The Evolution of Humans

Human evolution is an ongoing and complex process that began seven million years ago.

Human evolution began with primates. Primate development diverged from other mammals about 85 million years ago. Various divergences among apes, gibbons, orangutans occurred during this period, with Homini (including early humans and chimpanzees) separating from Gorillini (gorillas) about 8 millions years ago. Humans and chimps then separated about 7.5 million years ago.

Skeletal structure of humans and other primates.: A comparison of the skeletal structures of gibbons, humans, chimpanzees, gorillas and orangutans.

Generally, it is believed that hominids first evolved in Africa and then migrated to other areas. There were four main stages of human evolution. The first, between four and seven million years ago, consisted of the proto hominins Sahelanthropus, Orrorin and Ardipithecus. These humans may have been bipedal, meaning they walked upright on two legs. The second stage, around four million years ago, was marked by the appearance of Australopithecus, and the third, around 2.7 million years ago, featured Paranthropus. 

The fourth stage features the genus Homo, which existed between 1.8 and 2.5 million years ago. Homo habilis, which used stone tools and had a brain about the size of a chimpanzee, was an early hominin in this period. Coordinating fine hand movements needed for tool use may have led to increasing brain capacity. This was followed by Homo erectus and Homo ergaster, who had double the brain size and may have been the first to control fire and use more complex tools. Homo heidelbergensis appeared about 800,000 years ago, and modern humans, Homo sapiens, about 200,000 years ago. Humans acquired symbolic culture and language about 50,000 years ago.

Comparison of skull features among early humans.: A comparison of Homo habilis, Homo erectus, Homo floresiensis and Homo naledi skull features.

Neanderthals

A separate species, Homo neanderthalensis, had a common ancestor with humans about 660,000 years ago, and engaged in interbreeding with Homo sapiens about 45,000 to 80,000 years ago. Although their brains were larger, Neanderthals had fewer social and technological innovations than humans, and they eventually died out.

Theories of Early Human Evolution

The savannah hypothesis states that hominins were forced out of the trees they lived in and onto the expanding savannah; as they did so, they began walking upright on two feet. This idea was expanded in the aridity hypothesis, which posited that the savannah was expanding due to increasingly arid conditions resulting in hominin adaptation. Thus, during periods of intense aridification, hominins also were pushed to evolve and adapt.

The turnover pulse hypothesis states that extinctions due to environmental conditions hurt specialist species more than generalist ones. While generalist species spread out when environmental conditions change, specialist species become more specialized and have a greater rate of evolution. The Red Queen hypothesis states that species must constantly evolve in order to compete with co-evolving animals around them. The social brain hypothesis states that improving cognitive capabilities would allow hominins to influence local groups and control resources. The Toba catastrophe theory states that there was a near-extinction event for early humans about 70,000 years ago.

Human Adaptations

Bipedalism, or walking upright, is one of the main human evolutionary adaptations. Advantages to be found in bipedalism include the freedom of the hands for labor and less physically taxing movement. Walking upright better allows for long distance travel and hunting, for a wider field of vision, a reduction of the amount of skin exposed to the sun, and overall thrives in a savannah environment. Bipedalism resulted in skeletal changes to the legs, knee and ankle joints, spinal vertebrae, toes, and arms. Most significantly, the pelvis became shorter and rounded, with a smaller birth canal, making birth more difficult for humans than other primates. In turn, this resulted in shorter gestation (as babies need to be born before their heads become too large), and more helpless infants who are not fully developed before birth.

Larger brain size, also called encephalization, began in early humans with Homo habilis and continued through the Neanderthal line (capacity of 1,200 – 1,900 cm3). The ability of the human brain to continue to grow after birth meant that social learning and language were possible. It is possible that a focus on eating meat, and cooking, allowed for brain growth. Modern humans have a brain volume of 1250 cm3.

Humans have reduced sexual dimorphism, or differences between males and females, and hidden estrus, which means the female is fertile year-round and shows no special sign of fertility. Human sexes still have some differences between them, with males being slightly larger and having more body hair and less body fat. These changes may be related to pair bonding for long-term raising of offspring.

Other adaptations include lessening of body hair, a chin, a descended larynx, and an emphasis on vision instead of smell.

The Neolithic Revolution

The Neolithic Revolution and invention of agriculture allowed humans to settle in groups, specialize, and develop civilizations.

Before the Rise of Civilization: The Paleolithic Era

The first humans evolved in Africa during the Paleolithic Era, or Stone Age, which spans the period of history from 2.5 million to about 10,000 BCE. During this time, humans lived in small groups as hunter-gatherers, with clear gender divisions for labor. The men hunted animals while the women gathered food, such as fruit, nuts and berries, from the local area. Simple tools made of stone, wood, and bone (such as hand axes, flints and spearheads) were used throughout the period. Fire was controlled, which created heat and light, and allowed for cooking.

Humankind gradually evolved from early members of the genus Homo—
such as Homo habilis,
who used simple stone tools— into fully behaviorally and anatomically modern humans (Homo sapiens) during the Paleolithic era. During the end of the Paleolithic, specifically the Middle and or Upper Paleolithic, humans began to produce the earliest works of art and engage in religious and spiritual behavior, such as burial and ritual. Paleolithic humans were nomads, who often moved their settlements as food became scarce. This eventually resulted in humans spreading out from Africa (beginning roughly 60,000 years ago) and into Eurasia, Southeast Asia, and Australia. By about 40,000 years ago, they had entered Europe, and by about 15,000 years ago, they had reached North America followed by South America.

Stone ball from a set of Paleolithic bolas: Paleoliths (artifacts from the Paleolithic), such as this stone ball, demonstrate some of the stone technologies that the early humans used as tools and weapons.

During about 10,000 BCE, a major change occurred in the way humans lived; this would have a cascading effect on every part of human society and culture. That change was the Neolithic Revolution.

The Neolithic Revolution: From Hunter-Gatherer to Agriculturalist

The beginning of the Neolithic Revolution in different regions has been dated from perhaps 8,000 BCE in the Kuk Early Agricultural Site of Melanesia Kuk to 2,500 BCE in Subsaharan Africa, with some considering the developments of 9,000-7,000 BCE in the Fertile Crescent to be the most important. This transition everywhere is associated with the change from a largely nomadic hunter-gatherer way of life to a more settled, agrarian-based one, due to the inception of the domestication of various plant and animal species—depending on the species locally available, and probably also influenced by local culture.

It is not known why humans decided to begin cultivating plants and domesticating animals. While more labor-intensive, the people must have seen the relationship between cultivation of grains and an increase in population. The domestication of animals provided a new source of protein, through meat and milk, along with hides and wool, which allowed for the production of clothing and other objects.

There are several competing (but not mutually exclusive) theories about the factors that drove populations to take up agriculture. The most prominent of these are:

• The Oasis Theory, originally proposed by Raphael Pumpelly in 1908, and popularized by V. Gordon Childe in 1928, suggests as the climate got drier due to the Atlantic depressions shifting northward, communities contracted to oases where they were forced into close association with animals. These animals were then domesticated together with planting of seeds. However, this theory has little support amongst archaeologists today because subsequent climate data suggests that the region was getting wetter rather than drier.
• The Hilly Flanks hypothesis, proposed by Robert Braidwood in 1948, suggests that agriculture began in the hilly flanks of the Taurus and Zagros mountains, where the climate was not drier, as Childe had believed, and that fertile land supported a variety of plants and animals amenable to domestication.
• The Feasting model by Brian Hayden suggests that agriculture was driven by ostentatious displays of power, such as giving feasts, to exert dominance. This system required assembling large quantities of food, a demand which drove agricultural technology.
• The Demographic theories proposed by Carl Sauer and adapted by Lewis Binford and Kent Flannery posit that an increasingly sedentary population outgrew the resources in the local environment and required more food than could be gathered. Various social and economic factors helped drive the need for food.
• The Evolutionary/Intentionality theory, developed by David Rindos and others, views agriculture as an evolutionary adaptation of plants and humans. Starting with domestication by protection of wild plants, it led to specialization of location and then full-fledged domestication.

Effects of the Neolithic Revolution on Society

The traditional view is that the shift to agricultural food production supported a denser population, which in turn supported larger sedentary communities, the accumulation of goods and tools, and specialization in diverse forms of new labor. Overall a population could increase its size more rapidly when resources were more available. The resulting larger societies led to the development of different means of decision making and governmental organization. Food surpluses made possible the development of a social elite freed from labor, who dominated their communities and monopolized decision-making. There were deep social divisions and inequality between the sexes, with women’s status declining as men took on greater roles as leaders and warriors. Social class was determined by occupation, with farmers and craftsmen at the lower end, and priests and warriors at the higher.

Effects of the Neolithic Revolution on Health

Neolithic populations generally had poorer nutrition, shorter life expectancies, and a more labor-intensive lifestyle than hunter-gatherers. Diseases jumped from animals to humans, and agriculturalists suffered from more anaemia, vitamin deficiencies, spinal deformations, and dental pathologies.

Overall Impact of the Neolithic Revolution on Modern Life

The way we live today is directly related to the advances made in the Neolithic Revolution. From the governments we live under, to the specialized work laborers do, to the trade of goods and food, humans were irrevocably changed by the switch to sedentary agriculture and domestication of animals. Human population swelled from five million to seven billion today.