Pliocene Epoch

4-3 million years ago


Kenyanthropus and Australopithecus afarensis represent two putative ancestral hominids from the period of 4 to 3 million years ago.While their brains were small and the skulls were more apelike, they walked upright. Some features, such as the hips of Australopithecus afarensis, were essentially human in their structure.

Australopithecus afarensis
DATES: 3.9 to 3.0 million years ago
SITES: Hadar, Ethiopia; Middle Awash, Ethiopia; Omo, Kenya; Koobi Fora, Kenya; Laetoli, Tanzania; Chad (Morell, 1995; Johanson, 1982)
--The 1995 Chad discovery is interesting because it is 5400 km west of the Rift Valley sites (Brunet, 1995).
SPECIMENS: many individuals, cranial, dental, and post-cranial remains including Lucy and First Family
The cranial capacity was 375-500 cc which is larger than the chimp average. The brain weight/body size ratio is outside of the chimp range. A. afarensis had an apelike face with sloping, low forehead, a bony ridge over its eyes, a flat nose, and no chin. Most of its skull is apelike. The face did not protrude to the same extend that the face of a chimp does. The pelvis and leg of A. afarensis were very similar to those of humans. The legs were a little shorter than in humans. The pelvis allowed bipedal walking but there is no evidence of an enlarged birth canal that would have permitted enlarged fetal crania (Johanson, 1979; Susman, 1984; Hill, 1985; Leakey, 1972; Lovejoy, 1972; Lovejoy, 1993).

The hands and feet of A. afarensis were similar to those of humans although the long forearms, the curved finger and toe bones, and the angle of the shoulder socket may indicate it spent some time spent in trees. The narrow metacarpal heads (ends of the bones that make up the hand) may have prevented tool use (Skelton, 1986; Johanson, 1979; Gibbons, 1997b; Susman, 1994).
The height range was 3’3” to 5’7”. The females were smaller than males and Lucy (see next illustration) is the smallest specimen known. Australopithecus afarensis was bipedal given evidence from the pelvis, knee, and the position of the foramen magnum (where the spinal cord enters the skull). The images below indicate that australopithecines possess a more anterior foramen magnum, indicative of a bipedal stance. Footprint trackways known as the Laetoli footprints (2 sets: one of 5 prints, one of 12 prints) are consistent with the A. afarensis stature, date (3.6 to 3.8 mya), and surrounding fossils. The gait of the individuals that made these footprints is more shambling than in modern humans but it was certainly bipedal and unlike the quadrupedal baboon-like prints found nearby (White, 1980; Day, 1980; Leakey, 1979; Wolpoff).

The canines and jaw shape of A. afarensis were intermediate between apes and humans with large sexually dimorphic canines, a diastema (space) between teeth sometimes present, molarized premolars, large molars, and thick enamel on molars. The teeth represent a generalist’s denticia (as in humans) and lack the tooth specializations seen in some later australopithecines (Skelton, 1986; Johanson, 1979; Wolpoff, 1978). There is evidence of some carnivory by early australopithecines (de Heinzelin, 1999).
The habitat of A. afarensis seems to have been woodland and more open than that of A. ramidus. The appearance of this new habitat was caused by the changes in Miocene climate with its greater cooling, aridity, and increased seasonality (Johanson, 1982). The degree of sexual dimorphism in Australopithecus afarensis was similar to that observed in modern human populations.


Kenyanthropus platyops

DATE: 3.5 million years ago
SITE: Lake Turkana, Kenya
SPECIMENS: 1 complete cranium; isolated skull portions (temporal, maxillary) and teeth
Kenyanthropus was similar to Australopithecus afarensis in many respects including size of the braincase, the temporal fossa for lower jaw, other temporal bone characteristics, and the size of its canines. It lacked the venous sinus system observed in several Australopithecus species. It is more similar to Homo than A. afarensis with regards to its flatter face which doesn’t protrude as far and in some dental characteristics. The cranium is within the size range of gracile australopithecines. (Wong, 2001; Leakey, 2001) . Some have suggested classifying Kenyanthropus as the earliest member of Homo, ancestral to Homo habilis (Cela-Conde, 2003).