Encyclopedia of Evolution.pdf - Online Reading Center

australopithecines
the expanded end that connected with the knee joint, suggests
that this hominin walked upright. Other limb bones,
jaw fragments, and teeth were found as well. Orrorin presents
the same challenge to the interpretation of human evolutionary
history as does Sahelanthropus: Bipedalism may
have been present in the common ancestor of humans and
chimpanzees, and it calls into question whether the later
australopithecines were human ancestors. The place of origin
of the human lineage is likewise unclear: was it western
Africa, where Sahelanthropus lived, or eastern Africa, where
Orrorin lived? At the time, the extensive deserts of Africa
had not yet formed, and the forests in which both Sahelanthropus
and Orrorin lived may have been continuous across
the continent.
Ardipithecus
Ardipithecus ramidus (called “basic root ape” because it may
represent the branch point or beginning of the human lineage)
was discovered in Ethiopia in 1994 by anthropologist
Tim White and associates. The fossils are about 4.4 million
years old. The bones suggest that Ardipithecus was bipedal.
Anthropologists have not agreed whether Ardipithecus was
the ancestor of later australopithecines.
Australopithecus
The genus Australopithecus (“southern ape”) is represented
by several species that lived between about four million and
about two million years ago in various parts of Africa. The
three earliest species were A. anamensis and A. afarensis,
both named after the eastern African regions in which their
fossils were found, and A. bahrelghazali (“Bahr el Ghazal,”
the western African location at which the fossils were
found). These species lived between four and three million
years ago. Two other species, A. garhi (“surprise”) from
eastern Africa and A. africanus (“African”) from southern
Africa, lived between three million and two million years
ago. The evolutionary relationships among these species are
unclear. Genus Australopithecus is called the “gracile australopithecines”
because they were small (three to four feet
[one m] in height), had small faces, and jaws adapted to an
omnivorous diet.
Australopithecus africanus was the earliest prehuman
hominin to be discovered. A South African anatomist
(see Dart, Raymond) discovered this species in the 1920s.
The especially striking fossil of the Taung child (see photo
at right) showed a mixture of human and apelike features.
Anthropologist Robert Broom found many other A. africanus
specimens in the field. At first, British anthropologists disregarded
Dart’s and Broom’s discoveries, primarily because
they relied on Piltdown man as a guide to understanding
human evolution. The characteristics of Piltdown man suggested
that human brain size had begun to increase very early
in human evolution, and that this transition had occurred in
Europe. The Taung specimen suggested that ape skulls had
begun assuming modern characteristics before any increase
in brain size, and that this transition had occurred in Africa.
Dart lived to see Piltdown man revealed as a hoax, and the
Skull of the “Taung child” Australopithecus africanus. Discovered by
Raymond Dart in , this eventually directed attention of researchers to
Africa as the place where humans evolved. The specimen consists of part
of a juvenile skull and mandible and an endocast of the right half of the
brain (seen at center left). It is about . million years old. (Courtesy of
Pascal Goetgheluck/Photo Researchers, Inc.)
general acceptance of Australopithecus as a valid human
ancestral genus, even if A. afarensis may not have been on
the main line of human evolution.
Since the bones of A. africanus were found in association
with those of many prey mammals whose bones
had been crushed, it was at first thought that A. africanus
was a hunter. This is the image presented by writer Robert
Ardrey, upon which writer Arthur C. Clarke based the
opening of the movie 2001: A Space Odyssey. However, a
skull of a child of this species was found that had holes that
exactly matched the species of leopard that was present in
southern Africa at that time. According to the research
of South African paleontologist C. K. Brain, the piles of
crushed and broken bones, found in limestone caves, were
apparently leftovers from leopard meals, rather than australopithecine
hunts. Modern leopards eat their kills in
trees, which often grow out of limestone caves in the arid
South African landscape. A. africanus was the prey, not the
predator.
One of the most famous australopithecine fossils,
“Lucy,” was a nearly complete A. afarensis skeleton found
in the early 1970s by Donald Johanson (see Johanson,
Donald) (see figure on page 33). Since the discovery of
Lucy, both small and large individuals of this species have
been found, which may represent females and males of this
species or may indicate that there was more than one species
of australopithecine at that place and time. Because it was
so nearly complete, the Lucy skeleton revealed a great deal
about the movements of this species. The major adaptations
for bipedalism were present in Australopithecus afarensis:
The opening for the spinal cord was underneath the skull,