Page 302 - Earth's Climate Past and Future

278 PART V • Historical and Future Climate Change

western Indian Ocean caused by the partial closure of a open vegetation at or near the onset of the glacial cycles.
seaway near Indonesia and the resulting loss of warm Subsequently, forest vegetation repeatedly fluctuated up
tropical water from the western Pacific Ocean. and down the sides of the mountains at intervals of tens
Tectonic changes in southern Asia may have also of thousands of years. These oscillations appear to be
played a role. Continuing uplift of the northern and connected to the early northern hemisphere glacial
eastern parts of the Tibetan Plateau not only strength- cycles.
ened the Asian summer monsoon but also created a In addition, records from the eastern and southern
strong counterclockwise spiral of winds that drove hot, plateaus of Africa also show a widespread change from
dry air out of the interior of Asia across Arabia and into woodland-adapted browsing (leaf-eating or grass-eating)
northeastern Africa (Figure 15–6). These winds pro- animals to grazing animals strictly adapted to grasslands
duced much drier conditions in summer across a huge near and after 2.5 Myr ago. Many browsing animals
arc extending from west-central Asia across the Arabian became extinct, and new grazing animals appeared both
peninsula and into northeastern Africa. This drying by evolving and by emigrating from Asia. The evidence
trend may have been aided by the retreat of a vast inte- suggests a relatively rapid proliferation of open grass-
rior seaway that once occupied west-central Asia and lands at the expense of closed forests. This marked
moderated its temperatures. change from browsers to grazers (and from forest to
grassland habitat) has been attributed to the cooling and
IN SUMMARY, both the available data and model drying produced by northern hemisphere glacial cycles.
simulations confirms a gradual trend toward more One problem with this proposed link to the north-
open vegetation in Africa throughout the time of ern ice sheets is that GCM simulations suggest that the
human evolution. This evidence seems to support ice sheets would have had relatively small direct effects
the savanna hypothesis. on temperature and precipitation in North Africa. Even
the large ice sheets that existed at the last glacial maxi-
The actual record of hominin remains, however, mum, 20,000 years ago, cooled northern Africa by no
suggests that the picture is more complicated. Hominins more than 1 –4 C, with smaller changes farther south
°
°
occupied environments ranging from woodlands and and little change anywhere during the summer (mon-
grasslands to river margins throughout their history, soon) season. The smaller ice sheets that grew and
with no obvious trend toward greater occupation of melted during the glacial cycles that started 2.75 Myr
the savanna environment. This evidence indicates that ago would probably have had even smaller impacts on
hominins have long been resourceful in making use African climate.
of a range of environments and it has led to a different It is possible that these early glaciations had some
hypothesis of the effects of climate on human evolution. effect on vegetation and habitat in Africa because they
The basic premise of the variability selection were accompanied by drops in atmospheric CO levels,
2
hypothesis is that rapid evolution occurred because like those that have occurred during the last several
alteration of habitat put new demands on our ancestors, hundred thousand years (see Chapter 10). Lower CO
2
some of whom then took advantage of new opportuni- levels during these earlier glaciations could have caused
ties. Changes in climate favored hominims with traits some replacement of C3 trees and shrubs by C4 grasses
useful for survival in new environments, particularly during each glacial cycle, with fragmentation of forest
clans or tribes that had larger numbers of members with habitat.
such traits. Another problem with the variability selection
Climatic variability has greatly increased at high lat- hypothesis is that the summer monsoons that control
itudes during the last several million years, particularly annual rainfall and vegetation types across the savannas
with the onset of small glaciation cycles near 2.75 Myr south of the Sahara Desert and across most of the
ago and then the larger oscillations since 0.9 Myr ago plateaus of East Africa appear to have continued for
(Chapter 9). In addition, the earliest appearance of our many millions of years with no strong trend toward
genus Homo is dated to between 2.4 and 2.0 Myr ago, larger or smaller oscillations. It appears that the mon-
not long after the first glacial cycles. Perhaps these early soons did not contribute to a long-term increase in
glaciations created cycles of cooling and drying in climatic variability.
Africa that accelerated the pace of evolution by favoring Unfortunately, the fossil record of human remains is
individuals with greater adaptability. too sparse to determine any correlation between the
Several lines of evidence suggest that northern onset of glacial cycles and hominid evolution.
hemisphere glaciation could have affected climate and
hominin habitat in Africa. Pollen records from high ter- IN SUMMARY, the record of human remains is too
rain in East Africa show that the upper tree line began sparse to test the variability selection hypothesis.
to retreat up mountainsides and give way to more