100,000 years. Once in place, the glaciers became self-sustaining by control-
ling the climate.Then, mysteriously in only a few thousand years, the great ice
sheets collapsed and rapidly retreated to the poles.As the result of the Ice Ages,
many northern lands owe their unusual topographies to massive ice sheets that
swept down from the polar regions.
The Pleistocene, originally defined as a period of recent life based on the
fossil record of modern organisms, has become synonymous with a period of
glaciation that began some 3 million years ago. The Ice Age was so pervasive
that ice sheets 2 miles or more thick enveloped upper North America and
Eurasia, Antarctica, and parts of the Southern Hemisphere. In many areas, the
glaciers stripped off entire layers of sediment down to the bare bedrock, eras-
ing the entire geologic history of the region.
THE HUMAN ERA
About 3 million years ago, huge volcanic eruptions in the northern Pacific
darkened the skies and global temperatures plummeted, culminating in a series
of glacial episodes. The climate change prompted a shift from forested envi-
ronments to open savanna habitats in Africa. These changing conditions pro-
duced many new animal species and spurred the evolution of early humans,
who had to adapt to a new environment rapidly. Indeed, humans are products
of the ice ages, which spanned the whole of human experience.
Humans direct ancestors—the hominids—evolved in Africa, probably from
the same species that gave r ise to the great apes, including the gorilla and chim-
panzee. These species have 99 percent of the same genes as humans. Around 7
million years ago, much of Africa entered a period of cooler, drier climates when
forests retreated and were replaced with grasslands. Life on the savanna, where
humans’ancient ancestors roamed,was harsher and more challenging than life in
the forests, where the apes lived.To survive under these difficult conditions, early
humans rapidly evolved into intelligent, upright-walking species, whereas the
apes are much the same today as they were millions of years ago.
An early hominid species called Austr alopithecus(Fig. 190) first appeared
in Africa about 4 million years ago. It walked on two legs but retained many
apelike features, such as long arms in relation to its legs and curved bones in
its hands and feet. The species was quite muscular and considerably stronger
than modern humans. Males stood a little less than 5 feet tall and weighed
about 100 pounds. Females stood about 4 feet tall and weighed about 70
pounds.Two or more lines of australopithecine lived simultaneously in Africa
and survived practically unchanged for more than 1 million years. After a
lengthy period of apparent stability, all but one line became extinct, possibly
due to a changing climate or habitat.
QUATERNARY GLACIATION