PREHISTORIC LIFE HOME PREHISTORIC LIFE TABLE OF CONTENTS OBL HOME OBL REFERENCES
HOMINIDS 1

Orrorin

 The genus of the large ape Orrorin is known from the Late Miocene of Kenya.  Orrorin specimens have been dated at just older than 6 million years ago and others at 5.8-5.9 million years ago (Pickford, 2002). 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).

 

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).

orrorin
    Some have suggested classifying Orrorin and many of the gracile australopithecines in one single genus, Praeanthropus (Cela-Conde, 2003). 

Sahelanthropus tchadensis

 Sahelanthropus

DATE:  5-7 million years old

SITE: Chad, Central Africa

SPECIMENS: one cranium, probably male; jaw fragments

CHARACTERISTICS:

     Sahelanthropus was discovered in Central Africa from strata dated 5.2-7.4 million years old.  Hominid evolution has commonly been presented as an “East Side Story”—a process which occurred in East Africa, in more open habitats of the Rift Valley.  While most of the fossil excavations have occurred in East Africa, this find indicates that hominids/hominid ancestors were more widespread and that fossils should be sought in other areas of Africa (Coppens, 1994). Fossils of teeth and lower jaws of Sahelanthropus support both its age (about 7 million years old) and its classification as a hominid which evolved just after the split between chimp and hominid lineages (Brunet, 2005).

     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).

sahelanthropus
Sahelanthropus

Ardipithecus ramidus

(Australopithecus ramidus was renamed Ardipithecus ramidus; White, 1994).

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

CHARACTERISTICS:

     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). The locomotion of Ardipithecus seems to have been different than modern apes and humans; it may represent an extinct lineage (Kingdon, 2003).

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).

 

ardipithecus teeth

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

CHARACTERISTICS:

     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).

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

CHARACTERISTICS:

Skull:

     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).

australopithecus palate
apes
australopithecus
lower jaws
chimp and australopithecine
chimp and australopithecine
lower jaws lower jaws
premolars
     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).
chimp leg afarensis leg human leg
human chimp pelvis

      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).

     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).

 

foramen magnum foramen magnum
foramen magnum
foramen magnum

Teeth:

     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 (Reno, 2003).

lucy

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 (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).

garhi
garhi outline
Kenyanthropus platyops

DATE: 3.5 million years ago

SITE: Lake Turkana, Kenya

SPECIMENS: 1 complete cranium; isolated skull portions (temporal, maxillary) and teeth

CHARACTERISTICS:

    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).

 

kenyanthropus kenyanthropus