Biology - Chapter 16

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Biology - Chapter 16The Origin of Life

• Charles Page High School
• Stephen L. Cotton

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Section 16-1Spontaneous Generation

• OBJECTIVES
• Define spontaneous generation.

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Section 16-1Spontaneous Generation

• OBJECTIVES
• Describe the experiments of Redi, Spallanzani,
  and Pasteur.

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Section 16-1Spontaneous Generation

• Scientists wonder how life on Earth began
• for many years- life simply arose from
  nonliving matter
• mice from piles of grain
• maggots from decaying meat, etc.

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Section 16-1Spontaneous Generation

• The hypothesis that life arises regularly from
  nonlife is called spontaneous generation
• Lazzaro Spallanzani - born in Italy in 1729
• skeptical about life arising from nonlife

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Section 16-1Spontaneous Generation

• Spallanzani read a book by Francesco Redi, one of
  the first scientists to try to disprove
  spontaneous generation
• Redi hypothesized that the maggots actually arose
  not from the meat itself, but from the eggs of
  flies around it

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Section 16-1Spontaneous Generation

• Redi did some experiments, shown in Figure 16-1,
  page 339
• From this, Redi concluded that the maggots did
  not arise spontaneously from the decaying meat,
  but from the eggs laid by the flies

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Section 16-1Spontaneous Generation

• People now changed their belief about the flies
  but, what about the other examples?
• Redis conclusions were attacked by the
  Englishman John Needham, who claimed that
  spontaneous generation could occur

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Section 16-1Spontaneous Generation

• Needhams experiments are shown in Fig. 16-2,
  page 340
• his results showed the broth swarming with
  microorganisms
• this provided evidence to those who did believe
  in spontaneous generation!

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Section 16-1Spontaneous Generation

• Now back to Spallanzani- he was familiar with the
  work of both Redi and Needham
• he thought that Needham was wrong, because
  Needham did not boil the original solution long
  enough to kill everything

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Section 16-1Spontaneous Generation

• Spallanzani repeated the experiments of Needham
• But, he boiled the solution longer, and then
  sealed one set of jars while leaving the other
  set open to the air

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Section 16-1Spontaneous Generation

• After a few days, the open jar was loaded with
  microorganisms
• the closed jar contained no life
• Thus, he concluded that the microorganisms did
  not develop from the gravy (nonlife), but instead
  entered from the air

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Section 16-1Spontaneous Generation

• No one believed Spallanzani, and they said
• the solution was boiled too long
• the airtight seal prevented life

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Section 16-1Spontaneous Generation

• French scientist Louis Pasteur, 1864, did an
  elegant experiment that finally disproved
  spontaneous generation
• Similar to Needhams experiment, except Pasteur
  used a flask with an S-shaped neck

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Section 16-1Spontaneous Generation

• Note Fig. 16-4, page 341
• This flask didnt allow air to enter
• Boiled it thoroughly
• Result after a few days with that there was no
  microorganisms

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Section 16-1Spontaneous Generation

• He then broke off the neck, allowing air to enter
• Result? Very soon there was lots of
  microorganisms
• Pasteurs conclusion? Life comes only from life
  (same as Redi and Spallanzani)

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Section 16-2The First Signs of Life

• OBJECTIVES
• Describe conditions on ancient Earth.

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Section 16-2The First Signs of Life

• OBJECTIVES
• Define microfossil.

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Section 16-2The First Signs of Life

• OBJECTIVES
• Discuss various hypotheses on the evolution of
  cells.

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Section 16-2The First Signs of Life

• If life can come only from life, how did life on
  Earth first arise?
• Our planet started approximately 4.6 billions
  years ago as a great cloud of gas and dust,
  condensed into a sphere

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Section 16-2The First Signs of Life

• Gravity, heat from pressure and radioactivity
  melted into the first interior of rock
• first solid rocks about 4 billion years ago
• Then, for millions of years there was violent
  planet-wide volcanic activity

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Section 16-2The First Signs of Life

• 4 billions years ago, we would hardly recognize
  our planet
• harsh atmosphere, no water, and very hot
• atmosphere was water vapor, carbon monoxide and
  carbon dioxide, nitrogen, hydrogen sulfide, and
  hydrogen cyanide

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Section 16-2The First Signs of Life

• Atmosphere contained no free oxygen, and thus
  could not support life as we know it
• Geological evidence shows rocks from this time
  have no rust or other compounds that would
  require free oxygen in order to form

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Section 16-2The First Signs of Life

• About 3.8 billion years ago, the Earth cooled
  enough for the oceans to form as a liquid on the
  ground
• some microfossils have been found from about 3.5
  billion years ago- these were prokaryotes,
  similar to bacteria

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Section 16-2The First Signs of Life

• 1953- Stanley Miller and Harold Urey performed
  experiments
• gave us a glimpse of events leading to Earths
  first forms of life
• recreated what they thought might have been
  Earths earliest atmosphere

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Section 16-2The First Signs of Life

• They mixed methane (CH4), ammonia (NH3), water
  (H2O), and hydrogen (H2) in a flask
• then exposed these chemicals to an electric spark
  (to simulate sunlight and lightning)
• Figure 16-8, page 344

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Section 16-2The First Signs of Life

• In a few days, Miller found an organic soup
  formed
• it contained urea, acetic acid, lactic acid, and
  several amino acids
• all of this was repeated by others with similar
  results, in in the absence of oxygen!

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Section 16-2The First Signs of Life

• The conclusion? Over millions of years, at least
  some of the basic building blocks of life could
  have been created
• also have found similar chemicals in meteors,
  comets, and even cosmic dust

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Section 16-2The First Signs of Life

• A large meteorite crashed in Australia in 1969
  that even contained lipids and all 5 nitrogenous
  bases that form DNA and RNA
• thus, additional chemicals could have come from
  outer space to mix with those here

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Section 16-2The First Signs of Life

• However, a collection of chemicals such as amino
  acids is NOT life
• so, what happened next?
• Experiments by Alexander Oparin and Sidney Fox
  have shown that the organic soup did not remain
  simple

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Section 16-2The First Signs of Life

• The amino acids tend to link together, forming
  short chains
• chains of amino acids are called polypeptides
  (proteins)
• These molecules tended to group together into
  tiny round droplets- these could grow and even
  reproduce by themselves!

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Section 16-2The First Signs of Life

• At this stage, we would still not say that these
  droplets are alive
• but we might call them proto-life, because they
  have begun to perform tasks necessary for life

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Section 16-2The First Signs of Life

• The next step in the story is the most difficult
  to understand
• from this jumbled mixture of molecules formed
  organized structures such as RNA and DNA

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Section 16-2The First Signs of Life

• Some biologists believe that the first true cells
  arose in a shallow pool containing this organic
  soup
• when such a soup is dried, lipids form spheres
  around small DNA molecules
• and given enough time...

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Section 16-2The First Signs of Life

• Another hypothesis offered by G. Cairns-Smith and
  J. Bernal
• they noted that various attractive forces can
  concentrate amino acids and nucleic acids (RNA
  and DNA) onto the regular structures that make
  clay crystals

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Section 16-2The First Signs of Life

• These could then possibly form lengths of both
  DNA and protein
• Building on this idea, several scientists propose
  that the first cells formed near volcanic vents
• filled with energy-rich sulfur compounds

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Section 16-2The First Signs of Life

• Thus, these vents offer a set of favorable
  conditions
• a) assortment of chemicals
• b) strong currents to provide mixing
• c) a source of naturally formed chemical energy
• d) deposits of clay

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Section 16-2The First Signs of Life

• The oldest living prokaryotes (a group of
  bacteria that survive by obtaining energy from
  sulfur compounds) still live near these hot vents
  today!

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Section 16-2The First Signs of Life

• Although the origin of the first true cells is
  uncertain, we can identify several of their
  characteristics with certainty
• prokaryotes heterotrophs anaerobic (live
  without free oxygen)

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Section 16-2The First Signs of Life

• Heterotrophs could live without difficulty
  because there was plenty of the organic soup but
  it would soon run out
• organisms had to develop a way to make complex
  molecules from simpler ones

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Section 16-2The First Signs of Life

• The stage was set for the development of
  autotrophs
• some ancient form of photosynthesis evolved,
  different from modern plants
• they probably used hydrogen sulfide (H2S) like
  modern ones use water (H2O)

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Section 16-2The First Signs of Life

• The first autotrophs were enormously successful,
  spread rapidly, and were commonplace on Earth
  about 3.4 billions years ago
• They grew in layered, matlike formations called
  stromatolites
• Figure 16-11, page 346

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Section 16-2The First Signs of Life

• We earlier talked of experiments that disproved
  spontaneous generation, but now what are we
  saying? That it can occur. How?
• Todays Earth is much different than many years
  ago
• How is it different, lets see...

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Section 16-2The First Signs of Life

• On primitive Earth, there were no bacteria to
  break down organic compounds
• There was no oxygen to react with the organic
  compounds
• thus, the organic compounds could accumulate into
  making the organic soup (over time)

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Section 16-2The First Signs of Life

• Today, however, such compounds cannot remain
  intact in the natural world for a long enough
  period of time to give life another start

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Section 16-3 The Road to Modern Organisms

• OBJECTIVES
• Relate oxygen production in photosynthesis to the
  evolution of aerobic metabolism.

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Section 16-3 The Road to Modern Organisms

• OBJECTIVES
• Describe the importance of membrane-bound
  organelles and sexual reproduction to the
  evolution of life on Earth.

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Section 16-3 The Road to Modern Organisms

• The first great change occurred roughly 2.2
  billion years ago, when a more modern form of
  photosynthesis evolved
• it substituted H2O for H2S in their metabolic
  pathways
• Why was this important?

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Section 16-3 The Road to Modern Organisms

• It now released a deadly new gas into the
  atmosphere- a waste product of photosynthesis
  called oxygen!
• Over a period of 500 million years, our planet
  now has an atmosphere 1/5 (or about 20) oxygen

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Section 16-3 The Road to Modern Organisms

• Because oxygen was deadly to anaerobes (creatures
  that cannot tolerate oxygen), they were forever
  banished from the planets surface
• now only found deep in mud or other places where
  the atmosphere does not reach

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Section 16-3 The Road to Modern Organisms

• One effect of oxygen in the atmosphere, however,
  was beneficial to those organisms that did
  survive
• the formation of the ozone layer (O3) absorbs
  much of the ultraviolet rays from the sun, thus
  shielding living things

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Section 16-3 The Road to Modern Organisms

• The addition of oxygen to the atmosphere began a
  new chapter in the history of life on Earth
• now creatures developed that used oxygen in their
  metabolism (sum total of all the chemical
  reactions)

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Section 16-3 The Road to Modern Organisms

• These new aerobic pathways allowed organisms to
  obtain 18 times more energy from every sugar
  molecule than the anaerobic pathways did
• Between 1.4 and 1.6 billions years ago, the first
  eukaryotic cells evolved, fully adapted to an
  aerobic world

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Section 16-3 The Road to Modern Organisms

• Among the most important steps in the evolution
  of eukaryotic life was the emergence of sexual
  reproduction
• this catapulted the process of evolution forward
  at far greater speeds then ever before much
  better than binary fission (split)

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Section 16-3 The Road to Modern Organisms

• Sexual reproduction shuffles and re-shuffles
  genes in each generation
• this increases genetic variation, and this
  increases the chances for evolutionary change

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Section 16-3 The Road to Modern Organisms

• A few hundred million years after the evolution
  of sexual reproduction, evolving life forms
  crossed another great threshold
• the development of multicellular organisms from
  single-celled organisms

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Section 16-3 The Road to Modern Organisms

• As a result, Earths parade of life was well on
  its way.