Human Evolution Research Paper

One important way world historians have expanded their perspectives on humanity’s past is through the integration of hominid evolution with their customary consideration of events covering the last 5,000 years. To understand hominid evolution, they have turned to the findings of paleoanthropologists, who rely primarily on fossil evidence rather than written records to reconstruct the patterns of prehistory that stretch back 5 to 7 million years, when our earliest direct ancestors in the family of hominids, an amazing variety of distinct species, notably the australopithecines, or southern apes, diverged onto their own evolutionary line. Seventy million years of primate evolution set the parameters and determined the possibilities for the origins and development of hominids. From their primate heritage, human beings have derived a unique combination of physical attributes, including large brains relative to body size, eyes capable of binocular vision, nimble hands, smaller canine teeth, and a skeleton designed for upright walking. Human evolution also rested on long-standing trends in primate behavior, including social learning and cooperation, tool making, and group organization.

Before discussing human evolution in more detail, however, it’s important to mention the evidence used in paleoanthropology and the types of disputes that tend to arise. Given the paucity of evidence available to paleoanthropologists, the immense gaps in existing knowledge, and the number of recent archeological finds that call into question prevailing views, world historians should not be surprised to find the experts constantly disagreeing among themselves about the variables of hominid evolution. They utilize detailed site surveys, stratigraphic observations, chronometric methods of dating, and precise forms of climatological, biomolecular, and comparative anatomical analysis to begin to interpret what they have found. They have increasingly emphasized the diversity rather than the uniformity of early hominids and the possibility that early hominids lived widely throughout sub-Saharan Africa, but have remained divided over how to distinguish one species or genus from another, and hence over the use of proper nomenclature. Such fundamental aspects of human evolution as the development of walking, speaking, and the hunting-gathering adaptation remain subjects of considerable debate.

Earliest Hominids

Despite unresolved disagreements and uncertainties, and evidence for the development of a discernibly upright posture in primate evolution, paleoanthropologists have focused on bipedal locomotion, the peculiar human capacity to walk efficiently on two legs with a fully erect posture, as the key to understanding what separated our oldest forebears from other hominoids. Indisputably, Australopithecines living in eastern and southern Africa had ape bodies adapted to permit upright balance and a striding gait. Bipedalism represented a beneficial adaptive response to Africa’s expanding open woodlands and savanna environments. Compelled to traverse the considerable distances between shrinking wooded areas as they foraged for sustenance or sought a refuge as nightfall approached, our distant forebears must have benefited from the cooling effects of upright posture, the advantages of a panoramic view, especially when striding through tall grass, and the convenience of having free hands to carry food, artifacts, or children. The occasional bipedalism that natural selection steadily reinforced as australopithecines adapted to their special niche set the stage for the subsequent evolution of every other fundamental feature that separates us from our hominoid cousins. It probably contributed to such physical modifications as progressive hair loss, improved sweat glands, and the refinement of the human hand. By indirectly encouraging tool use and heightened levels of social interaction, it may even have served as a fountainhead for the development of cultural behavior.

For several decades after the discovery of Australopithecus afarensis in 1974, paleoanthropologists devoted considerable attention to the fossil evidence for this relatively small, gracile species known as Australopithecus afarensis, which lived in East Africa between 4 and 3 million years ago. Still viewed by most investigators as the common ancestor of all subsequent hominid forms, afarensis possessed an almost humanlike skeleton below the neck, but a braincase only about one-third that of the modern average. In the mid-1990s, archaeologists discovered the remains of two other previously unknown hominids, Australopithecus amanensis, an apelike creature that apparently lived in the dense forests of East Africa at least 4 million years ago, and what researchers originally called Australopithecus ramidus, another East African species remarkably similar to chimpanzees.

Ramidus inhabited flat, forested regions as many as 5.8 million years ago, and apparently did walk upright, at least on occasion, although doubts about its capacity for bipedalism have led some investigators to conclude that it should be treated as a distinct genus, Ardipithecus ramidus. If that judgment is sustained, then only amanensis will be viewed as a direct precursor of Australopithecus africanus, which itself completely died out 2.5 million years ago, but not before giving rise to other varieties of australopithecines as well as to the most ancient members of our genus almost 3 million years ago. One of these species, Australopithecus africanus, lived in southern Africa between 3 and 2.5 million years ago, giving way to a robust group that included Australopithecus aethiopicus and then Australopithecus boisei in East Africa and Australopithecus robustus in the far south. (Some paleoanthropologists classify aethiopicus, boisei, and robustus as members of the genus Paranthropus, which gives some impression of the degree of debate in the field.) Robustus died out about 1.5 million years ago, while boisei endured for another 500,000 years, at last succumbing to less burly creatures with larger brains relative to body size and greater dependence on tool use. The earliest forms of our genus, Homo, collectively known as Homo habilis or “handy man,” were indisputably present in East Africa somewhat more than 2 million years ago, coexisting with Australopithecus boisei for at least a million years and with robustus in South Africa for a much shorter period.

Appearance of the Genus Homo

What paleoanthropologists have lumped together as habilines may actually have been several closely related species. At a minimum, some experts insist on differentiating Homo rudolfensis from Homo habilis. Others explain substantial variations in terms of a pronounced sexual dimorphism (that is, pronounced differences in the physical appearance of the sexes). But all agree that an abrupt species transition apparently did take place about 2.6 million years ago, and that the brain size of habilines jumped appreciably around 600,000 years later. But beyond enlarged cranial capacity, Homo habilis remained remarkably apelike. Paleoanthropologists have used the earliest evidence for systematic tool manufacturing—a momentous accomplishment that gave birth to the development of culture as a primary adaptive mechanism for humankind—as the means for differentiating habilines from australopithecines.

No later than 2.4 million years ago, East African hominids who may have been the immediate precursors of Homo habilis rather suddenly started to modify water-smoothed pebbles and rocks gathered in river beds, fashioning them into rudimentary stone artifacts suitably shaped to fit immediate needs. Soon after 1.9 million years ago, habilines had at their disposal an array of simple but clearly recognizable choppers, scrapers, burins, and hammerstones, all made in set patterns created by striking a core with a smaller stone to chip off a few sharp-edged flakes. Known as the Oldowan industry, this basic technique eventually spread throughout much of Africa and Eurasia, continuing to be practiced in isolated areas as recently as 200,000 years ago. Intentionally designed Oldowan tools enabled habilines to broaden their subsistence strategies through increased dependence on meat. While the bulk of their diet still consisted of vegetable foods, habilines probably turned with growing frequency to opportunistic scavenging of predator kills and expert tracking of small game animals. Although their hunting activities remained limited, they were beginning to define the basic contours of the gathering and hunting adaptation.

As the social organization of their bands became more complicated, the demands placed on individuals in their relationships with others may have contributed to the development of the brain. Manual dexterity, bipedalism, and tool use simultaneously showed signs of improvement, all evolving together with enhanced mental capacity. What made the hominid brain truly distinctive was its division into two asymmetrically structured hemispheres, each containing identifiable areas designed to control particular mental and behavioral functions. Among other consequences now vital to our existence, habilines may have possessed a feature of the cerebral cortex known as Broca’s area, a region connected with speech in modern humans. Despite an immature larynx, which would have limited the range of sounds they uttered, habilines may have articulated a combination of vowels and consonants adequate for the exchange of some genuinely complex ideas.

Significance of Homo Erectus

As ever more elaborate cultural innovations augmented biological changes, the pace of hominid evolution quickened, starting with the appearance of Homo erectus, a species that apparently evolved from habilines in East Africa between 1.9 and 1.6 million years ago. Erectines were more humanlike in their anatomical features and behavior patterns than any of their hominid predecessors. What made them such a milestone in the biological evolution of our kind, however, was their impressive cranial capacity. The brain of Homo erectus was not simply bigger, but it was distinctly less symmetrical, since each part expanded at a different rate and was designed to perform specific functions.

Better control over the hand as well as the unprecedented ingenuity made possible by an increased brain-to-body ratio enabled erectines to become more sophisticated toolmakers and to create what paleoanthropoligists call the Acheulian tradition. Beginning about 1.6 million years ago, they gradually learned to devise a more patterned and versatile stone artifact known as the hand axe. Erectines discovered how to remove flakes from both sides (faces) of a shaped stone core, producing two straight, sharp edges that converged at a point. Erectines made another important technological innovation when they introduced the cleaver, and they turned to other materials, such as volcanic rock and flint. No later than a million years ago they possessed a standardized tool kit containing at least eighteen types of highly effective implements, each proportioned in accordance with its intended purpose.

Nearly 1.5 million years ago, Homo erectus took definite steps toward the controlled use of fire, the next great cultural advance after the fashioning of stone artifacts. The domestication of fire, for which archeologists have evidence dating back 400,000 years, gave hominids their first real command over a powerful force of nature. Still unable to ignite a fire on their own initiative, they nonetheless taught themselves to collect flaming debris from volcanic eruptions and lightning strikes and actively keep it burning in small hearths. Fires provided warmth as well as protection against dangerous animals. Erectines may have intermittently exploited fire for cooking as early as 700,000 years ago. As opportunities arose, they certainly drove game in a desired direction and opened up their hunting grounds with fire. Drawing on it in their toolmaking, they hardened green wood and occasionally bone or antler in the heat of their hearths. Along with warmth, fire provided light, permitting Homo erectus to lengthen the day.

Erectines learned how to turn animal skins into clothing, and at least 400,000 years ago started to build substantial, oval-shaped huts. Such attainments, taken together, underscore the extent to which they had become dependent on cultural rather than biological adaptations in comparison with their hominid predecessors. Until roughly a million years ago, the evolution of every living thing had unfolded largely in response to challenges emanating out of the natural environment. Then, uniquely with Homo erectus, shared understandings and norms of conduct, painstakingly learned and transmitted to the next generation, began to override behavior dictated strictly by genetic inheritance, more and more setting the context for everyday existence.

Given the size and complexity of their brains, erectines probably became more adept at sharing their thoughts through speech. Their nearly human linguistic potential enabled them to become active and perhaps even habitual hunters. Coming together in groups to improve their chances for successful expeditions aimed at big game strengthened and multiplied the patterns of interdependence between erectines, making their social structures more complex, tightknit, and more recognizably human than any network of relations that may have prevailed among habilines. They probably took a full million years to forge the enduring characteristics of the gathering and hunting adaptation, a lifeway at once highly flexible and extraordinarily stable.

Homo erectus was the first hominid to spread far beyond the African continent. Successive waves of a grand migration out of Africa may have started a million and a half years ago, but the first well-documented expansion into the frost-free zones of Eurasia occurred about 500,000 years later. Erectines funneled through the Near East, fanning out into southern Asia, where they encountered woodlands and savanna environments similar to those they had left behind. They simultaneously moved westward into parts of Europe without ever establishing an appreciable presence there. Reacting to mounting population pressures, tiny parties of the sole surviving hominid species dispersed by means of repeated budding—the splitting away of a small group from an existing band—in a continuous process which enabled them to reach southern China at least 800,000 years ago by way of India and Southeast Asia.

The tempo of evolutionary change again accelerated about 400,000 years ago. In what must have been a mutually supportive feedback process, erectines in Africa and Europe abruptly showed signs of perceptible cranial expansion at a time when they were mastering the deliberate ignition of fires and developing bona fide strategies for hunting large game animals. Just 200,000 years later, Homo erectus had virtually disappeared, having succumbed to competition from the most recent hominid species—Homo sapiens, or “wise man.” Isolated erectines did survive in Java until Homo sapiens intruded into the region between fifty and sixty thousand years ago. For world historians, the task of sorting out these issues has been complicated by experts who now argue that a hominid species called Homo ergaster was probably the forerunner of modern humans, while erectines should be regarded as a branch of humankind’s lineage.

Emergence of Homo Sapiens

Although gaps in the archaeological record have kept scholars from acquiring a clear understanding of how the emergence of anatomically modern humans actually unfolded, archaic Homo sapiens seem to have appeared first in north central and East Africa, but by 200,000 years ago, they had colonized much of Africa, and settled into the temperate latitudes of Eurasia. Always more populous in Africa than elsewhere, bands of archaic humans migrating out of that continent apparently displaced Homo erectus in one location after another, and subsequently diversified into many distinctive regional types without ever branching into more than a single species. From a cultural perspective, archaic Homo sapiens made few innovations beyond the sphere of stone toolmaking. About 200,000 years ago, however, an indisputable advance in stoneworking appeared in tropical Africa, and subsequently spread to Europe, most likely through cultural contacts. This more sophisticated technique, known as Levallois, relied on stone (typically flint) cores preshaped into set patterns, which allowed craftsmen to determine with reasonable consistency the dimensions of their flakes. The Levallois tool kit featured specialized scrapers and rough-hewn projectile points attached to wooden shafts among its more than fifty kinds of purposely designed implements.

The first signs of a cultural takeoff appeared with Neanderthals, or Homo neanderthalensis, which evolved in Europe and western Asia sometime after 250,000 years ago. Whether they constituted a distinct species or represented a subspecies of Homo sapiens continues to be another source of disagreement among the experts. In any case, learning to adapt to intensely cold climates, they extended the range of hominid settlement far beyond frost-free ecological zones, penetrating the great Siberian forests and unforgiving Arctic tundra. Neanderthals showed themselves to be outstanding toolmakers; they reinforced the trend toward smaller, lighter stone artifacts by improving the prepared-core techniques of the Levallois tradition. Never exploiting the possibilities of bone, antler, or ivory, they did utilize wood in assembling what could have been the first composite tools. Striving to increase the number of specialized devices, they developed the disc-core method to yield thin flakes easily trimmed at their sharp edges. With this approach, Neanderthals maintained a tool kit of more than sixty separate types of finely crafted implements, including haftable spearheads as well as delicate little saws and borers.

By 120,000 years ago, techniques characteristic of what paleoanthropologists call the Mousterian industry had spread to Africa, probably from southern France. Exploiting nature like no hominids before them, Neanderthals pushed into habitats as diverse as the Russian steppes and the immense mountain ranges of Asia. Their propensity for wearing animal furs for clothing, erecting skin-covered tents and hut-like structures made from bones and tree branches, constructing snares with sinews, and maintaining hearths within which to start encampment fires enabled them to gather in settlements as far north as the Arctic Ocean. Armed with heavy spears and a rudimentary spoken language more complex than anything Homo erectus had produced, Neanderthals became magnificent big-game hunters. Still lacking the cognitive capacity essential for modern speech, they nonetheless began to bury their dead in simple but clearly self-conscious ceremonies no later than 75,000 years ago. Experts have presumed that their ritualized treatment of death reflected at least an elementary awareness of mortality, a need to think about dying, and possibly even belief in an afterlife.

Somewhat later, Neanderthals created the first known ceremonial pieces and decorative objects out of pebbles, and the teeth and bones of animals. Personal ornamentation, whether accomplished with jewelry or body paint, presumably allowed them to express a hint of individuality and offered a means of declaring whatever social status they enjoyed. They could also unequivocally assert their kinship ties, thereby regularizing interactions within and between bands. However, none of these accomplishments meant that Neanderthals could match the overall cultural levels attained by anatomically modern humans, and thus make the behavioral adjustments that could have delayed, if not prevented, their extinction.

Anatomically Modern Humans

Experts who have advocated a multiregional interpretation of human origins envision distinct populations of Homo sapiens undergoing similar evolutionary alterations in different areas of the Eastern Hemisphere more or less simultaneously. Yet the preponderance of current archaeological, genetic, and fossil evidence seems to indicate that beings with a physiological and behavioral potential virtually indistinguishable from humans currently inhabiting Earth first evolved from African ancestors beginning about 200,000 years ago. Between 100,000 and 30,000 years ago, Homo sapiens appeared throughout most of the Eastern Hemisphere, gradually replacing the various populations of their archaic forebears. No later than 30,000 years ago, they had become the sole surviving hominid type, and the anatomical attributes as well as the cultural capabilities that have subsequently characterized our kind were fully formed.

The hallmark features of Homo sapiens were located from the neck up. They had delicately configured vertical faces with high foreheads, small brow ridges and teeth, reduced jaws, and prominent chins. Their modified vocal tracts made room for an enlarged pharynx and for the rearrangement of vocal cords, larynx, and tongue that together permitted them to make more varied sounds—in particular vowel sounds. The ability to speak fluently was interrelated with a further enlargement and internal reorganization of the human brain, now encased in a higher, rounder skull. Amazingly, the diverse facets of our mental potential are products of the brain’s structural complexity. While nearly four-fifths of the cortex remains uncommitted to particular operations, leaving substantial areas free to apply previous experience to immediate circumstances or forge links between seemingly unrelated ideas, certain parts of the brain perform highly specialized functions, the left hemisphere being organized for such tasks as sequential thinking and language, the right hemisphere for abstract and intuitive reflection.

Homo sapiens possess a unique ability to create symbols, and through symboling to infuse objects and ideas with discretionary meanings that transcend whatever may have been derived from sense experience. Symboling attained its ultimate expression in language, perhaps the most revealing outgrowth of our intelligence. Along with language came the earliest manifestations of culture based upon the symbolic expression of shared understandings. The capacity to create meaning and value and to infuse meanings and values into objects and events made it possible for human culture to reach full expression as a marvelously flexible, quickly modified, and endlessly cumulative adaptive mechanism that has almost replaced biological evolution as the primary determinant of our long-term survival.

Upper Paleolithic Takeoff

Beginning about 40,000 years ago, anatomically modern humans were already approaching the threshold of a cultural breakthrough that would mark a profound watershed in the human experience. Armed with spoken languages that improved the transmission of information, and probably driven for the first time by population pressures, they added to their cumulative heritage at an accelerated rate. Our Upper Paleolithic ancestors thus stood on the brink of making their culture—learned systems of behavior based on their symboling capacities—a complete way of life. Their intensified creativity peaked at the height of the last ice age 18,000 years ago, and lost momentum roughly 11,000 years ago. In crossing this Upper Paleolithic watershed, humans fundamentally transformed their social structures, making their gathering and hunting adaptations far more elaborate, diverse, and specialized than ever before. No later than 15,000 years ago, clusters of bands in regions with unusual population densities were organizing themselves into larger tribal units that could number up to eight hundred people.

As networks of interaction became more extensive, the growing need for a formal expression of beliefs that could help them preserve values, heighten social coherence, and overcome tension and uncertainty in their lives surely contributed to the relatively swift development of human aesthetic sensibilities. Artistic creativity rapidly assumed an incredible variety of forms. Rock engravings date back more than 35,000 years, while large quantities of sophisticated ornaments made of carved wood, bone, antler, ivory, shells, and stone date back at least that far. These ornaments include personal objects such as necklaces, pendants, and bracelets as well as tools and weapons used for ceremonial or decorative purposes. Archaeologists have unearthed artifacts more than 30,000 years old with patterned markings that may be lunar calendars and systems of mathematical notation. They have discovered remarkably beautiful cave paintings in France and Spain, sculpted figurines nearly as ancient, and the remains of musical instruments as varied as bone flutes, whistles, rattles, and drums. In addition, Upper Paleolithic peoples apparently expressed themselves through the rhythmic flow of poetry, dance, and song, although experts have little empirical evidence verifying such activities.

These arts derived their social potency from supernatural beliefs nearly as old as Homo sapiens themselves. Yet another manifestation of abstract thinking, human spirituality and religious responses to the world soon permeated every facet of daily existence. Homo sapiens discovered a means of comprehending perceived phenomena in terms of ubiquitous spirits, all instrumental in determining what happened in the world through activities almost everyone could understand. Through a holistic formulation of mythic concepts that embodied the actions of these spirits, they elaborated on what little they knew about the world from direct experience with visions that inserted humankind into a single, all-encompassing order of existence.

Like the spectacular burst of innovations in art and religion, radical advances in Upper Paleolithic toolmaking beginning roughly 35,000 years ago reflected the newly enhanced symbolizing capacity of modern humans. All of a sudden craftsmen were devising a wide range of more specialized, durable, and efficient implements. Along with at least 130 identifiable devices, the tailored clothing, control of fire, and insulated shelters that anatomically modern humans developed helped them expand throughout Earth’s habitable regions during the Upper Paleolithic.

Peopling of the Planet

Waves of migrations out of Africa, where populations were heaviest, began modestly around 100,000 years ago, exploded 40,000 years later, and by the start of the Holocene epoch 10,000 years ago gave to humankind a virtually global range. Pushed forward by frequent band splitting, Homo sapiens soon traversed the open grasslands of a Sahara region somewhat cooler and wetter than it is today and moved into the Near Eastern corridor. A progressively arid climate then drove excess populations into the steppe lands and tundra of southern Europe and western Asia. Anatomically modern humans appeared in East Asia 70,000 years ago at the very latest, reaching the heart of what is today China perhaps 30,000 years ago, not too long after Cro-Magnons (a particular population of anatomically modern humans) had established themselves in Europe and at approximately the same time that pioneers were starting to adapt to Arctic latitudes in earnest.

By moving along the southern shores of Asia, humans reached Australia some 40,000 years ago and perhaps as much as 20,000 years earlier. Around 45,000 years ago, the sea level in Australia was probably about 200 meters lower than it is now, meaning that mainland Australia and adjacent islands would have been connected. It is reasonably widely accepted that at this time people had already arrived in the region. There is also some evidence, such as at Lake Mungo in New South Wales, that suggests Australia was home to small populations as early as around 62,000 years ago. At the least there is some mitochondrial DNA evidence that lends support to the disputed theory that the peopling of Australia took place earlier than 45,000 years ago as a single founder population that was later cutoff or isolated from much of the rest of the world.

Although experts hold various theories about the peopling of the Americas, they agree that it happened at a relatively late date, and most likely involved a land bridge across the Bering Strait, known as Beringia, which appeared and vanished as ocean levels rose and fell. Although the earliest evidence for possible human settlement in the Americas has been problematic at best, isolated bands may have reached central Alaska as many as 40,000 years ago. Archaeologists have recently verified a site of human habitation at Monte Verde, located roughly 800 kilometers south of Santiago, Chile, that was certainly occupied 12,500 years ago and may date back more than 30,000 years. What experts have concluded with confidence is that between 14,000 and 12,000 years ago, scattered Paleo-Indian settlements with tool kits, languages, and cultural traditions resembling those common on the other side of the strait expanded throughout eastern Beringia and south of the retreating glaciers onto the Great Plains. Shortly after 12,000 years ago, the Paleo-Indian population suddenly exploded, pushing human beings to continuously occupy much of the Americas within several millennia.

Here it is worth noting a theory associated with the volcanic eruption at Lake Toba, on Sumatra (Indonesia) around 70,000 to 75,000 years ago. Probably one of the largest volcanic eruptions of the last twenty-five million years, it is thought to have plunged the Earth, already in the midst of an ice age, into an even darker and colder phase. Based on mitochondrial DNA evidence, it is thought that this cold snap might have reduced the world’s human population to less than 10,000 breeding pairs, effectively creating a sort of “bottleneck” in human evolution. Nevertheless, by the end of the most recent ice age, Homo sapiens had established themselves as the most numerous and widely scattered mammals on the planet, a dominant species culturally equipped to manipulate the natural environment in a manner potentially beneficial to their well-being, but simultaneously disruptive of the ecosphere’s sensitive balances. Although world population had probably reached 10 million in the aftermath of the Upper Paleolithic watershed, the gathering and hunting adaptation continued to prevail everywhere Homo sapiens resided. The attributes that now distinguish us as human beings had emerged at different stages in a prolonged and complicated evolutionary process as yet not fully explained, despite many recent scientific advances. Although the relatively hospitable savannas of sub-Saharan Africa invariably provided the setting for the emergence of our species, we cannot readily designate a precise period in world history when our behavior became truly human, unless it would be the tremendous evolutionary leap that occurred during the Upper Paleolithic.

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