Download Foundations of Social Studies HISTORY

Foundations of Social Studies
HISTORY
GEOLOGIC PAST
The geologic history of Earth follows the major events in Earth's past from the very beginning of the
history of the planet, long before animals or humans lived here. The geologic time scale is a system
of chronological measurements based on the study of the planet's rock layers. According to these
rocks, Earth was formed about 4.5 billion years ago from the dust and gas left over from the
formation of our sun.
Earth was initially liquid rock. Eventually, the outer layer of the planet cooled to form a solid crust.
Outgassing (loss of atmospheric gas to outer space) and volcanic activity produced our atmosphere.
Water vapor began accumulating in the atmosphere. The water vapor began to condense and,
increased by ice delivered from comets, produced the oceans.
Geologic Eras
Geologists study rock formations to understand the earth’s development. They have divided the
earth’s life into enormously long periods of time known as eras. These eras are based largely on
events that changed the earth’s crust, such as the breaking up of continents, their movements, and
the formation of mountains. As fossils of plants and animals appear in the rocks, eras can also be
based on the kinds of plants and animals that lived at the time.
Precambrian Era. The Precambrian is the earliest and longest of the geologic ages. Precambrian
time covers the vast bulk of the Earth's history. This started with the planet's creation about 4.5
billion years ago. The planet had cooled down from its original molten state. It developed a solid
crust and oceans were created from water vapor in the atmosphere. This era is marked by different
layers of sedimentary rock. It includes the fossilized remains of the very earliest plants and animals
buried when the sediments were formed. This era ended with the appearance of somewhat more
complex, multi-celled life-forms after lasting almost four billion years. It was followed by the
Paleozoic Era.
Paleozoic Era. Early in the Paleozoic era the continents were far apart, but moving tectonic plates
caused continents to move together into one large continent called Pangaea. During this time, ice
covered the northern part of Africa, which was located over the South Pole. According to fossils, a
large number of animals evolved during this time period. Invertebrates (animals without backbones)
ruled the oceans in the early and middle parts of the Paleozoic. The first vertebrates (including fish
and amphibians) began to flourish in the later Paleozoic era. Animals and plants began to populate
the land. At the end of the Paleozoic, many species in the oceans became extinct. They didn’t die all
at once. It took over 8 million years for the mass extinction to wipe out all those species. This was
the largest mass extinction of all time.
Mesozoic Era. The Mesozoic Era is marked as the beginning of land animals and plants. During this
era the first seed producing plants evolved. They moved deep into new territory away from shores
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where water was plentiful. As these new plants moved inland animals followed. The dinosaurs
became the largest life forms during this era and were abundant. Some were herbivores (eating
plants) while others were carnivores (eating meat). The first mammals and birds evolved during the
Mesozoic but there were relatively few species and they were small in size. During the Mesozoic,
mammals were eaten by carnivorous dinosaurs. The Mesozoic ended as the dominant dinosaurs
died after a meteor struck the Earth and the climate of the planet cooled.
Cenozoic Era. Mammals, which had been small and few during the Mesozoic, became more diverse
and much more numerous during this period. New mammal species evolved and were able to live in
areas and eat foods that had been used by dinosaurs during the Mesozoic. Grasses evolved and
were well adapted to the cooler climates of the late Cenozoic. Horses and other species of grazing
animals evolved and ate the newly-evolved grass. Human-like creatures called hominids appear
during in the fossil record of this era. Thousands of fossils of hominids have been found. Fossils of
hominids that are very similar to modern humans have also been found. The very last part of the
Cenozoic was a remarkable period. It is known as the Pleistocene Epoch.
The Pleistocene Epoch: The Ice Ages
It was during the Pleistocene, which began about 2.6 million years ago, that the most recent
episodes of global cooling took place. Much of the world's middle latitude zones were covered by
glaciers during cool periods, uncovered during the warmer periods when the glaciers retreated, and
then recovered by glaciers again. This happened four times. For this reason, the term Ice Ages is
more appropriate than Ice Age.
Plant and animal life at this time were extremely similar to modern ones. Many species of the
Pleistocene Epoch survive to this day: plants, insects, birds, and mammals. Yet the Pleistocene was
also characterized by the presence of distinctive large land mammals and birds. Mammoths and
their cousins the mastodons, long-horned bison, saber-toothed cats, giant ground sloths, and many
other large mammals characterized Pleistocene habitats in North America, Asia, and Europe. Native
horses and camels galloped across the plains of North America. Great birds with 25-foot wingspans
stalked prey. Around the end of the Pleistocene, all these giant creatures became extinct.
The Pleistocene also saw the appearance and expansion of our own species, Homo sapiens. By the
close of the Pleistocene, humans had spread through most of the world. According to a
controversial theory first proposed in the 1960s, humans hunting around the close of the
Pleistocene caused or contributed to the extinction of many of the Pleistocene large mammals. It is
true that the extinction of large animals on different continents appears to correlate with the arrival
of humans, but questions remain as to whether early human hunters were sufficiently numerous
and technologically advanced enough to wipe out whole species. It has also been hypothesized that
some disease wiped out species after species in the Pleistocene. The issue remains unsolved;
perhaps the real cause of the Pleistocene extinction was a combination of these factors. In any case,
human history on Earth begins in during the Pleistocene.
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