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Biology
Biology
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2. 17-3 Evolution of Multicellular Life
Photo credit: ©D.W. Miller
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Geologic Time Scale
Clock Model of Earth’s History
First humans
Radiation of mammals
First land plants
First prokaryotes
First multicellular organisms
Cenozoic Era Mesozoic Era Paleozoic Era Precambrian Time
Earth’s history is often compared to a familiar measurement, such as the twelve hours between noon and midnight. In such a comparison, notice that Precambrian Time lasts from noon until after 10:30 pm.
First eukaryotes
Accumulation of
atmospheric
oxygen
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Precambrian Time
Precambrian Time
Few fossils exist from Precambrian time because the animals were all soft-bodied.
Life existed only in the sea.
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Paleozoic Era
Paleozoic Era
Rich fossil evidence shows that early in the Paleozoic Era, there was a diversity of marine life.
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Trilobites
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Crinoid
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Paleozoic Era
The Paleozoic Era is divided into the following periods:
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
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Paleozoic Era
Cambrian Period 
The diversification of life at this time is called the “Cambrian Explosion.”
The first known representatives of most animal phyla evolved. These included:
invertebrates
brachiopods
trilobites
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Paleozoic Era
Carboniferous and Permian Periods 
Reptiles evolved from certain amphibians.
Winged insects evolved into many forms.
Giant ferns and other plants formed vast swampy forests.
Remains of ancient plants formed thick deposits of sediment that changed into coal over millions of years.
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Paleozoic Era
At the end of the Permian Period, there was a mass extinction in which many living things became extinct at the same time.
The mass extinction at the end of the Paleozoic affected both plants and animals on land and in the seas. As much as 95% of the complex life in the oceans disappeared.
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Mesozoic Era
Mesozoic Era
During the Mesozoic Era, dinosaurs became dominant. The Mesozoic is also marked by the appearance of flowering plants.
The Mesozoic Era is often called the Age of Reptiles.
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Mesozoic Era
Jurassic Period 
Dinosaurs became the dominant animals on land.
One of the first birds, Archaeopteryx, appeared.
Many paleontologists think that birds are close relatives of dinosaurs.
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Archaeopteryx
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Mesozoic Era
The Cretaceous Period ended with a mass extinction.
More than half of all plant and animal groups were wiped out, including all of the dinosaurs.
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Cenozoic Era
Cenozoic Era
During the Cenozoic, mammals evolved adaptations that allowed them to live in various environments—on land, in water, and even in the air.
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Cenozoic Era
The Cenozoic often is called the Age of Mammals.
The Cenozoic is divided into the two periods:
Tertiary Period
Quaternary Period
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17-3
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17-3
The first vertebrates to appear were the
jawless fishes.
eurypterids.
brachiopods.
reptiles.
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17-3
Coal was formed over millions of years from sediments deposited during the
Ordovician Period.
Carboniferous Period.
Cambrian Period.
Permian Period.
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17-3
The late Triassic Period saw the appearance of the first
reptiles.
giant ferns.
mammals.
fish.
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17-3
The term Cambrian Explosion refers to
the impact of a giant meteor that extinguished much of life on Earth.
the end of the Cambrian Period when many species were replaced by other species.
the first appearance of a great diversity of life representing most animal phyla.
a volcanic eruption that buried many of the fossils of Cambrian age.
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17-3
The fossil record indicates that mammals
were large meat-eaters that caused the extinction of the dinosaurs.
appeared in the Triassic Period but did not flourish until the dinosaurs became extinct.
successfully competed with the dinosaurs because they protected their young.
are the ancestors of modern birds.
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