The History of Life on Earth

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The History of Life on Earth
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21 The History of Life on Earth

• 21.1 How Do Scientists Date Ancient Events?
• 21.2 How Have Earths Continents and Climates
  Changed over Time?
• 21.3 What Are the Major Events in Lifes History?
• 21.4 Why Do Evolutionary Rates Differ among
  Groups of Organisms?

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21.1 How Do Scientists Date Ancient Events?

• Many evolutionary changes take place over long
  periods of time.
• To study long-term evolutionary change, we must
  think in time frames spanning millions of years
  and imagine conditions very different from
  todays.

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21.1 How Do Scientists Date Ancient Events?

• Fossils are preserved remains of ancient
  organisms, they tell us about body form or
  morphology, and where and how they lived.
• Earths history is recorded in rocks. Layers of
  rocks are called strata.

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21.1 How Do Scientists Date Ancient Events?

• Relative ages of rocks can be determined by
  looking at strata of undisturbed sedimentary
  rock. The oldest layers are at the bottom,
  youngest at the top.
• First observed in the 17th century by Nicolaus
  Steno.

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Chapter Opener 2 Younger Rocks Lie on Top of
Older Rocks
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21.1 How Do Scientists Date Ancient Events?

• In the eighteenth century, geologists realized
  that fossils could also be used to age rocks.
• Certain fossils were always found in younger
  rocks, others were found in older rocks.
• Fossils in more recent strata were more similar
  to modern organisms.

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21.1 How Do Scientists Date Ancient Events?

• Radioisotopes can be used to determine the actual
  age of rocks.
• Radioisotopes decay in a predictable pattern.
• Half-life is the time interval over which one
  half of the remaining radioisotope decays,
  changing into another element.

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Figure 21.1 Radioactive Isotopes Allow Us to Date
Ancient Rocks
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Table 21.1
Each radioisotope has a characteristic half-life.
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21.1 How Do Scientists Date Ancient Events?

• To date an event, we must know (or be able to
  estimate) the concentration of the radioisotope
  at the start of the event.
• For 14C, production in the upper atmosphere is
  about equal to its natural decay.
• In an organism, the ratio of 14C to 12C stays
  constant during its lifetime.

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21.1 How Do Scientists Date Ancient Events?

• When an organism dies, it is no longer
  incorporating 14C from the environment.
• The 14C that was present in the body decays with
  no replacement and the ratio of 14C to 12C
  decreases.
• This ratio can then be used to date fossils, up
  to about 50,000 years old.

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21.1 How Do Scientists Date Ancient Events?

• Sedimentary rocks can not be dated accurately
  the materials that form the rocks existed for
  varying lengths of time before being transported
  and converted to rock.
• But igneous rocks (e.g., lava or volcanic ash),
  that have intruded into layers of sedimentary
  rock can be dated.

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21.1 How Do Scientists Date Ancient Events?

• Other radioisotopes are used to date older rocks.
• Decay of potassium-40 to argon-40 is used for the
  most ancient rocks.
• Radioisotope dating is combined with fossil
  analysis.

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21.1 How Do Scientists Date Ancient Events?

• Other dating methods include paleomagnetic
  dating
• Movement and reversals of Earths magnetic poles
  are recorded in igneous and sedimentary rocks at
  the time they were formed, by alignment of
  mineral grains and other characteristics.

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21.1 How Do Scientists Date Ancient Events?

• The history of life is divided into geologic
  eras, which are subdivided into periods.
• Boundaries are based on changes in fossils.
• The eras were established before actual ages of
  rocks were known.

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Table 21.2 (Part 1)
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Table 21.2 (Part 2)
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• The idea that land masses have moved over time
  was first suggested by Alfred Wegener in 1912.
• By the 1960s, evidence of plate tectonics
  convinced geologists that he was right.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Earths crust is divided into solid plates about
  40 km thickcollectively, the lithosphere.
• The plates float on a fluid layer of liquid rock
  or magma.
• Heat from radioactive decay in Earths core
  causes the magma to circulate, setting up
  convection currents.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• The movement of plates is called continental
  drift.
• Where plates are pushed together, they move
  sideways past one another, or one is pushed
  underneath the other.
• Mountain ranges are pushed up, and deep rift
  valleys or trenches are formed.
• Where plates are pushed apart, ocean basins form.

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Figure 21.2 Plate Tectonics and Continental Drift
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Position of the continents has changed
  dramatically over time.
• Influences of ocean circulation patterns, sea
  level, and global climate
• Mass extinctions of marine animals have occurred
  when sea level dropped, exposing the continental
  shelves.

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Figure 21.3 Sea Levels Have Changed Repeatedly
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Earths atmosphere has also changed.
• Early atmosphere probably contained little or no
  free oxygen (O2).
• O2 began to increase when certain bacteria
  evolved the ability to use H2O as a source of H
  ions in photosynthesis. O2 was a waste product.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Cyanobacteria formed rock-like structures called
  stromatolites which are abundant in the fossil
  record.
• Enough O2 was liberated to allow evolution of
  oxidation reactions to synthesize ATP.

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Figure 21.4 Stromatolites (Part 1)
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Figure 21.4 Stromatolites (Part 2)
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• The evolution of life changed the physical nature
  of Earth.
• These changes in turn influenced the evolution of
  life.
• When O2 first appeared in the atmosphere it was
  poisonous to the anaerobic prokaryotes.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Some evolved the ability to metabolize the O2.
• Advantages aerobic metabolism is faster and more
  energy is harvested.
• Aerobes replaced anaerobes in most environments.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Atmospheric O2 also made possible larger and more
  complex cells.
• About 1 billion years ago, eukaryote cells
  appeared.

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Figure 21.5 Larger Cells Need More Oxygen
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Change in atmospheric O2 concentrations was
  unidirectional.
• Most physical conditions have oscillated over
  time in response to drifting continents, volcanic
  activity, and even extraterrestrial events such
  as meteorite impacts.
• Sometimes these events caused mass extinctions.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Earths climate has changed over time.
• Sometimes Earth was considerably hotter than
  today sometimes colder, with extensive
  glaciation.

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Figure 21.6 Hot/Humid and Cold/Dry Conditions
Have Alternated over Earths History
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• For Earth to be cold and dry, atmospheric CO2
  must have been much lower, but it is unclear what
  would cause low concentrations.
• Some climate changes have been very rapid.
  Extinctions caused by them appear to be
  instantaneous in the fossil record.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Today we are in a period of rapid climate change
  due to increasing CO2 concentrations, mostly from
  burning fossil fuels.
• Current CO2 concentration is greater than it has
  been for several thousand years.
• If CO2 concentration doubles, average Earth
  temperature will increase, causing droughts, sea
  increase, melting ice caps, and other major
  changes.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Volcanic eruptions can trigger major climate
  change.
• When continents came together to form Pangaea in
  the Permian period, many volcanic eruptions
  reduced sunlight penetration and thus
  photosynthesis.
• Massive glaciation resulted.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• Collisions with large meteorites are probably the
  cause of several mass extinctions.
• Evidence of impacts include large craters and
  disfigured rocks molecules with helium and argon
  isotope ratios characteristic of meteorites.

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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• A meteorite is thought to have caused or
  contributed to the mass extinction at the end of
  the Cretaceous period, 65 million years ago.
• First evidence was from a thin layer containing
  the element iridium. This element is very rare on
  Earth but abundant in some meteorites.

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Figure 21.7 Evidence of a Meteorite Impact
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21.2 How Have Earths Continents and
ClimatesChanged over Time?

• A large crater has been located beneath the
  northern coast of the Yucatán Peninsula, Mexico.
• A massive plume of debris from the impact heated
  the atmosphere, ignited fires, and blocked the
  sunlight.
• Settling debris formed the iridium-rich layer.

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An Artists Conception of the Presumed Meteorite
Impact of 65 Million Years Ago
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21.3 What Are the Major Events in Lifes History?

• Life first evolved about 3.8 billion years ago.
• Eukaryotic organisms had evolved by about 1.5
  billion years ago.
• The number of individuals and species increased
  dramatically in the late Precambrian.

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21.3 What Are the Major Events in Lifes History?

• The assemblage of all kinds of organisms alive at
  one time (or in one place) is called the biota.
• All the plants are the flora and all the animals
  are the fauna.

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21.3 What Are the Major Events in Lifes History?

• Although about 300,000 species of fossils have
  been described, they are only a tiny fraction of
  species that have existed on Earth.
• Only a tiny fraction of organisms become fossils,
  and only a fraction of those are studied by
  paleontologists.

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21.3 What Are the Major Events in Lifes History?

• Most organisms are decomposed quickly after
  death.
• If they are transported to sites with no oxygen,
  where decomposition is very slow, fossilization
  could occur.
• Many geologic processes transform rocks and
  destroy the fossils they contain.

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21.3 What Are the Major Events in Lifes History?

• A large number of fossil species are marine
  organisms that had hard shells or skeletons that
  resist decomposition.
• Insects and spiders are also well represented in
  the fossil record.

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Figure 21.8 Insect Fossils
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21.3 What Are the Major Events in Lifes History?

• The Precambrian Era
• For most of this era, life was microscopic,
  prokaryote cells living in oceans.
• Eukaryotes evolved about 2/3 through the
  Precambrian.
• By the late Precambrian, soft-bodied
  multicellular animals had evolved.

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Figure 21.9 Ediacaran Animals
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21.3 What Are the Major Events in Lifes History?

• Cambrian Period
• Beginning of the Paleozoic Era
• O2 concentration was approaching modern levels.
• Continents formed large land masses, the largest
  called Gondwana.

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Figure 21.10 Cambrian Continents and Fauna (Part
1)
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Figure 21.10 Cambrian Continents and Fauna (Part
2)
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21.3 What Are the Major Events in Lifes History?

• Rapid diversification of life took placecalled
  the Cambrian explosion.
• Most of the major groups of animals living today
  appeared in the Cambrian.
• Three different Cambrian fossil beds have
  preserved the soft parts of many animalsthe
  Burgess Shale, Sirius Passet, and Chengjiang site.

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21.3 What Are the Major Events in Lifes History?

• Ordovician Period
• A great radiation of marine organisms occurred,
  especially among the brachiopods and mollusks.
• At the end of the period, massive glaciers formed
  over Gondwana, sea levels were lowered, and a
  mass extinction occurred.

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21.3 What Are the Major Events in Lifes History?

• Silurian Period
• Marine life rebounded from the late Ordovician
  extinction.
• The first vascular plants appeared in the late
  Silurian, as well as some terrestrial
  arthropodsscorpions and millipedes.

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Figure 21.11 Cooksonia, the Earliest Known
Vascular Plant
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21.3 What Are the Major Events in Lifes History?

• Devonian Period
• The northern landmass (Laurasia) and southern
  landmass (Gondwana) moved towards each other.
• There were evolutionary radiations of corals and
  squid-like cephalopods.
• Jawed fishes replaced jawless forms.

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Figure 21.12 Devonian Continents and Marine
Communities (Part 1)
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Figure 21.12 Devonian Continents and Marine
Communities (Part 2)
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21.3 What Are the Major Events in Lifes History?

• Club mosses, horsetails, and tree ferns became
  common in terrestrial habitats.
• Their roots accelerated weathering of rocks and
  soil formation.
• Ancestors of gymnosperms appeared.
• First known fossils of centipedes, spiders,
  mites, and insects.
• Fish-like amphibians began to occupy land.

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21.3 What Are the Major Events in Lifes History?

• An extinction at the end of the Devonian resulted
  in loss of 75 percent of marine animals.
• Two meteorite impacts may have contributed to
  this extinction. The craters are in Nevada and
  western Australia.

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21.3 What Are the Major Events in Lifes History?

• Carboniferous Period
• Large glaciers formed over high latitudes but
  great swamp forests of horsetails and tree ferns
  grew on the tropical continents.
• These swamp plants became fossilized as coal.

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21.3 What Are the Major Events in Lifes History?

• Diversity of terrestrial animals increased.
  Snails, centipedes, scorpions, and insects were
  abundant.
• Insects developed wings. Flight gave them access
  to tall plants.
• Amphibians became larger their lineage split
  from the amniotesvertebrates with well-protected
  eggs.
• In the oceans, crinoids reached their greatest
  diversity.

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Figure 21.13 Evidence of Insect Diversification
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Figure 21.14 A Carboniferous Crinoid Meadow
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21.3 What Are the Major Events in Lifes History?

• Permian Period
• Continents came together to form the
  supercontinent Pangaea.
• Reptiles outnumbered amphibians at the end of
  that period.
• Ray-finned fishes diversified.

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Figure 21.15 Pangaea Formed in the Permian Period
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21.3 What Are the Major Events in Lifes History?

• Near the end of the Permian, massive volcanic
  eruptions poured lava over large areas of Earth.
• Volcanic ash blocked sunlight and caused climate
  cooling, resulting in the largest glaciers in
  Earths history.

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21.3 What Are the Major Events in Lifes History?

• O2 concentrations dropped to about 12
  percentmost animals would have been unable to
  survive at elevations above 500 m.
• A combination of factors resulted in the greatest
  mass extinction in Earths history.

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21.3 What Are the Major Events in Lifes History?

• At the start of the Mesozoic era, the surviving
  organisms inhabited a relatively empty world.
• The continents began to drift apart, sea levels
  rose, and flooded the continents forming large
  shallow seas.
• Three groups of phytoplankton became ecologically
  important dinoflagellates, coccolithophores, and
  diatoms.

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21.3 What Are the Major Events in Lifes History?

• New seed plants replaced the trees of the Permian
  forests.
• Earths biota became increasingly
  provincializeddistinct biotas evolved on each
  continent.

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21.3 What Are the Major Events in Lifes History?

• Triassic Period
• Pangaea began to break apart.
• On land, conifers and pteridosperms became
  dominant.
• A great radiation of reptiles began, which gave
  rise to crocodilians, dinosaurs, and birds.
• A mass extinction at the end may have been caused
  by a meteorite impact in present-day Quebec.

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21.3 What Are the Major Events in Lifes History?

• Jurassic Period
• Land once again in two continents, Laurasia and
  Gondwana.
• Ray-finned fishes began a great radiation.
• First salamanders, lizards, and flying reptiles
  (pterosaurs).

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21.3 What Are the Major Events in Lifes History?

• Dinosaur lineages evolved into predators on two
  legs, and large herbivores on four legs.
• Several groups of mammals appeared.
• Flowering plants appeared.

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Figure 21.16 Jurassic Parkland
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21.3 What Are the Major Events in Lifes History?

• Cretaceous Period
• A continuous sea encircled the tropics. Earth
  was warm and humid.
• Dinosaurs continued to diversify.
• Flowering plants began the radiation that led to
  their current dominance.

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Figure 21.17 Positions of the Continents during
the Cretaceous Period
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Figure 21.18 Flowering Plants of the Cretaceous
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21.3 What Are the Major Events in Lifes History?

• By the end of the period, many mammal groups had
  evolved.
• Another mass extinction at the end of the
  Cretaceous was caused by a meteorite.
• On land, all animals larger than about 25 kg
  became extinct.
• Many insects went extinct, perhaps because of
  lack of food plants.

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21.3 What Are the Major Events in Lifes History?

• The Cenozoic Era
• Characterized by an extensive radiation of
  mammals.
• Flowering plants came to dominate forests except
  in cool regions.
• Mutations in one group of plants allowed them to
  form symbiotic associations with N-fixing
  bacteria. This dramatically increased N available
  for terrestrial plants.

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Table 21.3
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21.3 What Are the Major Events in Lifes History?

• Tertiary Period
• Climate was hot and humid at the beginning, but
  became cooler and drier about half way through.
• Many flowering plants evolved herbaceous forms.
  Grasslands spread.
• Snakes, lizards, birds, and mammals underwent
  extensive radiations.

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21.3 What Are the Major Events in Lifes History?

• Three waves of mammals dispersed from Asia to
  North America across the Bering land bridge.
• Rodents, marsupials, primates, and hoofed mammals
  appeared in North America for the first time.

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21.3 What Are the Major Events in Lifes History?

• Quaternary Period
• Divided into Pleistocene and Holocene epochs.
• Pleistocene was a time of drastic cooling and
  climate fluctuation.
• During four major and 20 minor ice ages,
  continental glaciers spread, shifting the ranges
  of plants and animals towards the equator.

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21.3 What Are the Major Events in Lifes History?

• The last glaciers retreated from temperate
  latitudes about 15,000 years ago.
• The Pleistocene was also the time of hominid
  evolution and radiation.
• Many large mammal species became extinct in
  Australia and the Americas when Homo sapiens
  arrivedpossibly due to hunting pressure.

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21.3 What Are the Major Events in Lifes History?

• Three great evolutionary radiations occurred that
  resulted in major new faunas.
• The Cambrian explosion
• 60 million years later, the radiation that
  resulted in the Paleozoic fauna
• After the Permian extinction, in the Triassic

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Figure 21.19 Evolutionary Faunas
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21.4 Why Do Evolutionary Rates Differ among
Groups of Organisms?

• The rate of evolutionary change has varied
  greatly at different times and in different
  lineages.
• Changes in the physical and biological
  environment are likely to stimulate evolutionary
  change.
• Climate change can shift ranges of organisms,
  bringing them into contact with previously
  unknown competitors or predators.

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21.4 Why Do Evolutionary Rates Differ among
Groups of Organisms?

• Species whose morphology has changed little over
  millions of years are called living fossils.
• Examples horseshoe crabs, chambered nautilus,
  Gingko trees

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Figure 21.20 Living Fossils
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21.4 Why Do Evolutionary Rates Differ among
Groups of Organisms?

• On average, rates of evolutionary change are very
  slow.
• There are many series of fossils that show
  gradual change.
• Example Eight lineages of trilobites show
  gradual change in the number of rear dorsal ribs
  on the exoskeleton.

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Figure 21.21 Rib Number Evolved Gradually in
Trilobites (Part 1)
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Figure 21.21 Rib Number Evolved Gradually in
Trilobites (Part 2)
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21.4 Why Do Evolutionary Rates Differ among
Groups of Organisms?

• Gradual change appears to dominate the fossil
  record.
• One explanation is that climate change is usually
  slow.
• Ranges of organisms shift accordingly, so the
  environment in which an individual lived actually
  changed very little.
• As the climate warmed after the last glaciers,
  plants and animals shifted their ranges northward.

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Figure 21.22 Some Species Expanded Their Ranges
as Continental Glaciers Retreated (Part 1)
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21.4 Why Do Evolutionary Rates Differ among
Groups of Organisms?

• If the environment changes rapidly, some lineages
  may change rapidly.
• Example The house finch lived in semiarid
  regions of western North America.
• It was released in New York City in 1939 and
  formed a small population there.
• Now birds in the eastern populations have already
  evolved distinct differences.

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Figure 21.23 House Finches Changed Rapidly as
Their Range Expanded (Part 1)
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Figure 21.23 House Finches Changed Rapidly as
Their Range Expanded (Part 2)
101
21.4 Why Do Evolutionary Rates Differamong
Groups of Organisms?

• Rates of extinction have also varied.
• The five major extinction events reduced the
  biota, and were followed by high rates of
  evolution.
• But some groups have had high rates of extinction
  while others are proliferating.

102
21.4 Why Do Evolutionary Rates Differamong
Groups of Organisms?

• At the end of the Cretaceous, groups of related
  mollusk species with large geographic ranges
  survived better than groups with small ranges,
  even if individual species in the group had small
  ranges.

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21.4 Why Do Evolutionary Rates Differamong
Groups of Organisms?

• An organisms diet can also affect extinction
  rates.
• Animals with specialized diets are more
  vulnerable to loss of their food supply.
• Large, specialized carnivores may be more likely
  to go extinct than small carnivores with
  generalized diets.
• This hypothesis has been tested using canid
  fossils.

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Figure 21.24 Large, Specialized Canids Survived
Shorter Times (Part 1)
105
Figure 21.24 Large, Specialized Canids Survived
Shorter Times (Part 2)
106
21.4 Why Do Evolutionary Rates Differamong
Groups of Organisms?

• Although agents of evolutionary change are
  operating today as they have in the past, the
  dramatic increase in human population is driving
  major changes.
• Hunting has caused extinction of many species.
• Humans drastically alter the vegetation of
  Earthconverting forests and grasslands to
  agricultural land.

107
21.4 Why Do Evolutionary Rates Differamong
Groups of Organisms?

• Humans move thousands of species around the
  globe, deliberately and accidentally changing the
  ranges of species.
• Humans practice artificial selection and
  biotechnology that influences the evolution of
  some species.
• Humans have become a dominant agent of
  evolutionary change.