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22
The Changing Face of Earth
• Climatic conditions have fluctuated through
Earth’s history.
• At times, Earth was colder than it is today.
• Usually climates change slowly, but major climatic
shifts have taken place over periods as short as
5,000 to 10,000 years.
• For example, during one Quaternary interglacial
period, the Antarctic Ocean changed from being
ice-covered to being nearly ice-free in less than
100 years.
Figure 22.5 Hot/Humid and Cold/Dry Conditions Have Alternated Over Earth’s History
22
The Changing Face of Earth
• Although most volcanic eruptions produce only
local or short-lived effects, a few very large
eruptions have had major consequences for life.
• The collision of continents during the late Permian
(about 275 mya) created a single, giant land mass
called Pangea and caused massive volcanic
eruptions.
• Ash from the eruptions reduced the penetration of
sunlight to Earth’s surface, lowering temperatures,
reducing photosynthesis, and triggering massive
glaciation.
22
The Changing Face of Earth
• Collisions with large meteorites are rare, but they
have been responsible for several mass extinctions.
• Evidence for these collisions includes:
 Impact craters
 Rock disfigurations such as shocked quartz
crystals
 Abundant fern fossils suggesting that meteorite
impacts had scoured vast areas of Earth’s
surface
22
The Changing Face of Earth
• The first impact to be documented was that of a
meteorite 10 km in diameter that caused a mass
extinction at the end of the Cretaceous.
• Then a 180-km-diameter crater buried beneath
the northern coast of the Yucatán Peninsula of
Mexico was discovered.
22
The Fossil Record
• Fossils are a major source of information about
changes on Earth during the remote past.
• Periods of geological history are marked by mass
extinctions or by dramatic increases in diversity
called evolutionary radiations.
• Although the fossil record is fragmentary before
550 mya, it is still good enough to show that the
total number of species and individuals increased
dramatically in late Precambrian times.
22
The Fossil Record
• Most species exist, on average, for fewer than 10
million years; therefore, Earth’s species must
have turned over many times during geological
history.
• Combining data about physical events with
evidence from the fossil record, scientists can
compose pictures of what Earth and its
inhabitants looked like at different times.
22
Major Patterns in the History of Life on Earth
• For much of its history, life was confined to the
oceans.
• Shallow Precambrian seas teemed with life,
including protists and algae.
• By the late Precambrian, many kinds of softbodied invertebrates had evolved.
22
Major Patterns in the History of Life on Earth
• By the early Cambrian period (543–490 mya),
atmospheric O2 levels had nearly reached current
levels.
• The continental plates came together in several
masses. .
• The rapid diversification of life that took place at
this time is referred to as the Cambrian explosion.
• The best fossils of Cambrian animals are found in
China.
• A mass extinction occurred at the end of the
Cambrian period.
22
Rates of Evolutionary Change within Lineages
• Species that have changed little over millions of
years are known as “living fossils,” such as
Gingko from the Triassic.
• Horseshoe crabs living today are almost identical
to those that lived 300 million years ago.
• The lack of new environmental selective
pressures means that horseshoe crabs have not
needed new adaptations to continue flourishing in
these coastlines.
Figure 22.18 “Living Fossils”
22
Rates of Evolutionary Change within Lineages
• In some lineages, periods of gradual evolution
have been interrupted by periods in which
changes in the physical or biological environment
created conditions favorable for the rapid
evolution of new traits.
22
Rates of Evolutionary Change within Lineages
• In some cases, evolutionary change is rapid
enough to be measured directly. The house finch
provides an example.
22
Rates of Evolutionary Change within Lineages
• More than 99% of the species that have ever lived
are extinct.
• Each mass extinction changed the flora and fauna
of Earth.
• Traits that favor survival during normal times may
be different from those that favor survival during a
mass extinction.
• Because major changes on land and in oceans
did not always coincide, the mass extinctions had
different effects on different groups of organisms.
22
The Future of Evolution
• Evolution is taking place today.
• However, major changes are underway due to
human influence.
• Until recently, humans caused extinctions mostly
of large vertebrates.
• Small species are now more commonly being
rendered extinct due to human-caused changes
to Earth’s ecosystems.
• Artificial selection and biotechnology are also
important man-made evolutionary factors.
• Humans have become the dominant evolutionary
force on Earth today.