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The Earliest Hominines
The first undoubted hominine discovered thus far is Ardipithecus ramidus,
which was found in 1994 and is known from 17 fragments of teeth and bone. It
dates to approximately 4.4 million years ago.
Thought to be the descendent genus of Ardipithecus is the genus
Australopithecus; individuals of this genus were bipeds while on the ground and
had ape-like brains and dexterous hands. There are at least six species of
Australopithecus: A. anamensis, A. afarensis, A. aethiopicus, A. africanus, A.
boisei, and A. robustus.
In 1924 an unusual fossil was brought to Raymond Dart, an anatomist at the
University of Witwatersrand in Johannesburg. This fossil had a curious mix of
ape-like and human-like traits. Dart named the fossil Australopithecus africanus
and claimed that, based on the forward position of the foramen magnum, the
creature was a biped.
At least four species are recognized: A. afarensis and A. africanus being
smaller and lacking the massive jaws of the two larger species, A. boisei and A.
robustus. A. afarensis and A. boisei are from East Africa, while A. africanus and A.
robustus are from South Africa. An earlier species, A. anamensis comes from
Kenya, while a single representative of a sixth species, A. aethiopicus comes from
West Turkana and is known as the “black skull” for its distinctive black staining.
Australopithecus africanus has been discovered at three South African sites:
Taung, Makapansgat, and Sterkfontein. All of these sites range in date from 3 to
2.3 mya; however, a partial foot may be as old as 3.5 million years.
Australopithecus afarensis dates to between 3.9 and 2.9 mya, and was
discovered in the 1970s and 1990s in the Afar region of northern Ethiopia.
Included in this species are the two famous finds of Don Johanson: the remarkably
complete female skeleton AL 288-1, known as “Lucy”, and the collection of 13
individuals at Afar Locality 333 which has come to be known as the “First Family”.
Material nearly 4 million years old from Laetoli in Tanzania has also been
ascribed to A. afarensis, despite suggestions that the wide variation in size of
individuals may mean the presence of 2 species. It is likely, however, that A.
afarensis size differences represent sexual dimorphism similar to Miocene apes and
intermediate between the greatly dimorphic modern gorillas and the less dimorphic
Other East African sites have yielded fossils similar to A. afarensis or A.
africanus. These sites are all 2 million or more years old. The individuals ranged
between 3.5 and 5 feet, with weights of between 29 and 45 kg.
Australopithecus afarensis and A. africanus are considered to be “gracile” or
smaller australopithecines. These two species possessed small incisors, short
canines in line with the other teeth, and a rounded dental arch. No gap between the
canines and incisors in the upper jaw (diastema), as seen in apes, was present. The
molars and premolars were larger than those of modern humans, but were similar
in form. Tooth wear indicates that these species chewed as humans do, but with 2
to 4 times the force. The diet was largely tough, fibrous vegetation. A. afarensis
individuals tend to show more sivapithecine features, such as a less-rounded dental
arch, less shearing tooth wear, slight diastema, and some canine projection, than
the later A. africanus individuals. These sivapithecine features suggest a Miocene
Some sex differences have been noticed in these two australopithecines: males
seem to develop a bicuspid first lower premolar while females do not; and females
seem to possess skeletal features better suited to tree-climbing than males. These
differences suggest that males and females may have had slightly different foraging
strategies, with males spending more time on the ground and females exploiting
Cranial capacity of A. afarensis was 310 to 500 cc and that of A. africanus was
428 to 510 cc (roughly the size of a chimpanzee and 1/3 that of a human);
intelligence, however, is more a factor of the ratio of brain to body size.
Unfortunately, the vast rang of body sizes in these forms makes this ratio difficult
to assess. It is believed that these two australopithcines had mental capabilities
equivalent to those of the great apes of today.
What is significant is the fact that at 4 million years ago, there existed a bipedal
hominine. Evidence supporting this fact includes: forward placement of the
foramen magnum indicating a head balanced atop the spinal column; human-like
curvature of the spine; forearms shorter than those of an ape indications of a lower
centre of gravity than apes; the Laetoli footprints; and hip and knee anatomy. On
the other hand, these australopithecines still retained a slightly divergent great toe
and shoulder girdle well suited for climbing.
Bipedal locomotion preceded any increase in brain size; in fact,
australopithecines lacked the prolonged maturation of modern humans and likely
matured as apes do. Upright walking set the stage for larger brain sizes but was
not the sole cause of these later increases.
Robert Broom and John Robinson first discovered Australopithecus robustus,
a larger, more robust australopithecine, in 1948 at the sites of Swartkrans and
Kromdraai (South Africa). While no accurate dates are available, it is believed that
the deposits are 1.8 to 1 million years old.
A. robustus is similar to A. africanus, except for thicker bones relative to size,
with large muscle-markings that included an ape-like sagittal crest on the large
Cranial capacity of A. robustus was around 530 cc and the crest served to
anchor the massive chewing muscles of powerful jaws. Such chewing apparatus is
also seen in the modern gorilla and is an example of convergent evolution in these
In East Africa, a robust australopithecine species also exists: Australopithecus
First found at Ngorongoro Crater of Olduvai Gorge in 1959 by Mary Leakey,
the original specimen was named Zinjanthropus by Louis Leakey. Further study
revealed it was another australopithecine and it was renamed A. boisei.
Potassium-argon dating places the find at 1.75 million years old; however, another
specimen thought to be an earlier A. boisei is dated to 2.5 mya. The most recent A.
boisei fossils are only 1.3 million years old.
A. boisei is similar to A. robustus in form; however, the East African robust
australopithecine is even more robust than its South African relative. A. boisei s
cranial capacity was 500 to 530 cc and its weight was 34 to 49 kg, compared with
the 32 to 40 kg range of A. robustus.
The Black Skull from Kenya shares some primitive features with
Australopithecus afarensis and, being far earlier than the other A. boisei finds, has
been suggested as being descended from A. afarensis. While your text considers
this specimen to be an early A. boisei, other researchers prefer to place the Black
Skull in a separate species, Australopithecus aethiopicus.
It is uncertain whether A. robustus represents a southern offshoot of the A.
boisei lineage or an example of convergent evolution that evolved from A.
A. boisei and A. robustus were both highly efficient “chewing machines” which
had a diet of tough, uncooked vegetation.
The law of competitive exclusion states that when two closely related species
compete for the same niche, one will outcompete the other, bringing about the
“loser s” extinction. The specialized vegetarian robust australopithecines avoided
such competition with early Homo and these two genera were thereby able to
coexist for some 1.5 million years.
Older than Australopithecus afarensis is Australopithecus anamensis. This
species is a recent find, dated to 4.2 to 3.9 mya, and is represented by an arm bone
found in 1965 and two jaws and a tibia found in 1995. The jaws were ape-like with
a shallow palate and large canines, but the tibia was similar to that of A. afarensis
and is the oldest direct evidence of bipedalism yet found.
Older still than Australopithecus anamensis is Ardipithecus ramidus, a species
known from pieces of 17 individuals from Ethiopia dated to 4.4 mya. No pelvic,
foot, or leg bones are among the fossils; however, the shapes of the canines and
vertebral elements are hominine in appearance.
It is believed that the Australopithecus evolved, by way of Ardipithecus, from
some Miocene sivapithecine ancestor. Evolution of the hominines was not steady;
it was far more likely periods of stasis punctuated by short episodes of rapid
changes, as evidenced by long periods of little change between the diversification
of the various Australopithecus species.
At the close of the Miocene, the climates changed, with the Mediterranean Sea
drying up temporarily and the vast forests undergoing a reduction in size. More
open country was available, requiring the ancestors of the hominines, as well as the
early hominines themselves, to spend more time on the ground. New foods were
made available as older arboreal ones disappeared; dentition therefore was altered.
With the reduction of canines came, it is believed, a reliance on hands for defense,
using clubs or rocks. This use of objects for defense may have set the stage for
tool-use/tool-manufacture. There is no evidence of tool use or manufacture among
australopithecines, but modern chimps and orangs can and do make tools; in fact,
under experimental conditions, chimps have been able to make crude chipped stone
tools. Based on the abilities of modern chimps and orangs, it is believed that the
australopithecines used natural objects as tools.
There is a 2 million-year gap between the last sivapithecine and the first
australopithecine; the individuals in this gap likely were undergoing the transition
Bipedalism has drawbacks: exposure of soft belly to attack; slower running and
poorer ability to change direction instantly; back problems, hernias, circulatory
problems associated with the upright posture; and the consequences of serious leg
So why bipedalism? Perhaps for: carrying foraged foods from place to place
carrying infants so that fatal falls from mother were minimized; faster food
gathering and longer treks with less fatigue; spotting food sources or predators
from a distance with the increased height and visual perspective of standing on the
hind limbs; or freeing hands to fend off predators by using natural objects as
weapons. In any case, the bipedal adaptation was likely the result of several
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