From Tree Shrews To Primates

Primates have their earliest evolutionary ancestry in tree-shrew-sized proto-mammals that evolved in the shadow of dinosaurs about 200 million years ago. Only after their disappearance 65 million years ago did our (then barely larger than rat sized) mammalian ancestors slowly begin to evolve into primates. In the early part of their evolutionary history, most primates looked much like the modern-day tarsiers or lemurs. About 40 million years ago, however, new primate families arose: the monkeys. As the world cooled and forests increasingly gave way to grasslands, monkeys had to either adapt or disappear. They did disappear from North America, and they became largely restricted to tropical forest environments in equatorial regions. Africa was largely forested as late as about 15 million years ago, but soon afterward, its great tropical forests shrank. Between 5 million and 7 million years ago, the global climate gradually became warmer and drier. Forested areas began receding, making way for grassland savanna environments. Northern Africa gradually grew drier, while regions to the east and south became dominated by a savanna landscape. Indeed, the Mediterranean completely dried out 6 million years ago, and a great drop in the sea level occurred during that period, lasting for about 1 million years.

Eventually, the African primates that would evolve into Homo sapiens were forced down from the trees and made the open savanna their home. From paleontological work carried out over the last two or three decades we know that primates occasionally began to walk upright in the African savanna 5 million to 6 million years ago.11 It is important to emphasize the human "family tree" does not proceed in a straight line. Paleoanthropologists Ian Tattersall and Jeffrey Schwarz have presented convincing evidence that over 15 different species of humans or hominids have existed over the 6-million-year sojourn of the hominid family - and many of these species have existed simultaneously. Even at the beginning of the human sojourn there were at least three separate species of these early now-extinct ancestors.12 Thus, the diversity of extinct humans - and the consecutive number of hominid species - is much broader than many scholars have thought it to be.13

The earliest hominids were chimp-sized creatures that lived in the Ethiopian forests between 5 million to 6 million years ago. These earliest hominids were essentially tree-dwelling creatures, but they had developed an upright posture; its arms and shoulders, as well as its relatively small brain, show that it was still living a semi-arboreal life. In all likelihood, our early ancestors spent much of the daylight foraging on the ground in open semi-woodlands, seldom straying far from the safety of the trees. And, like modern chimpanzees, they still retreated to the trees at night. Slow and occasionally walking upright in an awkward gait, they would have been at the mercy of a variety of predators had they remained on the ground in the darkness.

At first blush, bipedalism just does not make sense. For our early ancestors, it would have been slower than walking on all fours, while requiring the same amount of energy. Several theories have been suggested to explain bipedalism and upright gait. Anthropologists Henry McHenry and Peter Rodman, for example, champion the idea that climate variation was part of the picture after all. When Africa dried out, they argue, the change left patches of forest widely spaced between open savannah. The first hominids lived mostly in these forest refuges but could not find enough food in any one place. Learning to walk on two legs helped them travel long distances overground to the next woodsy patch.14 Paleontologist Maeve Leakey, a member of the world's most famous fossil-hunting family, suspects the change in climate rewarded bipedalism, since a drier climate made for more grassland. Our ancestors, she argues, spent much of their time not in dense forests or on the savannah but in an environment with some trees, dense shrubbery, and a bit of grass. If a creature has to move into open country with grasslands and bushes, foraging on fruits and berries on low bushes, there must have been a strong advantage to being able to reach higher.15 A third explanation is offered by anthropologist Owen Lovejoy. He speculates that males who were best at walking upright would get more sex, leading to more offspring with those genetic advantages. Over time, female apes would choose to mate only with those males who brought them food — presumably the ones who were best adapted for upright walking.16 In short, there exist a variety of explanations for bipedalism.

Physical evidence for these distant relatives of our biological family was found in the Olduvai Gorge in Tanzania and consisted of the fossil relics of about 20 individuals. Subsequently, further physical evidence discovered at a site near Lake Rudolf in northern Kenya and later discoveries at Olduvai added to our sparse understanding of the activities of our ancestors. For example, we know that they used elementary tools, a notable step in the control of the environment. Tools found in Kenya are the oldest such evidence and consist of stones crudely fashioned by striking flakes off pebbles to give them a sharp edge. Frequently, the pebbles seem to have been transported purposefully and selectively from one site to another where they were further refined. In short, the conscious creation of tool implements had begun. About 1 million years ago, simple pebble choppers of the same type spread all over the Africa and Eurasia.17

In discussing the evolution of archaic humans, we must bear in mind that the period of 2.5 million years before the present marks the onset of the great climatic perturbations that culminated in the ice ages. For anthropologists, this period is characterized by a great diversification of hominids. Geological evidence indicates that massive layers of ice began to cover Antarctica. Eventually, great ice regions formed at the North Pole as well. Ice sheets began to move across North America, Europe, and Asia, until as much as a third of the area of those continents was buried under ice 1 mile thick. Huge glaciers descended from the great north-south mountain chains as well, and the Earth's climate changed rapidly. Rain forests dried, deserts became wet, and species began to die. Apart from the obvious effects on animals and plants, the severe cold locked up large quantities of sea water in ice sheets: sea levels fell, establishing a land connection between Britain and Europe, as well as between Indonesia and the Asiatic mainland. Periods of intense cold were interrupted by interglacial, usually warm, periods that produced heavy tropical rainfalls.

Climate change, in short, also figured in important ways in the evolution of other hominids. According to paleoecologist Stephen Stanley, the Isthmus of Panama was lifted up by movement of the planet's tectonic plates 2.5 million years ago. A new land bridge connected North and South America for the first time, causing major disruptions in the flow of the ocean currents and leading to a major ice age. In Africa, the climate grew cooler and drier, and the formerly large areas of open woodland began to disappear, forcing our ancestors to become ground dwellers. The results were predictable. Australopithecus died out, along with a large number of other species that were adapted to the woodlands. While the crisis eliminated many of the early hominids, it also freed them from an evolutionary dead end. As a result, at least one hominid group rapidly evolved into something new - an upright, large-brained hominid that could survive on the ground. From that group derived the genus Homo and, eventually, modern humans.18

In Africa, several new species of land-dwelling hominids appeared. Growing in stature, these creatures developed a distinct taste for meat.19 One of the most important stages of human evolution was reached: the appearance of Homo erectus ("man that walks upright"). So far, the earliest remains of a Homo erectus specimen are estimated to be about 1.5 million years old. Many signs point to its African origin and hence to its spread through Europe and Asia some half a million to a million years ago. Apart from fossils, a special tool used by Homo erectus helps us to plot the distribution of the new species by defining areas into which Homo erectus did not spread as well as those into which he did. This is the so-called stone "hand-axe," whose use may well have been mainly as a scraper and dresser of other materials.20 There can be no doubt of the historical success of Homo erectus, but the ecological impact of Homo erectus or other species through predation was comparatively minimal.21

Nevertheless, Homo erectus had an unprecedented capacity to manipulate the environment. Beside hand-axes, Homo erectus left the earliest surviving traces of constructed dwellings - huts, sometimes 50 feet long, built of branches, with stone-slab or skin floors - the earliest worked wood, the first wooden spear, and the earliest container, a wooden bowl. The existence of such artifacts hints strongly at a new level of mentality, at a preformed conception of the objects and perhaps an idea of process. Some researchers have argued that Homo erectus' early form of conscious intentionality might be viewed as the first budding of an aesthetic sense.22 Homo erectus -considered the proverbial missing link between apes and humans - was a bipedal creature, a social omnivore who could hunt and kill prey. Like modern humans, Homo erectus bore helpless young, and thus infant care was essential and the infant's brain could continue to expand during the first year. However, it has been firmly established that Homo erectus possessed only rudimentary linguistic abilities.23

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