Transcript Table of Contents - Milan Area Schools

The History of Life on Earth
22
The History of Life on Earth
• Defining Biological Evolution
• Determining Earth’s Age
• The Changing Face of Earth
• The Fossil Record
• Major Patterns in the History of Life on Earth
• Rates of Evolutionary Change within Lineages
• The Future of Evolution
22
Defining Biological Evolution
• Understanding evolution is important because the
features of all organisms are best understood in
the light of evolution.
• It is also important because humans are
becoming powerful agents of evolutionary
change.
22
Defining Biological Evolution
• Biological evolution is a change over time in the
genetic composition of a population of organisms.
• Some changes can occur rapidly enough to be
manipulated experimentally; others take place
over very long time frames.
• An understanding of the long-term patterns of
evolutionary change requires thinking in time
frames spanning many millions of years and
imagining conditions on Earth that are very
different from those we observe today.
22
Determining Earth’s Age
• Determining the actual age of rocks is difficult.
Determining the ages of rocks relative to one
another is easier.
• Geologists use the observation that in undisturbed
strata (layers), young rocks are found on top of
older rocks.
• Fossils are remains of ancient organisms
contained within rocks.
• In general, fossils of similar ages are found in
similar strata across the earth.
Figure 22.1 Young Rocks Lie on Top of Old Rocks
22
Determining Earth’s Age
• Radioactivity provides a way to date rocks.
• Radioactive isotopes decay in a predictable
pattern over long periods of time.
• The time it takes for half of a radioactive isotope
to decay is that isotope’s half-life.
• Each radioisotope has a characteristic half-life.
22
Determining Earth’s Age
• To use a radioisotope to date a past event, the
concentration of the isotope at the time of that
event must be known or estimated.
• In the case of 14C, we know that the ratio of 14C to
12C is relatively constant in the environment and
living organisms. When an organism dies, 14C is
no longer taken up by the cells, and the ration of
14C to 12C decreases through time.
•
14C
can be used to date fossils (and sedimentary
rocks they were deposited in) less than 50,000
years old.
22
Determining Earth’s Age
• Sedimentary rocks are unreliable for dating.
• To date sedimentary rocks, geologists look for
lava flows between sedimentary layers. The lava
can be dated by the decay of potassium-40 to
argon-40.
• When radioactive dating methods are not
applicable, alternative approaches and
observations are used, including paleomagnetism,
continental drift, sea level changes, and molecular
clocks.
22
Determining Earth’s Age
• Using information from these dating methods,
geologists have divided Earth’s history into eras
and periods.
• Boundaries between the divisions are based on
major differences in the fossil organisms
contained in the layers.
• The divisions were established before the actual
ages of the eras and periods were known.
• In the Precambrian era, early life evolved.
Table 22.1 Earth’s Geological History (Part 1)
Table 22.1 Earth’s Geological History (Part 2)
22
The Changing Face of Earth
• Earth’s crust consists of solid plates that float on a
fluid mantle.
• The mantle is heated by energy from radioactive
decay in the Earth’s core. Convection currents of
mantle fluid cause the crust plates to move.
• The process of plate movement is known as
continental drift.
• Throughout Earth’s history, the plates that carry the
continents have drifted apart and moved back
together numerous times.
• Plate movement has affected climate, sea level,
and the distribution of organisms.
Figure 22.2 Sea Levels Have Changed Repeatedly
22
The Changing Face of Earth
• Earth’s atmosphere has also changed since the
time the planet formed when little or no free
oxygen was present.
• Oxygen concentrations began to increase
significantly about 2.5 billion years ago when
some prokaryotes evolved the ability to split water
as a source of hydrogen ions for photosynthesis.
The waste product is O2.
• One lineage of these oxygen-generating bacteria
evolved into the cyanobacteria. These organisms
formed rocklike structures called stromatolites.
• The cyanobacteria liberated enough O2 to allow
the evolution of oxidation reactions as the energy
source for the synthesis of ATP.
Figure 22.3 Stromatolites
22
The Changing Face of Earth
• As life continued to evolve, the physical nature of the
plant was irrevocably changed.
• Living organisms not only added O2 to the atmosphere
but also removed CO2 from it.
• An atmosphere rich in O2 made possible the evolution
of larger cells and more complex organisms.
• About 1,500 mya, O2 concentrations became high
enough for large eukaryotic cells to flourish and
diversify.
• By 750–700 mya, O2 had increased to levels that
could support multicellular organisms.
Figure 22.4 Larger Cells Need More Oxygen
22
The Changing Face of Earth
• Unlike the unidirectional change in Earth’s
atmospheric O2 content, most physical attributes
on Earth have involved irregular oscillations.
• External events such as collisions with meteorites
have also affected Earth, sometimes resulting in
mass extinctions.
22
The Changing Face of Earth
• Climatic conditions have fluctuated through
Earth’s history.
• At times, Earth was colder than it is today; large
areas were covered with glaciers at the end of the
Precambrian and during the Carboniferous,
Permian, and Quaternary periods.
• 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
 Helium and argon within giant molecules that
have isotopic ratios characteristic of meteorites
 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.
 Abnormally high concentrations of iridium in a
thin layer separating the Cretaceous and
Tertiary rocks was found.
 Iridium is very rare on Earth but abundant in
some meteorites.
• Then a 180-km-diameter crater buried beneath
the northern coast of the Yucatán Peninsula of
Mexico was discovered.
Figure 22.6 Evidence of a Meteorite Impact
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.
• Evidence suggests that the major divisions in
many animal lineages predate the end of the
Precambrian by more than 100 million years.
• 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
• An organism is most likely to become a fossil if its
dead body is deposited in an environment that
lacks oxygen.
• About 300,000 species of fossil organisms have
been described.
• 1.7 million species of present-day biota have been
named.
• The actual number of living species is probably at
least 10 million.
• Most species exist, on average, for fewer than 10
million years; therefore, Earth’s species must
have turned over many times during geological
history.
22
The Fossil Record
• Among the nine major animal groups with hardshelled members, approximately 200,000 species
have been described from fossils.
• The fossil record is especially good for marine
animals that had hard skeletons.
• Insects and spiders are also well represented in
the fossil record.
• 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.
Figure 22.7 Insect Fossils
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, some of which
may be members of animal lineages that have no
living descendants.
Figure 22.8 Ediacaran Animals
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. Gondwana was the largest.
• 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.
Figure 22.9 Cambrian Continents and Animals (Part 1)
Figure 22.9 Cambrian Continents and Animals (Part 2)
22
Major Patterns in the History of Life on Earth
• During the Ordovician period (490–443 mya), the
continents were mostly in the Southern
Hemisphere.
• Evolutionary radiation of marine organisms was
intense. Animals lived on the sea floor or burrowed
in sediments.
• Ancestors of club mosses and horsetails colonized
wet terrestrial environments.
• At the end of the Ordovician, sea levels dropped
about 50 meters, and glaciers formed over
Gondwana. 75% of marine species became
extinct.
22
Major Patterns in the History of Life on Earth
• In the Silurian period (443-417 mya), northern
continents coalesced, but their general position
did not change.
• Marine organisms rebounded from the Ordovician
extinction, but few new species evolved.
• The tropical sea was uninterrupted by land
barriers; therefore, marine organisms dispersed
widely.
• The first known tracheophytes appeared on land
in the late Silurian.
Figure 22.10 Cooksonia, the Earliest Known Tracheophyte
22
Major Patterns in the History of Life on Earth
• During the Devonian period (417–354 mya), rates
of evolutionary change accelerated. Land masses
slowly moved northward.
• Evolutionary radiation of marine animals such as
coral and shelled cephalopods was high.
• All current major groups of fishes were present by
the end of the Devonian.
• On land, club mosses, tree ferns, and horsetails
became common and the first gymnosperms
appeared.
• Fishlike amphibians began to occupy the land.
• At the end of the Devonian, 75% of marine
species went extinct.
Figure 22.11 Devonian Continents and Marine Communities (Part 1)
Figure 22.11 Devonian Continents and Marine Communities (Part 2)
22
Major Patterns in the History of Life on Earth
• The Carboniferous period (354–290 mya) was
marked by large glaciers formed at high latitudes
and extensive swamp forests grew on the tropical
areas of the continents.
• Fossilized remains of the forests formed the coal
we now mine for energy.
• Diversity of terrestrial animals increased greatly.
• Snails, scorpions, centipedes, and insects were
abundant.
• Amphibians became larger, and reptiles evolved
from one amphibian lineage.
• Crinoids were plentiful on the seafloor.
Figure 22.12 A Carboniferous “Crinoid Meadow”
22
Major Patterns in the History of Life on Earth
• During the Permian (290–2458 mya), the
continents coalesced into a supercontinent called
Pangaea.
• Massive volcanic eruptions poured lava over large
areas of Earth.
• Ash produced from the eruptions blocked sunlight
and cooled the climate, resulting in the largest
glaciers in Earth’s history.
• By the end of the Permian, reptiles greatly
outnumbered amphibians.
• The lineage leading to mammals diverged from one
line of reptiles. Bony fishes radiated in the oceans.
Figure 22.13 Pangaea Formed in the Permian Period
22
Major Patterns in the History of Life on Earth
• At the end of the Permian, a large meteorite
crashed into northwestern Australia.
• Volcanic eruptions poured lava into the oceans,
which depleted O2 in deep oceans. Oceanic
turnover then carried the depleted water to the
surface where it released toxic CO2 and H2S.
• About 96% of all species on Earth became
extinct.
22
Major Patterns in the History of Life on Earth
• At the start of the Mesozoic era (248 mya), the
few surviving organisms found themselves in a
relatively empty world.
• Pangaea slowly separated, glaciers melted, and
shallow inland seas formed.
• Life proliferated and diversified.
• Earth’s biota diversified and became distinct on
each continent.
22
Major Patterns in the History of Life on Earth
• In the Triassic period (248–206 mya), vertebrate
lineages became more diverse.
• Conifers and seed ferns became the dominant
trees.
• Frogs and turtles appeared.
• A great radiation of reptiles began, which gave
rise to dinosaurs, crocodilians, and birds.
• The end of the Triassic was marked by a mass
extinction that eliminated 65% of the species on
Earth.
• A large meteor that crashed into Quebec may
have been responsible.
22
Major Patterns in the History of Life on Earth
• During the Jurassic (206–144 mya), two large
continents formed—Laurasia in the north and
Gondwana in the south.
• Ray-finned fishes began the great radiation that
culminated in their dominance of the oceans.
• Salamanders and lizards first appeared.
• Flying reptiles evolved.
• Dinosaur lineages evolved into bipedal predators
and quadrupedal herbivores.
• Mammals first appeared.
22
Major Patterns in the History of Life on Earth
• By the Cretaceous period (144–65 mya),
Gondwana was beginning to break apart, and a
continuous ocean circled the tropics. Sea levels
were high and the Earth was warm and humid.
• Flowering plants (angiosperms) evolved from
gymnosperms. Many groups of mammals had
evolved, but most were small.
• Another mass extinction marked the end of the
Cretaceous; it was probably caused by a large
meteorite colliding with Earth.
• All vertebrates larger than about 25 kg in body
weight, including all of the dinosaurs, apparently
became extinct as a result of this impact.
Figure 22.15 Positions of the Continents during the Cretaceous Period
Figure 22.16 Flowering Plants of the Cretaceous
22
Major Patterns in the History of Life on Earth
• By the early Cenozoic era (65 mya), the
continents were close to their present-day
positions; however, Australia was still attached to
Antarctica.
• The Cenozoic area was characterized by an
extensive radiation of mammals.
• Flowering plants diversified and dominated the
world’s forests.
• The Cenozoic is divided into two periods, the
Tertiary and the Quaternary.
22
Major Patterns in the History of Life on Earth
• In the Tertiary period (65–1.8 mya), Australia
began its drift northward. By 20 mya, it had nearly
reached its current position.
• The climate became drier and cooler.
• Grasslands spread over much of Earth.
• Invertebrates resembled those of today.
• Birds, mammals, and reptiles underwent
extensive radiations.
22
Major Patterns in the History of Life on Earth
• The current geological period is the Quaternary
(1.8 mya–present), and is divided into two
epochs, the Pleistocene and the Holocene.
• The Pleistocene epoch was a time of climate
fluctuations, including four major episodes of
glaciation; but there were few extinctions.
• The last of the Pleistocene glaciers retreated from
temperate latitudes less than 15,000 years ago.
• In the current Holocene epoch, some organisms
are still adjusting to climatic fluctuations.
• Many high-latitude ecological communities have
occupied their current locations for no more than
a few thousand years.
22
Major Patterns in the History of Life on Earth
• During the Pleistocene, hominids evolved and
radiated, resulting in the species Homo sapiens.
• Many birds and mammals became extinct in North
America, South America, and Australia when H.
sapiens arrived on those continents.
• Hunting may have caused the extinctions, but
there is still debate among paleontologists on the
topic.
22
Major Patterns in the History of Life on Earth
• Three great radiations have resulted in the
evolution of major new faunas.
• The Cambrian explosion marked the appearance
of all major present-day lineages.
• Paleozoic and Triassic explosions greatly
increased the number of families, genera, and
species, but no new organismal body plans
evolved.
Figure 22.17 Evolutionary Faunas
22
Rates of Evolutionary Change within Lineages
• The fossil record shows that no single pattern
characterizes evolutionary rates.
• In some species, there has been little change
over many millions of years.
• Other species have changed gradually over this
time period.
• Still other species have undergone rapid change
for short periods of time, followed by long periods
of slow change.
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 sandy coastlines where they spawn are harsh
environments that have changed little over
millennia.
• 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”
Figure 33.16 Minor Chelicerate Phyla (Part 2)
22
Rates of Evolutionary Change within Lineages
• Evolutionary changes have been gradual in some
lineages.
• The series of fossils showing changes in the
number of ribs on the exoskeleton of eight
trilobites during the Ordovician provides a good
example of gradual changes in lineages of
organisms over time.
Figure 22.19 Rib Number Evolved Gradually in Trilobites (Part 1)
Figure 22.19 Rib Number Evolved Gradually in Trilobites (Part 2)
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.
• The fossil record of stickleback fish demonstrates
how new conditions can lead to rapid evolutionary
change.
Figure 22.20 Natural Selection Acts on Stickleback Spines
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.
• Before 1939, these birds were confined to arid and
semiarid parts of western North America.
• In that year, some captive finches were released
into New York City, where some survived to form
small breeding populations.
• By the 1990s, the house finch had spread across
the eastern U.S. and southern Canada.
• By 2000, birds in finch populations that had been
separated for only a few decades had large
differences in body size.
Figure 22.21 House Finches Expanded their Range in North America (Part 1)
Figure 22.21 House Finches Expanded their Range in North America (Part 2)
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.