5–million–year period is a black hole in our knowledge of early human evolution. We can only guess at the nature of the apelike animals that flourished in Africa during these millennia. Paleoanthropologists David Pilbeam theorizes that these animals were mostly tree–living, with long arms and legs and a broad chest. They would have used all four limbs in the trees, occasionally scrambling on the ground and even standing on their rear limbs at times. About 5 million years ago, Pilbeam believes, the proto–chimpanzee, remained dependent on fruit and other tree foods, scattered resources that required a flexible social organizations. Intense controversy surrounds the relationship between humans, chimpanzees, and gorillas, but many biologists agree that chimpanzees are humans' closest relatives. Using molecular time clocks, they calculate that these three primate forms last shared a common ancestor about 6 to 7 million years ago.
“Coming Down from the Trees”
A fall in world temperatures after 20 million years ago resulted in increasingly open environments in tropical latitudes. With this reduction in forested environments probably came a trend toward ground–adapted species. Many species of living and now–extinct primates, including hominids, adapted to this kind of existence some time after 10 million years ago. In other words, “they came down from the trees.”
About 5 million years ago, the African savanna, with its patches of forest and extensive grassland plains, was densely populated by many mammalian species as well as by specialized tree–dwellers and other primates. Some of these flourished in small bands, probably walking upright, and conceivably making tools out of stone and wood.
Coming down from the trees created three immediate problems. First was the difficulty of getting around in open country. Hominids adopted a bipedal posture as a way of doing so at least 4 million years ago. Our ancestors became bipedal (walking on two feet) over a long period of time, perhaps as a result of spending more and more time feeding on food resources on the ground. Bipeadlism is a posture that is configured for endurance rather than power or speed. An upright posture and bipedal gait are the most characteristic hominid physical features.
Upright posture is vital because it frees the hands for other actions, like toolmaking. It contrasts with knuckle walking, which provides an excellent power thrust for jumping into a tree or a short sprint (think of a football lineman). It is a specialized way of moving around in which the backs of the fingers are placed on the ground and act as main weight–bearing surfaces. Knuckle walking was adaptive in the forest because long arms and hands as well as grasping feet were still vital climbing. Human arms are too short for us to be comfortable with this posture. Bipedalism favors endurance and the covering of long distances, important considerations in open country. It was a critical antecedent of hunting, gathering, and toolmaking.
Second, the savanna abounded in predators, making it hard for primates to sleep safely. Large hominids made these home bases, where they sheltered from the hot sun and slept in safety. What form these home bases took is a matter of great debate. Lastly, high–quality plant foods, abundant in the forests, were widely dispersed over the savanna. It is striking that later foragers subsisted off a broad range of game and plant food range to include more meat, perhaps during long periods of plant scarcity. Among mammals, these characteristics are associated with a trend toward larger brain size. As the brain size increased, so, gradually, the lifeways of evolving hominids became less apelike and closer to that of human foragers, a process that took hundreds of thousands of years to unfold.
Early hominids faced three major adaptive problems. They were large mammals;they were terrestrial primates;and they lived in a open, tropical savanna environment. Human beings are large and have additional food requirements due to higher metabolic rates. This means that each hominid has to range efficiently over a larger area to obtain food. Larger mammals are more mobile than their smaller relatives. They cover more ground, which enables them to subsist off resources that are unevenly distributed not only in space but also at different seasons. Mobility allows larger mammals like humans to incorporate unpredictable, often seasonal, resources in their diets. They can tolerate extremes of heat and cold, a capacity that may have contributed to the spread of humans out of the tropics later in prehistory. Bipedal humans have sweat glands and are heavily dependent on water supplies. These glands are a direct adjunct to bipedalism, for they enhance endurance for long–distance foraging.
These and several other factors—such as increased longevity and brain enlargement—created adaptive problems for emerging humans. The problems resulted in a number of solutions, among then wider territorial ranges, the need to schedule food gathering, a broadening of diet, a high degree of mobility, and much greater behavioral flexibility. This flexibility included enhanced intelligence and learning capacity, increased parental care, and new levels of social interaction.
