theory of the origins of the human species, Homo sapiens. Modern understanding of human origins is derived largely from the findings of paleontology, anthropology, and genetics, and involves the process of natural selection (see Darwinism). Although gaps in the fossil record due to differential preservation prevent the complete specification of the line of human descent, H. sapiens share clear anatomical, genetic, and historic relationships to other primates. Of all primates, humans bear particularly close affinity to other members of a group known as hominoids, or apes, which includes orangutans, gibbons, gorillas, chimpanzees, and humans. Humans and their immediate ancestors, known as hominids, are notable among hominoids for their bipedal locomotion, slow rate of maturation, large brain size, and, at least among the more recent hominids, the development of a relatively sophisticated capacity for language, tool use, and social activity.

The Evolutionary Tree

Humans are mammals of the Primate order. The earliest primates evolved about 65 million years ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrews. Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsiers, lemurs, and tree shrews, and are classified as lower primates or prosimians. During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental drift, diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys. The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 million years ago) or early Miocene (25 to 8 million years ago), a time period poorly represented in the fossil record. The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 million years ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 million years ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 million years ago and that chimpanzees and hominids diverged about 5 million years ago.

Hominid Evolution

The earliest known hominids are members of the genus Australopithecus, the earliest of which date to more than 4 million years ago. Unlike other primates, but like all hominids, australopithecines were bipedal. Their crania, however, were small and apelike, with an average cranial capacity of about 450 cc in the gracile species and 600 cc in the robust forms. Australopithecines that have been considered ancestral in the lineage leading to the human genus Homo include A. afarensis (an important skeleton of which is popularly known as Lucy) and A. africanus. The exact position of these and other early species on the hominid family tree continues to be disputed.

The first member of the genus Homo, a small gracile species known as H. habilis, was present in east Africa at least 2 million years ago. H. habilis was the first hominid to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominid evolutionary history. By about 1.6 million years ago, H. habilis had evolved into a larger, more robust, and larger-brained species known as Homo erectus. Cranial capacities ranged from about 900 cc in early specimens to 1050 cc in later ones. H. erectus persisted for well over a million years and migrated off the African continent into Asia, Indonesia, and Europe.

Between 500,000 and 250,000 years ago, H. erectus evolved into H. sapiens. Transitional forms between H. erectus and H. sapiens are referred to as archaic H. sapiens. With the exception of H. sapiens neandertalensis (see Neanderthal man), no additional subspecies are recognized. Indeed, some scientists consider Neanderthal a separate species. Archaic H. sapiens changed gradually, becoming somewhat larger, more gracile and larger-brained through time. Cranial capacity, for example, increased from about 1150 cc in early transitional forms to the current world average of just over 1350 cc. By 150,000 years ago in Africa and Asia and 28,000 years ago in Europe (see Cro-Magnon man), the transition to H. sapiens was complete, and fully modern humans became the single surviving hominid species.

The Evolution of Culture

Among hominids, a parallel evolutionary process involving increased intelligence and cultural complexity is apparent in the material record. Evidence of greater behavioral flexibility and adaptability presumably reflects the decreased influence of genetically encoded behaviors and the increased importance of learning and social interaction in transmitting and maintaining behavioral adaptations (see culture). Because the organization of neural circuitry is more significant than overall cranial capacity in establishing mental capabilities, direct inferences from the fossil record are likely to be misleading. Contemporary humans, for example, exhibit considerable variability in cranial capacity (1150 cc to 1600 cc), none of which is related to intelligence.

Tool use was once thought to be the hallmark of members of the genus Homo, beginning with H. habilis, but is now known to be common among chimpanzees. The earliest stone tools of the lower Paleolithic, known as Oldowan tools and dating to about 2 to 2.5 million years ago, were once thought to have been manufactured by H. habilis. Recent finds suggest that Oldowan tools may also have been made by robust australopithecines. The simultaneous emergence of H. erectus and the more complex Achuelian tool tradition may indicate shifting adaptations as much as increased intelligence.

While it is clear that H. erectus was much more versatile than any of its predecessors, adapting its technologies and behaviors to diverse environmental conditions, the extent and limitations of its intellectual endowment remain a subject of heated debate. This is also the case for both archaic H. sapiens and Neanderthals, the latter associated with the more sophisticated technologies of the middle Paleolithic. However impressive the achievements of H. erectus and early H. sapiens, most material remains predating 40,000 years ago reflect utilitarian concerns. Nonetheless, there is now scattered African archaeological evidence from before that time (in one case as early as 90,000 years ago) of the production by H. sapiens of beads and other decorative work, perhaps indicating a gradual development of the aesthetic concerns and other symbolic thinking characteristic of later human societies. Whether the emergence of modern H. sapiens corresponds to the explosion of technological innovations and artistic activities associated with Cro-Magnon culture or was a more prolonged process of development is a subject of archaeological debate.

Bibliography

See R. Lewin, Human Evolution (2d ed. 1989) and, with R. Leakey, Origins Reconsidered (1992); I. Tattersall, The Fossil Trail: How We Know What We Think We Know about Human Evolution (1995); A. Walker and P. Shipman, The Wisdom of the Bones: In Search of Human Origins (1996); C. Stringer and R. McKie, African Exodus: The Origins of Modern Humanity (1997); L. R. Berger and B. Hilton-Barber, In the Footsteps of Eve: The Mystery of Human Origins (2000); I. Tattersall and J. H. Schwartz, Extinct Humans (2000).

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