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CENOZOIC ERA

NEOGENE PERIOD

Pliocene Epoch

3-2 million years ago

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During the period of 3 to 1.6 million years ago, a variety of hominids existed including diverse australopithecines (including the robust australopithecines which were not directly ancestral to humans) and the first members of the genus Homo. Homo habilis is the most primitive species in the genus Homo.

Australopithecus garhi
DATE: 2.5, 2.6 million years ago
SPECIMENS: 3 specimens

Australopithecus garhi is very like A. afarensis with a small braincase of 450 cc. In some characteristics, it appears to be intermediate between A. afarensis and Homo habilis and it possessed humanlike leg proportions. It used tools, apparently to get the marrow out of bones. It is certainly not a robust australopithecine but its molars are large, like the robust australopithecines (the second molar is 1.8 cm which is actually larger than the average size for A. robustus). If this were a human ancestor, then this trend towards increased molar size would have had to have been subsequently reversed. Several individuals represented by a partial cranium, limb bones, and part of a foot. (Asfaw, 1999; Culotta, 1999).

Australopithecus africanus
DATES: 3 to 2.3 million years ago (Tobias, 1973)
SITES: Taung, Makapansgat, Sterkfontein, and Gladysvale, South Africa
SPECIMENS: many individuals
CHARACTERISTICS:
Skull:
The braincase of Australopithecus africanus was higher and rounder than in A. afarensis and less apelike. The cranial capacity is larger, 420-500 cc (Conroy, 1998; Conroy, 1998b). Teeth
There was a further reduction of canines and enlargement of the molars. The lower jaw projects less and approaches the shape of the jaw of modern humans. There was no diastema and the large molars and premolars were covered with a thick layer of enamel. The microscopic wear on the teeth suggests that there were tougher materials in the diet. Carbon isotopes in teeth suggest that their diet either included grasses or the animals which fed on grasses (Sponheimer, 1999; Vogel, 1999; Lewin, 1985; Leakey, 1974)
The pelvis and leg indicate bipedal gait but it may have been inefficient. The arms were longer than in A. afarensis, suggesting that A. africanus was adapted for both life in trees and on the ground (Shreeve, 1996). The South African habitat where the fossils were found seems to have included more woodland than in East Africa (Klein, 1977).
There is evidence of sexual dimorphism in size but it was probably not as great as in A. afarensis. Scanning electron microscopy has demonstrated that the patterns of bone remodeling are more like chimps than humans. Some researchers feel that A. afarensis and A. africanus are geographic variants of one species; others feel that A. africanus is not our direct ancestor but rather is related to the robust australopithecines (as the anterior pillars may indicate) (Skelton, 1986; Rak, 1985).

Homo habilis
DATES: 2.5 to 1.6 million years ago
SITES: Olduvai, Tanzania; Koobi Fora, Kenya; Omo, Kenya; Sterkfontein, South Africa; Uraha,; Longuppo, China? (Culotta, 1995; Larick, 1996; Wanpo, 1996)
CHARACTERISTICS:
Homo habilis differs from Australopithecus at the base of the skull. The foramen magnum (the opening for the spinal chord) is closer to the middle of the skull and the skull base is reduced in length but increased in width. The face decreased in width and the nasal opening was more sharply defined. The postcanine teeth were smaller than in Australopithecus.
Its cranial capacity was 500 to 800 cc. and values increase from the earliest specimens to latest ones. This range overlaps with Australopithecus at the low end and Homo erectus at the high end. It can be debated (indeed, there has been a debate several decades long) on whether early H. habilis should be classified as Australopithecus and late H. habilis should be called Homo erectus. H. habilis stood approximately 5 foot tall and weighed 100 pounds with females being smaller than males (Leakey, 1973b; Wood, 1987; Leakey, 1971b; Hughes, 1977; Johansen, 1987; Bilsburough, 1988; Tobias, 1972). Early Homo populations coexisted with australopithecines (Johanson, 1976).
In one brain endocast, there is a bulge corresponding to Broca’s area (an important speech center) in modern human brains. Two aspects of wrist bones (the scaphoid tubercle and the articular surface of the trapezium) were chimp-like. The thumb was similar to humans in the carpo-metacarpal joint and the flattened metacarpal surface. The foot was less flexible than in chimps and its degree of possible abduction was limited. Some characteristics of the lower leg were primitive and others were advanced, not found in any ape (Susman, 1982; Skelton, 1986).