Evolution And History

152 CHAPTER 7 | The First Bipeds

The Pliocene Environment

and Hominin Diversity

As described in the previous chapter, the Miocene epoch
was a time of tremendous geologic change. The effects
of these changes continued into the Pliocene. The steady
movement of geologic plates supporting the African and
Eurasian continents resulted in a collision of the two land-
masses at either end of what now is the Mediterranean Sea
(Figure 7.5). This contact allowed for the spread of species
between these continents.
A suite of geologic changes, known as the Great Rift
Valley system, are associated with this collision. This
system consists of a separation between geologic plates,
extending from the Middle East through the Red Sea and
eastern Africa into southern Africa. Part of rifting in-
volves the steady increase in the elevation of the eastern
third of the African continent, which experienced a cooler
and dryer climate and a transformation of vegetation from
forest to dry grassy savannah.
The system also contributed to the volcanic activity
in the region, which provides opportunities for accu-
rate dating of fossil specimens. Also in the Miocene, the
Indian subcontinent, which had been a solitary landmass
for many millions of years, came into its present position
through a collision with Eurasia, contributing further to
cooler, dryer conditions globally. In addition to causing
global climate change, these geologic events also provided
excellent opportunities for the discovery of fossil speci-
mens as layers of the earth became exposed through the
rifting process.

Diverse Australopithecine Species

Since Dart’s original find, hundreds of other fossil bi-
peds have been discovered, first in South Africa and
later in Tanzania, Malawi, Kenya, Ethiopia, and Chad. As
they were discovered, many were placed in a variety of
different genera and species, but now usually all are con-
sidered to belong to the single genus Australopithecus.
Anthropologists recognize up to eight species of the ge-
nus (Table 7.1). In addition, some other groups of fossil
bipeds from the Pliocene epoch (1.6 to around 5 mya)
have been discovered, including the earliest representa-
tives of the genus Homo.
First we will describe the australopithecines and their
contemporaries in the order in which they inhabited the
earth up to the middle Pliocene (2.5 mya) when the genus

Atlantic

Ocean

Indian

Ocean

CHAD

ETHIOPIA

KENYA

TANZANIA

SOUTH

AFRICA

MALAWI

Sterkfontein

Swartkrans

Taung

Hadar

East Turkana

West Turkana

MediterraneanSea

Olduvai

Gorge

Laetoli

Middle

Awash

Figure 7.5 Australopithecine fossils have been found in

South Africa, Malawi, Tanzania, Kenya, Ethiopia, and Chad.

In the Miocene the Eurasian and African continents made

contact at the eastern and western ends of what now is the

Mediterranean Sea. As these landmasses met, “rifting” also

occurred, gradually raising the elevation of the eastern third of

Africa. The dryer climates that resulted may have played a role

in human evolution in the distant past. This rifting also gives

us excellent geologic conditions for finding fossils today.

savannah Semi-arid plains environment as in eastern Africa.

Homo first appeared. The East African and South African

evidence will be presented separately because the dating

for East African sites is more reliable. Next we will exam-

ine late-appearing australopithecines, including a grade

of australopithecine found in both eastern and southern

Africa that coexisted with the genus Homo.

East Africa

The oldest australopithecine species known so far con-

sists of some jaw and limb bones from Kenya that date to

between 3.9 and 4.2 mya (see Australopithecus anamensis

in Table 7.1). Meave and Louise Leakey, daughter-in-law

and granddaughter of Louis and Mary Leakey, discov-

ered these fossils in 1995 and decided to place them in

a separate species from other known australopithecines.

Its name means “ape-man of the lake,” and it shows par-

ticularities in the teeth such as a true sectorial: a lower

premolar tooth shaped to hone the upper canine as seen

in apes. In humans and more recent ancestors, the pre-

molar has a characteristic bicuspid shape and does not

sharpen the canine each time the jaws come together. As