How Did Life Begin?

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How Did Life Begin?

• The Earth formed about 4.6 billion years ago
  according to evidence obtained by radiometric
  dating.

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½ life
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• The primordial soup model and the bubble model
  propose explanations of the origin of the
  chemicals of life.
• Scientists think RNA formed before DNA or
  proteins formed.

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Bubble model
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• Scientists think that the first cells may have
  developed from microspheres.
• The development of heredity made it possible for
  organisms to pass traits to subsequent
  generations.

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• Complex Organisms Developed
• Prokaryotes are the oldest organisms and are
  divided into two groups, archaebacteria and
  eubacteria.
• Prokaryotes likely gave rise to eukaryotes
  through the process of endosymbiosis.

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• Mitochondria and chloroplasts are thought to have
  evolved through endosymbiosis.
• Multicellularity arose many times and resulted in
  many different groups of multicellular organisms.
• Extinctions influenced the evolution of the
  species alive today.

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• Life Invaded the Land
• Ancient cyanobacteria produced oxygen, some of
  which became ozone. Ozone enabled organisms to
  live on land.
• .

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ozone
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• Plants and fungi formed mycorrhizae and were the
  first multicellular organisms to live on land

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fungi
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• Arthropods were the first animals to leave the
  ocean.

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• The first vertebrates to invade dry land were
  amphibians.
• The extinction of many reptile species enabled
  birds and mammals to become the dominant
  vertebrates on land.

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• The movement of the continents on the surface of
  the Earth has contributed to the geographic
  distribution of some species.

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• The fossil record also provides an amazing amount
  of evidence concerning common ancestors.
  Fossilized remains of invertebrates (animals
  without an internal skeleton), vertebrates, and
  plants appear in the strata or layers of Earth's
  surface in the same order that the complexities
  of their anatomy suggest.

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• The more evolutionarily distant organisms lie
  deeper, in the older layers, beneath the remains
  of the more recent organisms. Geologists are able
  to date rock strata with reasonable accuracy, and
  the age of a layer always correlates with the
  fossils discovered there. In other words, there
  would never be a stratum dating back 400 million
  years that contained fossils of mastodons, which
  evolved much later.

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• The Theory of Evolution by Natural Selection
• ? Charles Darwin concluded that animals on the
  coast of South America that resembled those on
  the nearby islands evolved differences after
  separating from a common ancestor.

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Darwins trip
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camouflage
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• ? Darwin was influenced by Thomas Malthus, who
  wrote that populations tend to grow as much as
  the environment allows.

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• ? Darwin proposed that natural selection favors
  individuals that are best able to survive and
  reproduce.

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• ? Under certain conditions, change within a
  species can lead to new species.
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• Evidence of Evolution
• ? Evidence of orderly change can be seen when
  fossils are arranged according to their age.
• ? ? Similarities of structures in different
  vertebrates provide evidence that all vertebrates
  share a common ancestor.

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• Though their ancestors ceased to walk on four
  legs many millions of years ago, snakes still
  possess vestigial hind limbs as well as reduced
  hip and thigh bones.

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• In some cases widely divergent organisms possess
  a common structure, adapted to their individual
  needs over countless generations yet reflective
  of a shared ancestor. A fascinating example of
  this is the pentadactyl limb, a five-digit
  appendage common to mammals and found, in
  modified form, among birds. The cat's paw, the
  dolphin's flipper, the bat's wing, and the human
  hand are all versions of the same original, an
  indication of a common four-footed ancestor that
  likewise had limbs with five digits at the end.

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Homologous structures
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• The embryonic forms of animals also reflect
  common traits and shared evolutionary forebears.
  This is why most mammals look remarkably similar
  in early stages of development. In some cases
  animals in fetal form will manifest vestigial
  features reflective of what were once functional
  traits of their ancestors.

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Differences in amino acid sequences and DNA
sequences are greater between species that are
more distantly related than between species that
are more closely related.
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• Examples of Evolution
• ? Individuals that have traits that enable them
  to survive in a given environment can reproduce
  and pass those traits to their offspring.
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Industrial Melanism and the Pepper Moth

• Both natural selection and mutation play a role
  in industrial melanism, a phenomenon whereby the
  processes of evolution can be witnessed within
  the scale of a human lifetime.
• Industrial melanism is the high level of
  occurrence of dark, or melanic, individuals from
  a particular species (usually insects) within a
  geographic region noted for its high levels of
  dark-colored industrial pollution.

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• With so much pollution in the air, trees tend to
  be darkened, and thus a dark moth stands a much
  greater chance of surviving, because predators
  will be less able to see it.
• At the same time, there is a mutation that
  produces dark-colored moths, and in this
  particular situation, these melanic varieties are
  selected naturally.
• On the other hand, in a relatively unpolluted
  region, the lighter-colored individuals of the
  same species tend to have the advantage, and
  therefore natural selection does not favor the
  mutation.

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• The best-known example of industrial melanism
  occurred in a species known as the pepper moth,
  or Biston betularia, which usually lives on trees
  covered with lichen.
• Prior to the beginnings of the Industrial
  Revolution in England during the late eighteenth
  century, the proportion of light-colored pepper
  moths was much higher than that of dark-colored
  ones, both of which were members of the same
  species differentiated only by appearance

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• As the Industrial Revolution got into full swing
  during the 1800s, factory smokestacks put so much
  soot into the air in some parts of England that
  it killed the lichen on the trees, and by the
  1950s, most pepper moths were dark-colored.
• It was at that point that Bernard Kettlewell
  (1907-1979), a British geneticist and
  entomologist (a scientist who studies insects),
  formed the hypothesis that the pepper moths'
  coloration protected them from predators, namely
  birds.

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• Kettlewell therefore reasoned that, before
  pollution appeared in mass quantities,
  light-colored moths had been the ones best
  equipped to protect themselves because they were
  camouflaged against the lichen on the trees.
• After the beginnings of the Industrial
  Revolution, however, the presence of soot on the
  trees meant that light-colored moths would stand
  out, and therefore it was best for a moth to be
  dark in color.
• This in turn meant that natural selection had
  favored the dark moths

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• In making his hypothesis, Kettlewell predicted
  that he would find more dark moths than light
  moths in polluted areas, and more light than dark
  ones in places that were unpolluted by factory
  soot.
• As it turned out, dark moths outnumbered light
  moths two-to-one in industrialized areas, while
  the ratios were reversed in unpolluted regions,
  confirming his predictions.

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• To further test his hypothesis, Kettlewell set up
  hidden cameras pointed at trees in both polluted
  and unpolluted areas.
• The resulting films showed birds preying on light
  moths in the polluted region, and dark moths in
  the unpolluted oneagain, fitting Kettlewell's
  predictions

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• Experiments show that evolution through natural
  selection has occurred within populations of
  antibiotic-resistant bacteria and in Darwins
  finches.
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• Speciation begins as a population adapts to its
  environment.
• ? Reproductive isolation keeps newly forming
  species from breeding with one another.

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• when the Colorado River cut open the Grand
  Canyon, separating groups of squirrels who lived
  in the high-altitude pine forest. Eventually,
  populations ceased to interbreed, and today the
  Kaibab squirrel of the northern rim and the Abert
  squirrel of the south are separate species.