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Human evolution is a field of science that falls within the larger area of physical anthropology. Human evolutionary studies are broadly synonymous with paleoanthropology, although paleoanthropology is a slightly wider concept that covers the host of fields contributing to the understanding of the human biological past in all its varied aspects. The central concern of human evolution involves sorting anatomical and behavioral differences within and among hominid species in order to delineate their ranges of variation through geological time and across geographical space. Hominid is often used as a colloquial term to indicate membership of fossil forms in the family Hominidae, the taxonomic group that includes anatomically and behaviorally modern humans and their precursors of the last six million years. The term human is a more subjective notion, whose limits can be debated. Some writers use it to include all members of the hominid family, while others restrict it to the genus Homo or to the species Homo sapiens.

In pre-evolutionary times, the Swedish naturalist Carolus Linnaeus (1707–1778), in his first edition of the Systema Naturae (1735), classified all organic organisms into a natural order using a hierarchical system with binominal nomenclature. He included humans (under the genus Homo and the species sapiens, derived from the Latin words for "man the wise"), along with lemurs, monkeys, and apes, in the order Primates. Intriguingly, in place of supplying physical characteristics to define this new species, Linnaeus avoided controversy by simply writing nosce te ipsum ("know thyself"). More than two and a half centuries later, physical anthropologists are still unable to agree on what constitutes modern humanity.

In terms of the morphological definition of modern humans, only a small number of unique anatomical characteristics stand out: (1) Homo sapiens is the only surviving member of the family Hominidae, a group anatomically committed to terrestrial bipedalism; (2) Members of this species have (not uniquely) relatively large brains—averaging 1,350 milliliters—with the most complex neocortex of all primates; (3) their chin-bearing faces are small compared to their neurocrania; and (4) they have a brow region structured into two parts. Behaviorally, modern humans are identified by the unique presence of: (1) a spoken language; (2) the cognitive faculties to generate mental symbols, as expressed in art; (3) the ability to think, reason, and plan; and (4) a bizarre inability to sustain prolonged bouts of boredom. Are anatomically modern humans and behaviorally modern humans the same thing? Not entirely. Anatomically and behaviorally modern humans appear in the archaeological and fossil records at different times.

Approximately one hundred thousand years ago, or perhaps somewhat earlier, anatomically modern humans appear in the fossil records of the Middle East and Africa; they are similar both cranially and postcranially to modern humans today, yet these earliest forms left no archaeological evidence to lead us to believe they had incorporated a modern behavioral repertoire. At seventy to fifty thousand years ago, we detect no change in the morphology of early anatomically modern humans, but there is dramatic evidence of a change in behavior. Splendid murals painted on the walls and ceilings of caves, musical instruments, and elaborate notations, together with a complex technology of stone and bone, are known from western Europe beginning about thirty thousand years ago. But these dramatic expressions were rather late, compared to the suggestions of similar symbolic behaviors known from as long ago as seventy thousand years, and maybe even more, in Africa. Similarly, modern humans had arrived in Australia by sixty thousand years ago, and an effectively modern level of cognition must have been present in these people to have allowed them to cross at least fifty miles of open ocean to get there. Obviously, a cognitive gulf was breached at some time after about seventy thousand years ago (perhaps earlier). This arose first of all in Africa, and spread thence to other parts of the world. Once Homo sapiens was in this behavioral mode, the speed of technological and other behavioral innovation (formerly episodic and rare) increased out of all proportion to what had gone before. At what point religious awareness was acquired is not known, but it was probably part of an overall biological potential for modern cognition that was achieved as a single "package." The huge range of behaviors made possible by this potential was only gradually discovered—and indeed, Homo sapiens is still enlarging its behavioral range today.

The human species and religious doctrine

By nature humans are inquisitive beings with an unquenchable thirst to understand and explain the meaning of life, especially their own. Since the days of the ancient Greek philosopher Aristotle (384–322 B.C.E.), the organic world had been looked upon as stable and unchanging, ascending steadily from the simplest forms to the most complex. Under the doctrine of the "Great Chain of Being," humans were perceived as godly creations and were positioned just below the angels on the top branch of a nicely ordered tree of life. All flora and fauna were designed for the purposes in nature that they were perceived to fulfill. The humanistic ideas of the Renaissance period centered all philosophy on human values and exalted human autonomy and superiority to the rest of nature. By the late seventeenth century, René Descartes's (1596–1650) philosophical idea that animals were complex machines with no higher sense of purpose had been expanded by French and German philosophers to create new foundations for a human social order. Morality was no longer considered to descend from an absolute truth enshrined in Christian beliefs, nor was the notion of accountability in the afterlife. The study of human nature became the key to understanding moral order in decent, complex societies. At a later period some struggled to integrate humans and nature with materialistic philosophy, but this view lost support during the turmoil of the French Revolution.

From Cuvier to Darwin

It would not be until the eighteenth century that the study of human origins became an approachable, but still controversial, topic within the budding science of natural history. Doubts raised by some natural historians questioned the interpretations of biblical literalists as to how humans came to exist on Earth, especially as increasing fossil discoveries in recognizably ancient sediments came to reveal that Earth's fauna did indeed appear to have a biological past. It was evident to naturalists that the Earth bore scars of an ancient history that contained puzzling geological phenomena, such as fossil fish on the tops of mountains, that were inexplicable within the boundaries even of the rudimentary scientific understanding that then existed.

It was impossible, then, to avoid the question as to where humans fitted into the picture. In 1830, the French naturalist Georges Cuvier's (1769–1832) treatise on fossil fauna and flora that was discovered in ancient geologic strata reported no evidence of human fossils coeval with these ancient genera. Since the geologic deposits involved varied greatly from one layer to the next, with bony evidence of once living creatures present in places where they had either gone extinct or now existed only on other continents, Cuvier reasoned that divinely instigated catastrophes and re-creations were responsible for the many extinction and replacement events he perceived. He argued that human fossils could be found if one were to look under the deepest of oceans, as suggested by the Old Testament's story of the great flood. Other naturalists, like Etienne Geoffroy Saint-Hilaire (1772–1844) and Jean Baptiste de Lamarck (1744–1829), provided strict evolutionary reasons for the drastic changes observed in the fossil record. Lamarck, for example, postulated that anatomical and behavioral changes acquired in a creature's lifetime might be passed on to its descendants. However, the Lamarckian paradigm of evolution would shift when two important events took place: (1) the 1858 announcement of Charles Darwin's (1809–1882) and Alfred Russel Wallace's (1823–1913) mechanism of natural selection to explain how species gradually change over time; and (2) the 1856 discovery (and the 1864 naming) of an extinct human species.

Charles Darwin, who rejected the basic tenets of the inheritance of acquired characteristics, enormously popularized a different evolutionary explanation for life on Earth with his the On the Origin of Species, published in 1859. Darwin proposed that biological organisms gradually evolve over time by adapting to their environments. Those individuals who are optimally suited to their environments end up producing more descendants than those who are not. If the features that make them better "adapted" are passed along by biological inheritance to their offspring, those features will become more common in the population, whose aspect will thus change over time. Keenly aware of the controversy it would generate, the retiring Darwin minimized any reference to humans in his publication, and did not broach the problem of human origins until many years later. Darwin's theory of "descent with modification" generated a great deal of controversy within religious and scientific communities. The highly public and politico-religious uproar that resulted centered on the distasteful suggestion that humans and apes share a common ancestor, especially in view of the long held belief that other animals are unable to think and are effectively nothing more than soulless automatons. Coming to Darwin's defense, Thomas Henry Huxley (1825–1895) fervently defended the tenets of Darwinian evolution, most publicly in his debate with Bishop Samuel Wilber-force (1805–1873) in 1860. In his influential 1863 publication of a series of public lectures titled Evidence as to Man's Place in Nature, Huxley argued that humans should be seen as biological organisms, and subject to the same natural laws that all other organic entities obey.

Interpreting the hominid fossils

The second epochal event for human evolutionary studies was the 1856 discovery of a fossil human at the Feldhofer Grotto in the Neander Valley, Germany. Most authorities of the day dismissed this find as the remains of a "barbarous" type of Homo sapiens. However, in 1864 the anatomist William King named the new form Homo neanderthalensis, thereby implying that there had been at least one ancient human extinction and speciation event. With further discoveries of the remains of extinct fossil humans, evolutionary concepts were more palatably applied to modern humans. The British geologist Charles Lyell (1797–1875), once a firm believer in God's role, abandoned many of his theological notions and accepted Darwin's theory of descent with modification after examining the remains of the Feldhofer Neanderthal.

At the turn of the twentieth century, the rediscovery of Mendelian genetics provided a basis for Darwin's evolutionary mechanism. Nonetheless, some paleontologists continued the attempt to integrate Christian beliefs with the idea of evolution. One such was the French Jesuit Pierre Teilhard de Chardin (1881–1955). While in Jesuit training in England, Teilhard also trained in paleontology and archaeology, and became embroiled in the Piltdown controversy that was just erupting. In 1912, he was invited to the Piltdown site in Sussex, which had yielded fossil bones including those of a human, and flint tools. Upon arrival he found a tooth. Reconstruction of the fragmentary hominid pieces seemingly offered the perfect transitional candidate from apes to humans—perhaps too perfect.

In 1912, "Piltdown Man" was introduced to the world as Eoanthropus dawsoni. At that time, the large brain was considered to be the hallmark of humanity; and for forty years British anatomists would disregard many significant fossil human discoveries because of their prized and large-brained Piltdown fossil. Teilhard later continued his paleontological research at the "Peking Man" site of Zhoukoudian in China. The Chinese fossils helped Teilhard to reconcile his now expansive knowledge of the human fossil record with his Christian beliefs. In The Phenomenon of Man (1938–1940), Teilhard proposed a theory of human evolution in which humans were evolving towards a final spiritual unity, also known as Finalism. This notion elicited the disapproval of his Jesuit superiors.

Early in the 1950s, Piltdown was exposed as a hoax—the doctored remains of a human and orangutan—and Teilhard has even been fingered as the hoaxer, though he remains only one of the more unlikely suspects of many. By the late 1950s the human fossil record had greatly expanded, as had the plethora of names used to describe it. A tidying-up was in order, and this was gradually achieved under a gradualist and progressivist model of human evolution.

In the 1970s and 1980s, new systematic methods began to transform the understanding of the constantly expanding human fossil record. Further, molecular studies were providing new perspectives. In particular, the "molecular clock" shortened the ape-human divergence to as little as five to six million years ago (from maybe twelve to fourteen). From around 1970 researchers uncovered bipedal but otherwise rather apelike hominids from sites in eastern Africa. These joined the Australopithecus fossils already known from southern Africa in the 2.5 to 1.5 million years ago range, and dated mostly from about 3.5 to 2.0 million years ago. Interpreted using an underlying gradualist model, these archaically-proportioned fossil hominids mostly reflected the search for an "earliest ancestor."

The situation at the beginning of the twenty-first century

Over the following few decades, hundreds of fossil human discoveries offered fuel for systematic debates. The "single species hypothesis," which stated that the human ecological niche was so wide that only one species of hominid could have existed at any one time, was rapidly invalidated by new finds, but still lingers in models of human origins that find deep roots in time for contemporary geographical groups of humankind. Evolutionary theory, as well as the rather sparse fossil record, imply in contrast that the species Homo sapiens must have had a single origin at one time and in one place, probably Africa. All of the human diversity familiar today has apparently appeared within the past 150 thousand years or so.

Despite minor differences of opinion, it is clear that the diversifying pattern of human evolution is similar to that of other mammalian taxa. Hominid phylogeny is a story of evolutionary experimentation, with multiple speciations and extinctions. The hominid family comprises at least five genera and eighteen known species (see Fig. 1, p. 302), some of which shared territories in both time and space. At present, all geographical varieties of modern humans occupy the single surviving twig of what appears once to have been a densely branching bush.

See also ANTHROPOLOGY; EVOLUTION; EVOLUTION, BIOCULTURAL; EVOLUTION, BIOLOGICAL; EVOLUTION, THEOLOGY OF; PALEOANTHROPOLOGY; PALEONTOLOGY; SOCIOBIOLOGY; TEILHARD DE CHARDIN, PIERRE

KENNETH MOWBRAY IAN TATTERSALL