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The genus of the large ape Orrorin is known from the Late Miocene
Orrorin possessed a number of features which link it to australopithecines and Homo such as an elongated neck of the femur, a prominent gluteal tuberosity, and other features. The femur structure indicates that it was bipedal although its arms and fingers indicate that it was also adapted for climbing (Pickford, 2002). While the arms of Orrorin were more similar to those of apes, the femur and teeth were more similar to those of humans (Kingdon, 2003).
|Some have suggested classifying Orrorin and many of the gracile australopithecines in one single genus, Praeanthropus (Cela-Conde, 2003).|
DATE: 5-7 million years old
SPECIMENS: one cranium, probably male; jaw fragments
Sahelanthropus was discovered in
Its primitive features include a small, ape-sized braincase (320-380 cc), small incisors, a small sagittal crest, a massive brow ridge, shape of basioccipital bone, and the orientation of its petrous bone. Its advanced features include an enamel thickness intermediate between chimps and Australopithecus, a face which protrudes less than chimps or Australopithecus, small apically worn canines, its basicranium length and orientation, the position of the formamen magnum, and its large supraorbital torus . The basicranium is similar to that of Ardipithecus (Brunet, 2002, Wong, 2003). Although the occipital region of Sahelanthropus was damaged, the structure of the foramen magnum suggests that it was bipedal (Cela-Conde, 2003; Zollikofer, 2005).
Although Sahelanthropus does share some features with chimps, it is most similar to Australopithecus. Several features of its face and the base of the skull identify it as a hominid. It was probably bipedal given the more anterior position of its foramen magnum (Guy, 2005).
(Australopithecus ramidus was renamed Ardipithecus ramidus; White, 1994).
DATE: 4.4 million years ago
SITE: Middle Awash,
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).
Analysis of the lower jaw have led some to conclude that while Ardipithecus could represent the ancestral form for hominids, Australopithecus afarensis could be ancestral to the robust australopithecines (Rak, 2007).
3.9 million years ago (from a strata ideal for argon dating at
SITES: 2 sites near
12 specimens from
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).
DATES: to 3.0 million years ago
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 (see illustration below).
| One of the first changes to lead to bipedal locomotion might have been a greater mobility of the lumbar region of the spinal column. Australopithecus afarensis possessed a lumbar region of the spine which unlike that of apes and similar to humans. The hip of A. afarensis was very similar to that of modern humans. The major in hips since A. afarensis has been a widening of the pelvic inlet to accommodate the enlarged fetal skull during birth (Lovejoy, 2005). 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). In all known apes (modern and fossil), the big toe (hallux) diverges from the rest of the foot for increased grasping ability. In all hominids known to date (including Australopithecus and Homo habilis) the hallux is convergent and has lost its opposition. The South African fossil of an australopithecine foot (perhaps as old as 3.5 million years) is the oldest foot to show a human-like convergence for bipedal locomotion although it is intermediate between human and ape toes in this regard (McHenry, 2006).
In all known apes (modern and fossil), the big toe (hallux) diverges from the rest of the foot for increased grasping ability. In all hominids known to date (including Australopithecus and Homo habilis) the hallux is convergent and has lost its opposition. The South African fossil of an australopithecine foot (perhaps as old as 3.5 million years) is the oldest foot to show a human-like convergence for bipedal locomotion although it is intermediate between human and ape toes in this regard (McHenry, 2006).
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, 1983). Australopithecines possessed enlarged gluteal muscles and flattened condyles of the femur and tibia for upright posture (Kingdon, 2003). The femur of Australopithecus afarensis was adapted for bipedal locomotion with a longer neck, a reduction of the shaft angle, a more superior greater trochanter, and modified muscle attachment points). Only minor differences separate it from the femora of modern humans (Lovejoy, 2002). The lumbar vertebrae were modified for bipedal locomotion in australopithecines and lost the features which allowed the flexion of the back (lordosis) which allows knuckle walking in apes (Martelli, 2003). Bipedal walking requires less energy than quadrupedal knucklewalking and would have been a more efficient form of locomotion in early hominids (Sockol, 2007).
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. In modern
primates, the degree of sexual dimorphism is correlated with social structure
and if this is true of Australopithecus,
it suggests that monogamy was the main reproductive strategy (
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 (described later) 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).
DATE: 3.5 million years ago
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). Kenyanthropus had facial and dental similarities to Homo rudolfensis (Kingdon, 2003).