Encyclopedia of Evolution.pdf - Online Reading Center

• Proterozoic Eon (“earlier life”; also called Proterozoic
era). During the Proterozoic, oxygen gradually accumulated
in the atmosphere, and complex cells evolved (see
eukaryotes, evolution of). In the middle of the Proterozoic,
a few multicellular organisms such as seaweeds
evolved. Toward the end of the Proterozoic, during the
Ediacaran period, there were a few animals, as evidenced
by embryos and burrows. Most of the inhabitants of the
Ediacaran period were the enigmatic Ediacaran organisms.
By the end of the Proterozoic, the atmosphere had
its modern concentration of oxygen. At least two worldwide
glaciations occurred during the Proterozoic (see
Snowball Earth). The Ediacaran period (defined in
2004) is the first new geological period to be defined in
over a century. It lasted from about 600 to about 540 million
years ago.
All of Earth history after the Precambrian comprises the
Phanerozoic Eon (“visible life”). This eon, which consists of
three eras, includes the evolution of all complex organisms
that were related to modern plant and animal groups:
• Paleozoic era (“ancient life”). Animal life proliferated in the
oceans (see invertebrates, evolution of; fishes, evolution
of). Land plants and animals evolved (see seedless
plants, evolution of; gymnosperms, evolution of;
amphibians, evolution of; reptiles, evolution of). The
land had been barren at the beginning of the Paleozoic era
but by the end was covered with extensive forests, which
had modern groups of plants except the flowering plants.
The end of the Paleozoic era was marked by the greatest of
the mass extinctions, in which over 95 percent of the species
died (see Permian extinction). In most major groups
of organisms, a few representatives survived this extinction.
• Mesozoic era (“middle life”). Life proliferated in the
oceans. On land, flowering plants evolved (see angiosperms,
evolution of), and dinosaurs were the largest
and most diverse of the vertebrates. Mammals and birds
also evolved during the Mesozoic era (see birds, evolution
of; mammals, evolution of). The Mesozoic era
ended when a gigantic asteroid hit the Earth (see Cretaceous
extinction).
• Cenozoic era (“recent life”). A cooler, drier Earth was
dominated by flowering plants, and the mammals were the
largest vertebrates. Toward the end of the Cenozoic era,
particularly cool, dry conditions caused the ice ages.
Eras (particularly the last three) are divided into periods,
most of which also reflect major events in Earth history. All
of the periods are divided into epochs, although most scientists
are familiar only with the epochs of the Tertiary and
Quaternary periods. Because of the intense interest of humans
in their own history, the Recent epoch has been defined to
begin just 10,000 years ago, coinciding with the end of the
most recent ice age and the beginning of villages, agriculture,
and then civilization. The Earth is currently in the Phanerozoic
Eon, the Cenozoic era, the Quaternary period, and the
Recent epoch.
The Encyclopedia of Evolution contains entries for each
of the eras and periods of the Phanerozoic Eon. The epochs
Goodall, Jane
Geological Time Scale with Selected Periods
and Epochs (to nearest million years)
Duration
(million
Eons Eras Periods Epochs years ago)
Hadean 4500–3800
Archaean 3800–2500
Proterozoic 2500–540
Ediacaran 600–540
Phanerozoic 540–
Paleozoic 540–250
Cambrian 540–510
Ordovician 510–440
Silurian 440–410
Devonian 410–360
Carboniferous 360–290
Permian 290–250
Mesozoic 250–65
Triassic 250–210
Jurassic 210–140
Cretaceous 140–65
Cenozoic 65–
Tertiary 65–2
Paleocene 65–55
Eocene 55–35
Oligocene 35–25
Miocene 25–5
Pliocene 5–2
Quaternary 2–
Pleistocene 2–0.01
Holocene 0.01–
Note: The complete list of periods is presented only for the Phanerozoic
Eon.
Note: The complete list of epochs is presented only for the Cenozoic era.
Note: The Holocene epoch is also called the Recent epoch.
Note: The Cenozoic era is now frequently divided into the Paleogene
period (Paleocene, Eocene, Oligocene epochs) and the Neogene
period (Miocene, Pliocene, Pleistocene, and Holocene epochs).
of the Tertiary and Quaternary periods are included with
their respective periods. In the accompanying table, as well
as in each entry, the time periods have been rounded to the
nearest five million years.
Further Reading
Museum of Paleontology, University of California, Berkeley. “The
paleontology portal.” Available online. URL: http://www.
paleoportal.org/. Accessed May 3, 2005.
Goodall, Jane (1934– ) British Primatologist Jane Goodall
is a leading world authority on the species that is one of
humankind’s closest relatives, the chimpanzee (see primates).
In order to understand how humans are different from other