Page 298 - Earth's Climate Past and Future

274 PART V • Historical and Future Climate Change

Climate and Human Evolution 0 Homo sapiens brain size: 1100–1500 cm 3
?
Anthropologists agree that humans evolved in Africa. ?
Most of the earliest evidence for our ancient ancestors
comes from plateaus along the eastern side of the conti- Homo erectus
nent (Figure 15–1). Volcanic activity in this region over 1 brain size:
millions of years deposited basalt layers that can be 800–1000 cm 3 ?
dated by radiometric methods. These dated basalts ? ?
bracket the ages of intervening sediment layers that
hold much of the record of human evolution. ? ? ?
2 Genus ?
15-1 Evidence of Human Evolution Homo ? ?
?
Anthropologists focus either on distinctive events that Genus ?
break the continuous process of evolution into separate ? ? Paranthropus Stone tools
stages or on quantitative traits that can be measured as Age (Myr) 3
they gradually change. Human evolution is marked by
five distinctive developments (Figure 15–2): (1) the ini- Brain size:
tial branching off from primitive apes between 6 and 4 400–500 cm 3
Myr ago; (2) the onset of bipedalism (a preference for ?
?
moving upright on two legs) near 4 Myr ago; (3) the use 4 ?
of stone tools beginning near 2.5 Myr ago; (4) the Genus Prevalent
branching of the prehuman line into the genus Homo Australopithecus two-legged
and other forms by 2 Myr ago; and (5) the development movement
of large brains since 2 Myr ago. Divergence from
5 primitive apes
Appearance of Human Ancestors Human evolu-
(6–4 million years ago)
tion can be traced back to small shrewlike mammals that
evolved during the millions of years after the massive
FIGURE 15-2 Human evolution The last 5 million years
span the evolutionary line from primitive apes to the
australopithecines (“southern apes”), our own genus, Homo,
and finally our own species, Homo sapiens (“intelligent man”).
The onset of two-legged walking appeared early in this
evolutionary progression, followed by the first use of stone
tools more than a million years later. (Adapted from P. B.
20°N deMenocal, “Plio-Pleistocene African Climate,” Science 270
[1995]: 53–59.)
Savanna

Forest
0° extinctions at the time of the asteroid impact 65 million
years ago (Chapter 5). These primitive mammals eventu-
ally evolved features we now associate with monkeys,
such as grasping front paws and long prehensile tails,
which led to the lemur family (a monkey-like tree-climb-
20°S ing animal). Modern lemurs are shown in Figure 15–3.
By 10 Myr ago, one such line had further evolved to
primitive apes. Subsequently, a group of apes that
Elevation >1 km included both our human ancestors and chimpanzees
branched off from the primitive apes, with modern apes
evolving from the other branch. Our prehuman ances-
20°W 0° 20° 40°E
tors are thought to have foraged for food in and near
FIGURE 15-1 African topography and rain forests Eastern woodlands, moving at times on two legs. Radiometric
Africa is a region of broad plateaus at elevations not far above dating of volcanic rocks in East Africa indicated that
1 km. Most rain forest vegetation occurs today in the wet this branching occurred sometime between 10 and 5
intertropical convergence zone near the equator, encircled by a Myr ago, but for a long time the timing was difficult to
broad band of drier savanna. constrain more precisely.