The History of Life on Earth

1
The History of Life on Earth
2
The History of Life on Earth

• Defining Biological Evolution
• Determining Earths 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

3
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.

4
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.

5
Determining Earths 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.

6
Figure 22.1 Young Rocks Lie on Top of Old Rocks
7
Determining Earths 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 isotopes half-life.
• Each radioisotope has a characteristic half-life.

8
Determining Earths 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.

9
Determining Earths 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.

10
Determining Earths Age

• Using information from these dating methods,
  geologists have divided Earths 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.

11
Table 22.1 Earths Geological History (Part 1)
12
Table 22.1 Earths Geological History (Part 2)
13
The Changing Face of Earth

• Earths crust consists of solid plates that float
  on a fluid mantle.
• The mantle is heated by energy from radioactive
  decay in the Earths core. Convection currents of
  mantle fluid cause the crust plates to move.
• The process of plate movement is known as
  continental drift.
• Throughout Earths 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.

14
Figure 22.2 Sea Levels Have Changed Repeatedly
15
The Changing Face of Earth

• Earths 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.

16
Figure 22.3 Stromatolites
17
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 750700 mya, O2 had increased to levels that
  could support multicellular organisms.

18
Figure 22.4 Larger Cells Need More Oxygen
19
The Changing Face of Earth

• Unlike the unidirectional change in Earths
  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.

20
The Changing Face of Earth

• Climatic conditions have fluctuated through
  Earths 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.

21
Figure 22.5 Hot/Humid and Cold/Dry Conditions
Have Alternated Over Earths 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 Earths surface, lowering
  temperatures, reducing photosynthesis, and
  triggering massive glaciation.

23
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 Earths surface

24
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.

25
Figure 22.6 Evidence of a Meteorite Impact
26
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.

27
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, Earths species must
  have turned over many times during geological
  history.

28
The Fossil Record

• Among the nine major animal groups with
  hard-shelled 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.

29
Figure 22.7 Insect Fossils
30
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
  soft-bodied invertebrates had evolved, some of
  which may be members of animal lineages that have
  no living descendants.

31
Figure 22.8 Ediacaran Animals
32
Major Patterns in the History of Life on Earth

• By the early Cambrian period (543490 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.

33
Figure 22.9 Cambrian Continents and Animals
(Part 1)
34
Figure 22.9 Cambrian Continents and Animals
(Part 2)
35
Major Patterns in the History of Life on Earth

• During the Ordovician period (490443 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.

36
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.

37
Figure 22.10 Cooksonia, the Earliest Known
Tracheophyte
38
Major Patterns in the History of Life on Earth

• During the Devonian period (417354 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.

39
Figure 22.11 Devonian Continents and Marine
Communities (Part 1)
40
Figure 22.11 Devonian Continents and Marine
Communities (Part 2)
41
Major Patterns in the History of Life on Earth

• The Carboniferous period (354290 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.

42
Figure 22.12 A Carboniferous Crinoid Meadow
43
Major Patterns in the History of Life on Earth

• During the Permian (2902458 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 Earths 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.

44
Figure 22.13 Pangaea Formed in the Permian Period
45
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.

46
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.
• Earths biota diversified and became distinct on
  each continent.

47
Major Patterns in the History of Life on Earth

• In the Triassic period (248206 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.

48
Major Patterns in the History of Life on Earth

• During the Jurassic (206144 mya), two large
  continents formedLaurasia 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.

49
Major Patterns in the History of Life on Earth

• By the Cretaceous period (14465 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.

50
Figure 22.15 Positions of the Continents during
the Cretaceous Period
51
Figure 22.16 Flowering Plants of the Cretaceous
52
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
  worlds forests.
• The Cenozoic is divided into two periods, the
  Tertiary and the Quaternary.

53
Major Patterns in the History of Life on Earth

• In the Tertiary period (651.8 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.

54
Major Patterns in the History of Life on Earth

• The current geological period is the Quaternary
  (1.8 myapresent), 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.

55
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.

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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.

57
Figure 22.17 Evolutionary Faunas
58
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.

59
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.

60
Figure 22.18 Living Fossils
61
Figure 33.16 Minor Chelicerate Phyla (Part 2)
62
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.

63
Figure 22.19 Rib Number Evolved Gradually in
Trilobites (Part 1)
64
Figure 22.19 Rib Number Evolved Gradually in
Trilobites (Part 2)
65
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.

66
Figure 22.20 Natural Selection Acts on
Stickleback Spines
67
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.

68
Figure 22.21 House Finches Expanded their Range
in North America (Part 1)
69
Figure 22.21 House Finches Expanded their Range
in North America (Part 2)
70
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.

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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
  Earths ecosystems.
• Artificial selection and biotechnology are also
  important man-made evolutionary factors.
• Humans have become the dominant evolutionary
  force on Earth today.