162 CHAPTER 7 | The First Bipeds
(Figure 7.13). Though paleoanthropologists debate which
species is ancestral to humans, they agree that the robust
australopithecines, though successful in their time, ulti-
mately represent an evolutionary side branch.Environment, Diet, and
Origins of the Human Line
Having described the fossil material, we may now consider
how evolution transformed an early ape into a hominin.
Generally, such paleoanthropological reconstructions and
hypotheses about the origin of bipedalism rely heavily on
the evolutionary role of natural selection. The question at
hand is not so much why bipedalism developed as much
as how bipedalism allowed these ancestors to adapt to
their environment.
Hypotheses about adaptation begin with features
evident in the fossil evidence. For example, the fossil record
indicates that once bipedalism appeared, over the next sev-
eral million years the shape of the face and teeth shifted
from a more apelike to a humanlike condition. To refineAustralopithecines and the
Genus Homo
A variety of bipeds inhabited Africa about 2.5 mya, around
the time the first evidence for the genus Homo begins to
appear. In 1999, discoveries in East Africa added another
australopithecine to the mix. Found in the Afar region
of Ethiopia, these fossils were named Australopithecus
garhi from the word for “surprise” in the local Afar lan-
guage. Though the teeth were large, this australopithecine
possessed an arched dental arcade and a ratio between
front and back teeth more like humans and South African
gracile australopithecines rather than like robust groups.
For this reason, some have proposed that A. garhi is ances-
tral to the genus Homo. More evidence will be needed to
prove whether or not this is true.
Precise relationships among all the australopithecine
species (and other bipeds) that have been defined during
the Pliocene are still not settled. In this mix, the question of
which australopithecine was ancestral to humans remains
particularly controversial. A variety of scenarios have been
proposed, each one giving a different australopithecine
group the starring role as the immediate human ancestor
A. anamensisA. afarensisA. africanusEarly Homo
Robust
australopithecinesAA. anamensis?A. afarensisA. garhi?A. africanusEarly Homo Robust
australopithecinesBA. afarensisKenyanthropus
platyopsEarly HomoDA. africanus Robust
australopithecinesA. afarensisEarly HomoCCA. africanusA. aethiopicusA. robustus A. boiseiA. afarensisEarly HomoEA. africanus
A. garhiA. boiseiA. aethiopicusA. robustusFigure 7.13 The relationship among the various australopithecine (and other) Pliocene groups, and
the question of which group is ancestral to the genus Homo, are debated by anthropologists. Several
alternative hypotheses are presented in these diagrams. Most agree, however, that the robust
australopithecines represent an evolutionary side branch. Ardipithecus ramidus has been proposed
as ancestral to the australopithecines.