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australopithecines
This skull of a robust australopithecine (Paranthropus boisei) that
lived in Africa about . million years ago has a sagittal crest across
the top, which served as an attachment surface for large chewing
muscles. The sagittal crest and the large molars indicate that the robust
australopithecines primarily ate coarse vegetation. (Courtesy of Pascal
Goetgheluck/Science Photo Library)
In late 2006, two significant new Australopithecus discoveries
were announced. A baby A. afarensis was found
in Ethiopia by anthropologist Zeresenay Alemseged. The
“Dikika Baby” fossil is even older than the Lucy fossil, and
its shoulder blades indicated that this species retained some
ability to swing in trees. The “Little Foot” australopithecine
fossil, found in Sterkfontein, South Africa, had previously
been represented only by a foot. In late 2006, the discovery
of more of this same individual was announced.
Kenyanthropus
Kenyanthropus platyops (“flat-faced man of Kenya”) was discovered
by anthropologist Meave Leakey. This species lived
about three and a half million years ago and resembled Australopithecus
in many ways, including upright posture. As its
name suggests, it had a much flatter face than any other australopithecine,
a feature it shares with Homo. For this reason,
some anthropologists suggest that Kenyanthropus, rather than
any species of Australopithecus, was the ancestor of humans.
Paranthropus
Three species of the genus Paranthropus (“alongside human”)
were the “robust australopithecines.” They all lived in eastern
Africa and eventually became extinct without leaving
evolutionary descendants. They contrasted with the gracile
australopithecines in having huge jaws that allowed a diet of
coarse vegetable materials. The first robust australopithecine
to be discovered, by anthropologist Louis Leakey (see Leakey,
Louis), was even called “Nutcracker man.” The large
teeth and prominent sagittal crest of robust australopithecine
skulls indicated that they could chew powerfully. The sagittal
crest, across the top of the skull, allowed attachment sites for
large chewing muscles (see photo above). Chemical analysis
of the tooth enamel of robust australopithecines suggests that
they ate plants with the C 4 photosynthetic pathway, probably
grasses, or termites that had eaten grass (see isotopes).
Detailed laser analysis of layers of tooth enamel in P. robustus
specimens indicate that they were able to switch between
food sources based on C 4 plants and those based on C 3 plants
such as fruits and nuts. The conclusion is that robust australopithecines
had dietary flexibility, and that their extinction
was probably not due to the disappearance of food supplies.
This conclusion raises the possibility that robust australopithecines
were driven to extinction by competition with early
modern humans, rather than by environmental changes. With
their big jaws, the robust australopithecines may have looked
fierce but were probably gentle, as are most herbivores.
Paranthropus aethiopicus (“of Ethiopia”) lived about two
million years ago. This species may have been the ancestor of
P. robustus (“robust”) and P. boisei (named after Mr. Boise,
a benefactor of Louis Leakey), which lived between two million
and one million years ago.
When climates change, species evolve and proliferate, but
in many different ways. About two and a half million years
ago, the climate of east Africa became even drier, with more
grassland and less forest cover. This occurred at the beginning
of the Pleistocene epoch, when the Earth began its ongoing
cycle of ice ages. While Ice Ages caused very cold climates
in the north, they caused dry climates in tropical areas such
as those inhabited by the African hominins. According to
paleontologist Elisabeth Vrba, this change, while not sudden,
was sufficiently severe that many species of mammals (such
as species of antelopes) became extinct, and others evolved
that were better able to survive in open grasslands. This is
also approximately when baboons evolved from arboreal
monkeys. The hominins were no exception to this pattern.
This was about the time of the last gracile australopithecines,
which apparently evolved in two different directions. Some of
them evolved into a more humanlike form, with larger brains
and use of tools. These early humans, which may have been
several species, are often referred to as Homo Habilis. Others
evolved into the robust australopithecines.
A half dozen or more hominin species may have lived
in Africa all at the same time, about two million years ago.
It is presently impossible to determine which of them may
have been ancestral to modern humans, although some
(such as Paranthropus) can be dismissed. The last of the
australopithecines were the robust australopithecines,
which did not become extinct until after the evolution of
Homo. Homo and Paranthropus lived at the same time and
general location. Evolutionary biologist Stephen Jay Gould
suggested that, rather than a ladder of upward progress, the
human evolutionary story more closely resembles a bush,
with branches leading in different directions, only one of
which became the modern genus Homo (see progress,
concept of).