An outline of human phylogeny

Bernard Campbell

There are no fossils now believed to lie within our hominid lineage before c.6.0 m.y.a. The earliest group of well-known undoubted hominid fossils comes from Laetoli in Tanzania, and dates from c.3.7 m.y.a. These belong to the genus Australopithecus, which is considered to range in time from c.5 m.y.a. to 1 m.y.a., and appears to have been confined to the continent of Africa. Australopithecus was a bipedal, small-brained hominid, which later diversified into 2-3 more robustly built species, as well as probably giving rise to members of our own genus, Homo.

With regard to brain reorganization, left-right cerebral hemispheric asymmetries exist in extant pongids and the australopithecines, but neither the pattern nor direction is as strongly developed as in modern or fossil Homo. KNM-ER 1470 shows a strong pattern that may be related to handedness and tool-use/manufacture. The degree of asymmetry appears to increase in later hominids.

The earliest fossil remains classified as Homo, and thought to be our direct ancestors, come from south-west Ethiopia and adjacent Kenya. They are dated to c.2 m.y.a. This species, named Homo habilis, possessed a somewhat larger brain than Australopithecus, and appears at approximately the same time as the earliest stone tools. The successor to Homo habilis was the much more modern-looking Homo erectus. The earlier specimens are from Kenya, and date to c.1.5-1.8 m.y.a. Only after c.1.0 m.y.a. do we find that this species of Homo has spread into Eurasia. Archaic forms of Homo sapiens are variously recognized from Afro-Eurasian specimens dated to c.300 000 years ago. Recent biochemical data suggest that modern humans, Homo sapiens sapiens, arose in Africa c.200 000 years ago. This fits in well with the available fossil evidence from Africa and the Near East, where human skeletal material with completely modern features is known from an earlier date than elsewhere [eds].

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For early dispersals, see The African Emergence and Early Asian Dispersals of the Genus Homo by Roy Larick and Russell L. Ciochon

Caption: Hominids now known as Homo erectus  were found on Java, Indonesia, in 1891, and at Zhoudoudian, near Beijing, in the 1930s. As Homo erectus  was clearly more primitive than hominid fossils known in Europe, human beings were initially thought to have emerged in East Asia and dispersed westward. Since the early 1960s, numerous fossils from African localities in the eastern Rift Valley, Lake Malawi and South Africa have demonstrated an African emergence for Homo . In the 1990s, advances in dating methods and new finds at Dmanisi (Georgia), Riwat (Pakistan), two Javanese sites and Longgupo (China) show that early Homo  had arrived in East Asia by 2 million years ago. Areas in pale blue indicate land masses submerged since those early dispersions.

Caption and image Copyright © American Scientist, magazine of Sigma Xi, The Scientific Research Society.