Temperature was also a critical environmental factor, but
ancient peoples discovered that they were highly adaptable to
temperature extremes, unlike some other species. Th e earliest
settlers of the Americas, who probably crossed from Asia to
Alaska along a land bridge that is now covered by the Ber-
ing Sea, found themselves in an inhospitable environment,
with bitter subzero temperatures and large amounts of snow
and ice. Th eir descendants migrated to desert regions of the
American Southwest and Mexico, where they faced hot, dry
conditions, with temperatures oft en exceeding a hundred de-
grees. And their descendants migrated farther south to cold
and forbidding regions of South America.
At the same time, human beings discovered that they
were also able to adapt to widely diff erent elevations. Th e
people of the Pacifi c islands lived essentially at sea level. Th e
early inhabitants of the Andes range in South America dis-
covered that they could adapt to elevations well over 10,000
feet, where the air is thin and their lungs had to expand
to take in more oxygen. In time, humans, because of their
adaptability, were able to occupy virtually every niche in
the ecosystem, including mountains, forests, grassy steppes,
deserts, tropical islands, and every region between. In doing
so, they adapted their tools, technologies, and ways of life to
the climate and terrain.
A major consideration for ancient civilizations was travel
and transport. In an age of eff ortless air travel, it is easy to
forget that geography and natural barriers were formidable
obstacles as people tried to immigrate to new regions or sim-
ply to follow food supplies. Th e early Romans, for example,
were unable to cross the mountains that divided Italy from
the rest of Europe, though in time they were able to do so and
expand their empire farther to the north. Later, they found
that major rivers in northern Europe blocked their expansion
in that direction, allowing the tribes that would eventually
overrun Rome to settle and amass on the empire’s northern
border. Geographical obstacles also inhibited contact be-
tween peoples. In the Andes of South America the forbid-
ding terrain left the communities that settled there isolated
from other groups of people. By remaining isolated, they
were unable to learn from other people, in contrast to other
civilizations that enriched themselves by contact with others.
Similarly, the Pyrenees Mountains cut off the people of the
Iberian Peninsula from contact with other Europeans.
To solve the transportation problem, many early peoples
settled along bodies of water. Th e Egyptians used the Nile to
travel up and down much of the African continent, and vari-
ous civilizations in such places as Mesopotamia and China
navigated the rivers for trade and cultural contact. Using boats
and raft s, these people were able to obtain goods—spices, salt,
foodstuff s, ores, precious metals, fabrics, lumber—that were
unavailable at home. Of course, one of the main reasons that
the ancient Greeks and then the ancient Romans were able to
extend their infl uence over a wide geographic region was that
they controlled the Mediterranean Sea. (It is worth noting
that Mediterranean means “middle of the world.”) Like other
peoples, including many Asians and especially Pacifi c island-
ers, geographical conditions made the Greeks and Romans
excellent boat builders and mariners.AFRICA
BY LEAH A. J. COHEN
Many systems (such as the celestial systems that dictate the
movement of the earth around the sun and around its own
axis) have driven climate change in Africa over the span of
Earth’s four-billion-year existence. Since most of these sys-
tems change very slowly (cycling on the scale of tens of thou-
sands to millions of years), many of the systematic conditions
that have aff ected climate change in Africa during the past
10,000 years have remained relatively similar.THE GEOGRAPHY AND TOPOGRAPHY OF AFRICA
Africa’s physical geography has been a key factor in its cli-
matic conditions over this period of time. Africa is a large
expanse of land, covering 11.7 million square miles. It is the
second-largest continent (behind Asia) and represents 20
percent of the world’s land. Th is is relevant to the study of
climate in Africa, because land that borders large bodies of
water tends to show less variation in temperature than areas
at the same latitude (the east-west global location) that are
landlocked. In other words, locations near the center of large
continents (such as Africa) have much wider climate vari-
ability than locations at the same latitude that receive the
same amount of solar radiation (sunlight) but are near large
expanses of water. Because Africa is a large, consolidated
landmass, much of the continent does not benefi t from the
tempering eff ect of water. Th us (all else being equal) tem-
peratures inland in Africa tend to be either higher or lower
than those on the coastlines.
Africa’s major lakes include Lake Victoria, bordering Ke-
nya, Tanzania, and Uganda (East Africa); Lake Tanganyika
in western Tanzania (East Africa); Lake Malawi in western
Malawi (southern Africa); and Lake Chad bordering Niger,
Chad, Nigeria, and a small part of Cameroon (western Af-
rica). Paleoclimatologists (scientists who study past climatic
conditions on Earth) have learned a great deal about past cli-
mate change by collecting lake-bottom sediment cores from
these and other African lakes. Sediment cores are samples of
sediment taken using a long cylindrical tube that is driven
into the lake bottom and closed, so that the layers of sedi-
ment, which represent diff erent time periods, are preserved
for the purposes of research. Th e data collected from sedi-
ment core sampling and other research techniques are set to a
timeline using at least one of a variety of dating methods. Th e
most commonly employed method for dating sediment core
layers is radiocarbon (or carbon-14) dating.
Th e topography of Africa is varied, with lowlands, high-
lands, mountains, and valleys, but in general the continent
of Africa is a large plateau. Africa is not known for an abun-
dantly mountainous landscape. Its two tallest mountains areclimate and geography: Africa 235