Pliocene Epoch

5-2 million years ago


Several primitive hominid fossils are known from the period between 7 million years ago and about 4 million years ago.

The genus of the large ape Orrorin is known from the Late Miocene (6 million years ago) of Kenya. It is known from 19 specimens which include jaws, teeth, leg, arm, and finger bones. It femurs possessed a spherical head, long neck, and a groove for the obturator externus muscle which chimps lack. These adaptations indicate that Orrorin was a frequent if not obligate biped. It possessed pointed canines and its arm and hand indicate that it was adept at climbing as well. Its teeth were covered with thick enamel, like humans but unlike Ardipithecus and chimps. A ridge in its teeth is similar to that of fossil and modern apes but not australopithecines or humans. (Wong, 2003; Senut, 2001).


Ardipithecus ramidus
DATE: 4.4 million years ago
SITE: Middle Awash, Ethiopia; 2 five million year old fossils from Kenya may also belong to
SPECIMENS: original description based on 17 specimens (teeth and jaws, occipital fragments, humerus, radius, ulna) found in 1994
There are 6 characteristics of the teeth that are unlike any hominid, living or extinct. The canines were large and the post-canine teeth were relatively small. There was no honing facet of the canines as in apes. The premolars were not molarized while, in later hominids, the premolars are similar in structure and function to the molars. The dental enamel is a thin layer. The arrangement of the teeth (the dental arcade) is similar in shape similar to A. afarensis but the canines line up with postcanine teeth as in apes. The first deciduous molar (dm1) in the mandible of a young A. ramidus is more similar to that of a chimp’s than to that of any known hominid.
The size and structure of the canines, the enamel thickness, the P3 tooth, and the structures of the temporal and occipital bones are more primitive than A. afarensis. The skull foramina (holes) and the structure of the arm differ from those characteristics of living apes. Ardipithecus was not a knuckle-walker and its arm was intermediate between apes and A. afarensis. Although the precise size of the brain is unknown, it appears apelike (Andrews, 1995; Leakey, 1995; White, 1994; WoldeGabriel, 1994; Haile-Selassie, 2001).
Fossils found near Ardipithecus suggest Ardipithecus may have been a forest dweller. A later find of Ardipithecus ramidus kadabba toe bones possibly suggest a human-like gait (Wong, 2003). Some have suggested that Ardipithecus may be ancestral to chimps rather than humans, while others dispute this, claiming that Ardipithecus is a member of the hominid clade, although close to the divergence of human and chimp ancestors (Senut, 2001; Haile-Selassie, 2001).


Australopithecus anamensis
DATES: 3.9 million years ago (from a strata ideal for argon dating at Allia Bay; those of Kanapoi are not as datable and fall into the range of 3.9 to 4.2 million years ago); new finds date at 4.07-4.17 mya
SITES: 2 sites near Lake Turkana Bay, Kenya (Allia Bay and Kanapoi)
SPECIMENS: 12 specimens from Allia Bay, 9 from Kanapoi; teeth and jaws, tibia, humerus, temporal, cranial fragments; both juvenile and adult
Australopithecus anamensis had a primitive jaw with large canines like apes and Ardipithecus, a small external ear openings as in chimps, and an apelike skull. The structure of the knee indicates that it was bipedal (this is significant since it was older than the Laetoli footprints described shortly). There was less flexibility in ankle and big toe compared to chimps, perhaps indicating that it spent less time in trees. The humerus was similar to that of humans and the tibia resembles that of A. afarensis and Homo . Some new specimens seem to be intermediate between Ardipithecus and the first A. anamensis specimens found (Culotta, 1995b; Leakey, 1997; Leakey, 1998; Tattersall, 1997).