Homo sapiens and human diversification

All human beings on Earth are highly adaptive animals of the genus Homo, species Homo sapiens, and subspecies Homo sapiens sapiens (Latin for "wise, wise human"). In terms of physical structure and physiological function, Homo sapiens—modern humans—are classified taxonomically as members of the order Primates, which is part of the class Mammalia. Since humans and other members of Primates (monkeys and apes) are biologically related, scientists presume both groups to be the products of an evolutionary process similar to other divergent categories of animals. The evolution of Homo sapiens from previously existing species is also believed to account for diversifications within the modern human population, such as racial differentiation (though race, as popularly understood, is largely a social construct).

Modern humans and modern apes (the two most closely related of modern primate species) are believed to possess a common biological ancestry or line that diverged perhaps five or six million years ago. The scanty fossil record of this early period, in conjunction with modern genetic studies, means that the proposed evolutionary tree is continually debated and revised. For a time, the fossil primate called Ramapithecus (Rama's Ape) or Bramapithecus (Brahma's Ape) was thought to be the earliest known direct ancestor of modern humans, but this view was discredited, and the extinct ape species was classified in the genus Sivapithecus. Discovered in West Africa in 2001, Sahelanthropus tchadensis is the earliest known potential hominin, dating back six or seven million years. The term "hominin" is generally understood to refer to modern and extinct humans as well as all immediate ancestral species, while "hominid"—which used to have that meaning and is still used as such by some sources—is now considered a broader category referring to hominins as well as other Great Apes (chimpanzees, orangutans, gorillas, and extinct relatives) and their ancestors.

While there is much to discover about the Sahelanthropus tchadensis, scientists generally agree that the species likely walked upright and had ape and human-like features

The Ardipithecus ramidus is also widely acknowledged as an early hominin that lived in East Africa around 4.4 million years ago. While still quite ape-like and likely tree-dwelling, fossils suggest Ardipithecus had a relatively upright posture. This points to the gradual evolution of bipedalism. By 2009, scientists had collected enough fossils to construct a partial skeleton of this hominin, which they nicknamed Ardi. Since the 1990s, the genera Orrorin and Keyanthropus have also been identified as early hominins, expanding the view of evolution in the Hominini tribe as a diverse, continually branching "bush" rather than the traditional model of a "tree" with modern humans as the culmination.

Several million years ago, during the late Pliocene epoch, early forms of the hominin Australopithecus appeared in Africa. They share certain characteristics with both humans and apes. Their brains are larger than those of apes but smaller than those of humans. Several species of Australopithecus have been identified, including A. afarensis, a famous example of which is the fossil known as Lucy. Another genus, Paranthropus, has also been identified as a similarly ape-like bipedal hominin, including P. aethiopicus, P. robustus, and P. boisei.

The Emergence of the Genus Homo

Examples of the first undisputed members of the genus Homo—true humans (though not sapiens)—appear in the fossil record about 2.1–1.5 million years ago. Samples of Homo habilis ("handy human"), the oldest known species discovered in the Homo genus, have been found in East Africa, marking the first known hominin use of tools. However, its classification has often been challenged. More widespread was Homo erectus ("upright human"), of which fossils have been found in China, Africa, Asia, and Europe. This creature habitually walked upright, made shelters, and used sophisticated tools.

Homo erectus is also very important since it is believed to have been the first hominid to have used fire purposefully. It was suggested by John E. Pfeiffer, in a 1971 article entitled "When Homo erectus Tamed Fire, He Tamed Himself," that this first domestication of a natural force was a tremendous evolutionary step, changing the fundamental rhythms of life and human adaptability to environments. Most scholars accept the premise that Homo erectus was a hominid grade intermediate between the australopithecines and Homo sapiens.

Exactly when, where, and how advanced members of the species Homo erectus evolved into Homo sapiens are key questions in the study of human evolution, and they are questions that resist resolution. Homo heidelbergensis has been suggested as evolving from H. erectus in Africa, with some migrating out into Europe and the Middle East, where they may have diverged into Neanderthals (Homo neanderthalensis) and Denisovans. H. heidelbergensis populations remaining in Africa may then have given rise to Homo sapiens, who eventually spread out of Africa in one or more waves. Genetic evidence suggests they also interbred with Neanderthals and other archaic humans, further obscuring any concept of directly progressive evolution.

However, even this general theory is unclearly documented and often contested. It might be thought that the closer one comes, in terms of time, to modern humans, the easier it would be to find the answers. In actuality, such is not the case. The ancestral line or lines leading to modern humans have become hazy beginning approximately 500,000 years ago. Direct fossil evidence of the earliest members of the species Homo sapiens is scarce; moreover, finds of modern human fossils in the Middle East have intensified the debate about the immediate ancestry of Homo sapiens sapiens. Still, much evidence indicates that the middle to upper Pleistocene epoch (beginning about 350,000 years ago), known as the Paleolithic or old stone age in archaeological terms, witnessed the emergence of early Homo sapiens.

The Earliest Homo Sapiens

In 1965, hominid fossil remains were found at a site named Vértesszöllös, near Budapest. They consisted of some teeth and an occipital bone (a bone at the back of the skull). The site also yielded stone tools and signs of the use of fire. Several features of the find recall Homo erectus, but the estimated cranial capacity of 1,400 cubic centimeters is well into the normal range for Homo sapiens. The age of the site was established at 350,000 BCE. These remains have been attributed to a Homo erectus intermediate type on the grounds that the remains, and the site, show a mixture of elements reflective of the transitional hominid evolutionary process. Such an assessment placed the Vértesszöllös specimens at the root of the Homo sapiens evolutionary line, some 100,000 years earlier than other specimens.

A better-known example of early Homo sapiens comes from a gravel deposit at Swanscombe near London, England. In 1935, 1936, and 1955, three related skull pieces were unearthed that fit together perfectly to form the back of a cranial vault with an advanced (over Homo erectus) cranial capacity of about 1,300 cubic centimeters. This has been dated to around 275,000 to 250,000 BCE. A more complete skull of approximately the same age (dated to the Mindel-Riss interglacial period about 250,000 BCE) was found at Steinheim, in southern Germany, in 1933. Swanscombe’s and Steinheim’s advanced morphological characteristics, in combination with relatively primitive ones, such as low braincase heights, suggest that they are primitive members of the species sapiens and are representatives of a population intermediate between Homo erectus and Homo sapiens.

The finds at Swanscombe and Steinheim have been augmented by others from France and Italy, and especially from the Omo River region in southern Ethiopia. One Omo skull displays more mixed features (between erectus and sapiens), including flattened frontal and occipital areas, a thick but rounded vault, large mastoid processes (pointed bony processes, or projections, at the base of the skull behind the ears), and a high cranial capacity. Another skull is more fully sapiens or modern in appearance. Some paleoanthropologists assert that the Omo group of fossils also helps bridge the gap between advanced Homo erectus and Homo sapiens.

Neanderthals

The best-known examples of what have been considered early Homo sapiens come from a group of fossils known collectively as Neanderthals. Their name derives from the place where the first fossil type was discovered in 1865, the Neander Valley near Düsseldorf, Germany. Similar Neanderthal fossil types have been found at more than forty sites in Europe, North Africa, Asia, and the Middle East.

Neanderthal fossils tend to show an aggregate of distinctive characteristics that has led to their being regarded as a separate human species, Homo neanderthalensis. However, some scientists have considered them as a subspecies of humans, with the designation Homo sapiens neanderthalensis. The characteristic features of their morphology include large heads with prominent supraorbital tori (thick brow ridges), receding jaws, stout and often curved bones, and large joints.

Most important Neanderthal fossils disclose large brain capacities and, contrary to early belief, are found in sites revealing complex and sophisticated cultures. These two facts clearly separate Neanderthal humans from earlier species that exhibit some of the same morphological features. Neanderthals generally stood fully erect between 1.5 and 1.6 meters in height; they were not the stoop-shouldered brutes of early characterizations. They lived during the last glaciation (the Würm glacial stage) in Eurasia. The sites from which most examples of the Neanderthals have been recovered have commonly yielded tools of the Mousterian complex, a stone-tool industry named for the kind found at Le Moustier, France, and dating from about 90,000 to about 40,000 BCE.

In fact, according to some researchers, two groups of Neanderthal humans seem to have existed. The first is referred to as classic Neanderthals from such sites as Germany, France, Italy, Iraq (the Shanidar 1 fossil), and the former Soviet Union. The second group, known as either generalized or progressive Neanderthalers, lived contemporaneously with, as well as later than, classic Neanderthal humans. They display a combination of modern H. sapiens features and typical Neanderthal characteristics (especially the prominent supraorbital torus, the forehead ridge). Included in this category for the sake of simplification are those specimens termed Neanderthaloid. However, later research has suggested rethinking these categories, including the classification of Homo rhodesiensis as a separate species.

Cro-Magnons

Neanderthals were a successful group for many thousands of years, flourishing from about 127,000 BCE to 37,000 BCE, with a wide distribution geographically. Neanderthal traces suddenly and mysteriously disappear from the fossil record, however, and they seem to have been superseded in Europe around 37,000 BCE by Homo sapiens with different morphology. These people became known as Cro-Magnons, so named for the Cro-Magnon cave near Les Eyzies in southwestern France, where the first skeletons were found in 1868 and where more than one hundred skeletons have since been discovered. However, that term is largely avoided in the scientific community in favor of more specific taxonomic names, including European early modern humans (EEMH). Indeed, Cro-Magnon skeletal anatomy is virtually the same as that of modern European and North African populations. The skull is relatively elongated, with a large cranial capacity; the brow ridges are only slightly projecting. The average height of Cro-Magnons has been estimated between 1.75 and 1.8 meters.

Cro-Magnon humans left more evidence of a highly developed culture than any of their predecessors. They made weapons and tools of bone and stone, stitched hides for clothing, and lived in freestanding shelters as well as caves. Some Cro-Magnon people produced beautiful cave paintings (they have been found in southwestern France and northern Spain) and bone carvings, and they modeled in clay. Though Cro-Magnon samples are the best-known examples of early Homo sapiens sapiens, mounting fossil evidence from sites outside Europe as well as genetic research from the 1980s onward, suggests a much older date of origin for the emergence of modern man.

At Qafzeh, a cave near Nazareth, Israel, anatomically modern fossils classified as Homo sapiens sapiens were discovered in 1988 and reliably dated to 92,000 BCE. In addition, newer fossil finds of progressive Neanderthals from Kebara Cave in Israel, taken together with earlier Neanderthal finds from the caves of et-Tabun and es-Skhul, also in Israel, makes it certain that Neanderthals and modern humans coexisted for many thousands of years.

Anthropologists have puzzled over the disappearance of the Neanderthals and, more importantly, over where they fit in the human family tree. It appears unlikely that classic Neanderthal humans were in the direct ancestral line of modern Homo sapiens sapiens. Reasons for their sudden disappearance are believed to include a combination of factors: extinction because of disease, lack of adaptation to the warmer climate following glaciation, and annihilation by other human groups.

Many scholars have considered the classic Neanderthals to be a cold-adapted, specialized side branch from the modern human line that became extinct as the climate became warmer. The generalized or progressive Neanderthals are considered by some to have avoided this specialization, perhaps continuing to exist through adaptation and ultimately being absorbed by flourishing modern human populations during the late Pleistocene epoch.

The Emergence of Modern Humans

Although an exact time, place, and mode of the origin of the modern human species cannot be determined, early genetic studies pointed to a date before 100,000 BCE. Examination of mitochondrial DNA (mtDNA) from a sampling of present-day humans representing five broad geographic regions allowed researchers to propose a genetic family tree and calculate roughly (assuming a fairly constant mutation rate) a temporal origin for the modern human population. Further studies seemed to indicate that the modern human ancestral line emerged between 280,000 years and 140,000 BCE. Later research tended to push this date back, with fossil discoveries in Morocco announced in 2017 suggesting Homo sapiens appeared as much as 300,000 years ago. In general, however, genetic evidence, in concert with fossil finds, make it plausible that a common ancestral population for Homo sapiens sapiens appeared in sub-Saharan Africa or the Levant (in the eastern Mediterranean region). Regional differentiation occurred, followed by radiation outward to other areas. The range of genetic and anatomical variability exhibited by fossil remains of modern humans is no greater than that known for the extant populations of modern times.

During the late Pleistocene epoch (approximately 40,000 to 11,000 BCE), five different groups seem to have developed on the Eurasian and African landmasses. The last glaciation, approximately 30,000 to 10,000 BCE, absorbed enough water to lower the oceans ninety meters below present levels. Emerging land bridges allowed people to move from Asia into North America, Australia, and elsewhere. In time, the major groups became subdivided into smaller ones that diverged into the human phenotypes seen today.

This view of racial diversification emphasizes the effectiveness both of geographic barriers in reducing free gene flow among varied groups of Homo sapiens and of environmental pressure in selecting different adaptive responses from the gene pool. These are also key factors in the entire evolutionary process by which modern humans developed over epochs into their present taxonomic position in the animal kingdom.

The Study of Paleoanthropology and Physical Anthropology

The study of human evolution is primarily the concern of the physical anthropologist and the paleoanthropologist. Evolution may be defined as change in the genetic composition of a population through time. Because evolution is thought to operate according to several principles and factors, modern human evolutionary theory is studied in the light of ideas and practices taken from different disciplines, including archaeology, biochemistry, biology, cultural anthropology, ecology, genetics, paleontology, and physics.

Early investigations into human evolution sought to establish the sequence of the human ancestral line through chronological and morphological analyses of hominid fossil remains (bones and teeth), thus placing them in their proper phylogenetic context (their natural evolutionary ordering). This has been augmented and, in some cases, superseded by sophisticated techniques in fossil dating and new avenues of exploration into the evolutionary process, such as genetic research.

Determination of the accurate age of a fossil is most important since it sets the fossil in a correct stratigraphic context that allows comparison with remains from the same geologic layer or level a great distance away. Accurate dating also has helped determine the order of succession for fossils that could not be established on morphological grounds alone.

The most valuable absolute dating methods are the radioactive carbon technique, which can effectively date specimens between 60,000 BCE and the present; the potassium-argon technique, which most easily dates material older than 350,000 BCE; and the fission-track method, which helps bridge the gaps between other methods. These methods are based on the constant or absolute rates at which radioactive isotopes of carbon, potassium, and argon decay. When absolute dating is impossible, investigators have ascribed a relative age to fossil remains by noting the contents of the layer of rock or the deposit in which the remains were found. A layer containing remains of extinct animals is likely to be older than one containing remains of present forms.

In conjunction with dating, anatomical studies of fossil remains and comparisons with the morphological features of known hominid types, as well as comparisons to primate skeletal structures, have been primary approaches to the study of the evolutionary path of Homo sapiens. The species Homo sapiens (of which the modern human races compose a number of geographical varieties) may be defined in terms of the anatomical characteristics shared by its members. In general, these include a mean cranial capacity of about 1,400 cubic centimeters, an approximately vertical forehead, a rounded occipital (back) part of the skull, jaws, and teeth of reduced size, and limb bones adapted to fully erect posture and bipedalism. Scientists assume that any skeletal remains which conform to this pattern and cannot be classified in other groups of higher primates must belong to Homo sapiens.

It is striking that the anatomical differences observed between Homo erectus and Homo sapiens have been confined to the skull and teeth. The limb bones thus far discovered for both are similar (though H. erectus appears more robust). Cranial capacity and morphology continue to be the dominant determining boundary separating sapient and presapient human species.

The Contribution of Other Sciences and Social Sciences

Human adaptability studies, using techniques from physiology, demographics, and population genetics, investigate all the biological characteristics of a population that are caused by such environmental stresses as altitude, temperature, and nutrition. It is believed that these normal stresses acted as genetic selectors in prehistoric times and continue to do so. Such variants as skin color and body hair are observable products of these stresses. The investigation of climatic changes during prehistoric epochs, as revealed in the geologic record, is important for understanding those pressures affecting the evolutionary history of humans.

Genetic studies have become indispensable to the study of human evolution. Four forces have been identified as fundamental in the evolutionary process: mutation, natural selection, gene flow, and genetic drift. Since mtDNA is inherited through the female, it is possible to calculate how much time has elapsed since the mutations that gave rise to present variations originated in prehistoric populations.

Also important to the study of the evolution of Homo sapiens is the examination and classification of cultural remains preserved at hominid fossil sites. Not only can the relative date of a fossil be supported, but sometimes, it is also possible to reconstruct the environmental situation that may have influenced the evolutionary process operating in a population. Cultural response is an integral part of hominid adaptation and, in turn, influences natural selection. Technology changes the physical and economic environment, and economic changes alter the demographic situation. Humans continue to promote or influence their own evolution by willingly or unwillingly altering the environment to which they must adapt.

Modern technology has been very useful in the study of early humans and has allowed researchers to identify and date fossils more accurately. One of these technologies, ZooMS, identifies human bone using collagen peptide mass fingerprinting. To distinguish between Homo sapiens and other members of the Homo genre, the Micro-CT and synchrotron technologies are practical tools to examine the identifying dental microstructures of these hominins. For dating fossils, argon-argon dating and paleomagnetic dating analysis are proven to be relatively accurate.

The modern methods useful for investigating the evolutionary history of Homo sapiens are multidisciplinary. While each of them reveals an aspect of the emergence of modern humans and complements the other methods of study, emphasis is placed on careful fieldwork, accurate dating, and comparative morphological analyses of hominid fossil remains. Increasing in importance, however, is the accumulating wealth of genetic data on human population relationships.

Human Evolution in the Context of Animal Evolution

Increasing attention is being given to the biological and behavioral changes that led to the emergence of Homo sapiens sapiens—the last major event in human evolution. Mounting evidence continues to push backward in time to the point at which modern Homo sapiens made its appearance in the evolutionary scheme. A clearer understanding of the evolutionary history of modern Homo sapiens has not only helped to define the place of the modern human species more accurately in relation to the rest of the animal kingdom, but also helped to illuminate the pressures, adaptations, and changes that have made humans what they are.

Through the pressures and process of evolution (including adaptation and natural selection), Homo sapiens has become one of the most successful and adaptive animals that ever lived because it came to possess an elaborate culture (culture is based on learned behavior). The key is Homo sapiens' advanced mental capacity. Humans exhibit an exceptional ability to assign arbitrary descriptions to objects, concepts, and feelings and then communicate them unambiguously to others. In the late Middle Pleistocene, the hominid branch that gave rise to early Homo sapiens witnessed an increase in brain size, complex social organizations, continual use of fire, and perhaps even language. As to what initiated these changes, many have suggested tool use and, in turn, a hunting economy.

In a classic article published in 1960, entitled "Tools and Human Evolution," Sherwood Washburn argued that the anatomical structure of modern humans is the result of the change, in terms of natural selection, that came with the tool-using way of life. He stated that tools, hunting, fire, and an increasing brain evolved together. Washburn also argued that effective tool use led to effective bipedalism—another significant characteristic of Homo sapiens: humans are different from all other animals because of their use of increasingly complex tools. While these assertions have been challenged, the general connection between human evolution, intelligence, and success is strong.

The other behavioral pattern that is seen to have been of utmost importance to sapient evolution is big-game hunting. Early Homo sapiens was undoubtedly a big-game hunter, as were all successors until approximately 8,000 years ago. It has been argued that human intellect, interests, emotions, and basic social life are the evolutionary products of the success of hunting adaptations. Success in hunting adaptation dominated the course of human evolution for hundreds of thousands of years. The agricultural revolution and the industrial and scientific revolutions are only now releasing human beings from conditions characteristic of 99 percent of their evolutionary history. It has been argued that, although most humans no longer live as hunters, they are still physically hunter-gatherers. Some investigators in the field of stress biology—the study of how the human body reacts to stressful situations—suggest there may be some link between an emotional reaction, such as explosive aggression, and human evolutionary history. Again, these theories have faced important challenges, but indicate the consensus that evolutionary history plays an ongoing important role. It is clear that tools and more efficient hunting helped produce great change in hominid evolution and made humans what they are. Humans continue to be users of increasingly complex tools, such as computers, and perhaps this continued development of technology may determine the future evolutionary path of Homo sapiens.

Principal Terms

Gene Pool: The total collection of genes available to a species

Generalized: Not specifically adapted to any given environment; used to describe one group of Neanderthal humans

Hominid: Any living or fossil member of the taxonomic family Hominidae ("of man") possessing a human form

Hominoid: Referring to members of the family Hominidae and Pongidae (apes) and to the taxonomic superfamily of Hominoidae

Morphology: The scientific study of body shape, form, and composition

Natural Selection: Any environmental force that promotes reproduction of particular members of the population that carry certain genes at the expense of other members

Pleistocene Epoch: The sixth of the geologic epochs of the Cenozoic era; it began about three million years ago and ended about ten thousand years ago

Würm Glaciation: The fourth and last European glacial period, extending from about seventy-five thousand years ago to twenty-five thousand years ago

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