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Deep Time |
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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Cenozoic Era: (248 mya-present)
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Paleocene | Eocene | Oligocene | Miocene | Pliocene | Pleistocene | Holocene
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Pleistocene Epoch (1.8-0.01 mya)
During the Pleistocene, glaciers repeatedly advance from the Arctic
north over Europe and North America, then retreat. The first major glacial flow occurs
about 1.6 mya. Ice, up to a mile thick in places, spreads from Greenland over the Arctic
Sea into northern Europe and Canada. As the ice advances, temperatures ahead of the flow
drop significantly.
The temperature change has a profound impact on life. Mammoths, rhinos,
bison, reindeer, and musk oxen all evolve to have warm, woolly coats to protect them from
frigid conditions. These new mammals feed on the small bushes and hardy grasses that
tolerate cold as they follow the moving line of glaciers. Glacial retreats allow for the
temporary return of warm-weather plants such as oak and beech trees, lush grasses, and
flowers. During these "interglacial" periods, species that sought shelter in the warmer
south return to their old habitats.
The hominid line continues to evolve during the Pleistocene. About
100,000 years ago, anatomically modern humans first appear. Many scientists think early
humans impact other mammal species through hunting, as human populations expand and
disperse into new habitats. By the end of the Pleistocene, nearly all large mammals are
extinct, including woolly mammoths, giant wolves, saber-toothed cats, giant ground sloths,
and massive wombats.
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0.6 mya: Early humans
Homo heidelbergensis displays physical characteristics of
modern humans: increased brain capacity, smaller teeth, and a face that slopes less
than that of other hominid ancestors. A fossil jaw found in Mauer, Germany, dates
these early humans to approximately 600,000 years ago.
0.1 mya: Modern humans
Modern humans (Homo sapiens) disperse throughout Africa,
the Middle East, and Europe. They have larger brain cases than most, if not all
other, hominids and higher foreheads than their immediate ancestors. Early European
H. sapiens, called Cro-Magnons, have lean builds. Cave paintings suggest
they have brains capable of sophisticated behavior.
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Late Pleistocene extinction
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Date:
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0.01 mya
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Intensity:
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3
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Affected:
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More than 200 genera
of very large herbivores (2 tons or more) go extinct, including 95 percent in
North America
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Hypotheses:
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Human intervention
(over-hunting), climate change
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Summary:
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The late Pleistocene extinction
is uncharacteristic of other mass extinction events. Extinction takes place at
different times on different continents, but always targeting a specific kind
of animal. What's more, climate change alone probably did not cause it. Over
about 40,000 years, 200 or more groups of large herbivores are wiped out. With
their prey absent from the food chain, many carnivores and scavengers also die
off. Temperature fluctuates throughout the last ice ages, but the fossil record
does not indicate an unusual concentration of death among smaller animals, plants,
or marine creatures, at least some of which would likely have been affected by
climate change. Instead, the loss of very large fauna almost always
coincides with the arrival of humans to a continent. This suggests the possibility
of over-hunting by early human settlers.
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Read more
A smoking gun? (0.01 mya)
The pattern of large mammal extinction toward the end of the
Pleistocene suggests that humans, as they spread across the planet, hunt these
animals out of existence. There is little evidence of extinction among smaller
animals, plants, or marine life at this time.
From their roots in Africa and southeast Asia, humans spread
first to Australia about 55,000 years ago. Shortly after their arrival, giant
marsupials, large reptiles, and 450-pound flightless birds begin dying en
masse. This trend continues throughout Eurasia and the Americas.
By 11,000 years ago -- only 1,000 years since humans are known
to have arrived in North America -- mammoths, mastodons, horses, camels, and ground
sloths go extinct, followed closely by their predators, which include lions, giant
wolves, great bears, and saber-toothed cats. Interestingly, moose, caribou, and
grizzly bears, which had earlier crossed over from Eurasia, survive, perhaps having
been better conditioned to avoid humans.
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-> Go to the Holocene Epoch
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Intro | Precambrian Eon | Paleozoic Era | Mesozoic Era | Cenozoic Era |
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