Page 300 - Earth's Climate Past and Future

276 PART V • Historical and Future Climate Change

and leopards, and that they constantly had to contend
with hyenas and other scavengers for this food. Bran- TABLE 15-1 Growth in Size (Volume) of
dishing tools as weapons may have helped them drive Hominid Braincases
off competitors. Type of Age Braincase
Use of tools for butchering implies an important hominid (Myr ago) (cm )
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change in diet, which must have previously consisted
mainly of items collected from their environment: fruits, Homo sapiens 0.2–0 1100–1500
nuts, leaves, and small insects (mainly grasshoppers and Homo erectus 2.4–1.8 800–1000
termites). The use of crude cutting tools would have Australopithecus 4.1–3.1 400–500
allowed hominins to take greater advantage not just of
the meat but also the bone marrow and other internal
animal parts. These protein-rich food sources would
have more readily satisfied their energy needs. Stone intelligent man) first appeared between 200,000 and
tools could also have helped hominins dig out buried 100,000 years ago, with braincases ranging in size
roots and tubers that were otherwise difficult to reach. between 1100 and 1500 cm . A tripling of brain size in
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The available evidence suggests that tool making
followed more than 1 million years after the ability to 4 million years is unusually rapid compared to many
evolutionary changes.
walk upright. Some scientists infer that tool making was
a natural evolutionary development for creatures whose 15-2 Did Climate Change Drive Human Evolution?
hands were freed for other uses when they began to
walk on two legs. Many competing hypotheses have been proposed to
Appearance of Homo Sometime after 2.5 Myr explain human evolution. Some hypotheses focus on
ago, the ancestral australopithecines evolved into several social factors, such as the need of early humans to
new forms (see Figure 15–2). One line led to the genus gather food for their infants, who spent several years in
Paranthropus, stout creatures with large teeth and strong a defenseless state. Others focus on the impact of new
jaws used to crush protein-rich palm nuts and other technology, such as the creation of tools to facilitate
hard food in their vegetarian diet. This group became food gathering and to serve as weapons for defense and
extinct by 1 Myr ago. hunting. Other scientists propose that climate change
The other major group carries the name of our own has altered human evolution. Climatic hypotheses of
genus, Homo. These were more graceful (lean-bodied) human evolution fall broadly into two groups.
creatures with larger heads and braincases. The earliest According to the savanna hypothesis, human evo-
of these creatures is dated (with some uncertainty) to lution was driven by a long-term drying of African cli-
about 2.4 to 2.3 Myr ago, and our human ancestor mate in which tropical rain forests became interspersed
Homo erectus (upright man) was definitely present by with semiarid grasslands, which gradually spread
just after 2 Myr ago (see Figure 15–2). The wear on between groves of trees. Fragmentation of once contin-
their teeth indicates a broader-based diet of meat, fruits, uous forest caused our ancestors to move on the ground
and vegetables. for longer distances, requiring more rapid movement to
The first appearance of these human-like creatures cover longer distances and also greater resourcefulness.
appears to follow closely after the earliest use of stone This drying trend began well before the divergence of
tools, but the precise timing is difficult to determine humans and chimpanzees from great apes 6 to 4 Myr
because the fossil record is fragmentary. The use of ago, and it has continued in subsequent times.
tools is likely to have favored those prehumans who Remarkably little evidence from the earlier part of
were able to move easily across the landscape in pursuit this interval has been found in North Africa, because
of a large variety of seasonally changing food sources. aridity is so unfavorable to permanent deposition and
Frequent movement may also have called on a greater preservation of sediment. Much of the information
use of intellect and imagination. about the longer-term climate of Africa comes from
Brain Size Over time, our human ancestors devel- ocean sediments that contain material blown out from
oped larger brains, shown by the increasing size of the continent. Cores from the tropical eastern Atlantic
preserved skulls that encased and protected the brain. Ocean show gradual increases in the rate of influx of
In broad outline, the volume of the braincase tripled continental dust (mostly quartz and clay) from North
over the last 3 or 4 million years (Table 15–1). Africa after 4.5 Myr ago (Figure 15–5 left). This trend,
The bipedal australopithecines had braincase vol- also evident in sediments from the western Indian
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umes of 400–500 cm . The Homo erectus ancestors who Ocean, has been interpreted as a sign of progressive
used stone tools had braincases twice as large, roughly drying that reduced the vegetation cover and exposed
800–1000 cm . Fully modern humans (Homo sapiens, or larger areas to erosion by winds. Dust transport may
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