The History of Life on Earth

							The History of Life on
       Earth
Life on Earth has evolved slowly over
      the past 3.5 billion years.
        Unit 9: Key learnings
1. Earth's atmosphere, climate and
   landforms have changed dramatically
   over the past 4.6 billion years.
2. Geological, anatomical and biochemical
   clues indicate that life has evolved over
   the past 3.8 billion years.
    Unit 9: Essential question
How does the pattern of life's emergence on
Earth indicate that evolution has occurred?
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
    Essential question 1.1:
How do scientists determine the age of the
       Earth or preserved fossils?
I. The Earth is approximately 4.6 billion years
old.

1. The early Earth was a barren and lifeless fiery
   ball of rock with a metal core.
2. The Earth formed from nebular material left
   over when our star (the Sun) formed.
3. The oceans were formed from volcanic venting
   and comet collisions.
   a) The water vapor released from volcanic eruptions
      condensed into a liquid form.
   b) Frozen water from comets probably contributed the
      bulk of our ocean water.
II. Evidence for Earth’s extreme age is held in
the radioactive decay of rocks (radiometric
dating).
Question: If the half life of a substance is 3000 years,
how old is a fossil that holds 12.5% of the radioactive
isotope and 87.5% of the decay element?
    Essential question 1.2:
What does uniformitarianism tell us about
geologic changes over the Earth’s history?
III. Uniform change has been occurring on Earth
from the beginning (uniformitarianism).
 1.  Tectonics, weathering, erosion and deposition have
     constantly re-shaped the Earth’s surface.
 2. Since these processes have occurred since the
     beginning, Earth’s life forms have been greatly
     affected.
    a) Change has forced constant adaptation and re-
         adaptation.
    b) Change has caused extinctions and mass extinctions.
    c) Living things are now causing uniform change and
         affecting the physical world as well as the biological
         world.
 3. Geologic, atmospheric and biological change is evident
     in the fossil record.
IV. Gradual sedimentation and fossil
preservation leads to superposition and a
relative method of dating objects.
1.   Superposition means that older objects lay beneath
     younger objects.
2.   Radiometric dating is the only way to determine
     actual ages.
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
   Essential question 2.1:
What three explanations are there for the
 appearance of the Earth's first forms of
                  life?
I. Abiogenesis is the process through which non-living
matter reacts chemically to form simple organic compounds,
and later living cells.
1.  Scientists believe that life originated in the oceans since
    water is the best place for lifeless matter to combine at
    random.
2. The possibility of this has been confirmed by a series of
    laboratory experiments beginning in the 1920s.
   a) 1920s: Oparin (Russian) and Haldane (British)
       hypothesized that the Earth’s early oceans were rich
       with organic compounds (Primordial Soup) that may
       have lead to the first cells.
        •   They suggested that these molecules were formed when
            energy from solar radiation, volcanoes, or lighting
            caused their spontaneous origin.
     b) 1950s: Urey hypothesized that while oxygen was
        absent in the atmosphere, it was rich in gases such as
        hydrogen (H2), nitrogen (N2), ammonia (NH3) and
        methane (CH4).
II. Urey’s assistant, Miller tested Urey’s
hypothesis.
1.   Urey believed that the electrons in these molecules
     would constantly be pushed to higher energy
     levels.
2.   Since no gaseous oxygen was present to “oxidize”
     the high-energy electrons, the molecules holding
     them would combine ferociously!
3.   In 1953, Miller tested this in the lab by simulating
     early Earth conditions.
4.   He successfully caused the production of many
     organic compounds including amino acids and
     lipids.
Miller-Urey Experiment
III. Reevaluation of early Earth’s conditions has
caused many scientists to reject the primordial
soup idea on the basis of two arguments.
1. Timeframe: Since fossils dating back 3.5
     billion years have been found, it’s unlikely
     that the Miller-Urey reactions could have
     occurred quickly enough to meet this
     timeline.
2. Photo-disassociation: The early atmosphere
     which lacked ozone, would not have
     allowed ammonia and methane to exist in
     the presence of UV light (destruction of
     molecules by solar radiation).
IV. In 1986, Louis Lerman suggested that Urey's
chemistry happened in bubbles that were emitted
from undersea vents.
1.   Ammonia, methane and other key gases bubbled up
     from undersea volcanoes.
2.   The bubbles protected the gases from UV damage
     and concentrated the chemistry.
3.   Bubbles burst at the surface and released new
     organic compounds into the atmosphere.
4.   The new compounds were altered by reactions with
     UV light and lightning and fused into larger
     compounds.
5.   Complex organic molecules rained down with
     precipitation.
6.   Laboratory experiments using UV light yield results
     similar to Miller and Urey's.
Lerman’s Bubble model
VI. The current consensus is that the molecules necessary
to spawn the formation of the first cells came from many
different areas of pre-biotic chemistry.
There are three tenets to the heterotrophic theory of life.
1. Endogeneous (on Earth) sources
   a) Atmospheric synthesis via Miller-Urey
       synthesis
   b) Deep sea hydrothermal vent synthesis via
       Lerman’s Bubble model
2. Exogeneous (off Earth) sources
   a) Delivery from sources such as comets,
       meteorites, and interplanetary dust.
VII. Once formed, RNA may have played a role
in promoting and catalyzing the first replicating
structures.
1. RNA molecules can form spontaneously in
   water and replicate themselves.
2. RNA molecules are able to act as enzymes in
   the cutting and splicing of introns and exons
   (ribozymes).
3. RNA could have catalyzed the production of
   the first proteins.
4. When RNA is replicated it mutates and creates
   variation allowing for the first form of chemical
   evolution.
Spontaneous assembly of RNA
VIII. Some of the newly formed organic
compounds may have provided the first form of
cellular structure.
1.  There are many proposed candidates for the first
    cell membranes.
   • Lipids gather together in water and form a
        vesicle structure similar to a cell membrane (oil
        bubbles in salad dressing).
   • Amino acids and carbohydrates spontaneously
        form spheres.
2. Once these structures captured RNA molecules, the
    first cell-like structures may have been formed.
IX. These vesicle structures could not be considered
living until they were able to transfer their characteristics
to their offspring (heredity).

1. DNA probably developed after RNA and
    provided a stable form of information storage
   • DNA is less likely to mutate and destroy an
       effective replicator once it has formed
2. Molecules that could replicate themselves
    would have survived more often than “here
    today-gone tomorrow ones”
   • Richard Dawkins Selfish Gene Theory
   • This idea has been supported by modern
       chemical evolution studies.
Question: What kind of organism am I (20
questions)?
X. Summary of key concepts in abiogenesis

• The building blocks of life could have spontaneous
  originated from early Earth conditions (Heterotrophic
  theory of origin of life).

• RNA is the most likely candidate for the first replicator
  since it has the qualities of both DNA and enzymes.

• Cellular structures may have originated independently,
  but through metabolic integration, joined early RNA
  molecules as the first reproducing “cells”.
 A final note on the origin of
           cells…
While the material offered in this presentation
concerning the progression of life from the
cellular level forward is well documented, there
are three explanations offered to explain the
presence of those initial cells:
    • Abiogenesis (Preceding description and our
      current assumption)
    • Creation/Intelligent design (Un-testable, so not
      for public schools to address)
    • Panspermia (possibly by cosmic ancestry) (See
      me after school!)
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
  Essential question 3.1:
How did Prokaryotic cells give rise to
         eukaryotic Protists?
I. The oldest fossils we’ve found are from
simple prokaryotic cells (Eubacteria).
 1.    The oldest fossils date back 3.5 billion years, but
       chemical evidence extends to 3.85 bya. Bacteria fall
       into two categories:
 2.    Eubacteria appeared first with the divergence of
       Archaens coming soon after.
      a)   Eubacteria are the most common type of bacteria today, but
           have evolved considerably over the years.
      b)   Archaebacteria are more closely related to eukaryotes than
           eubacteria and may be the link between the two.
 3.    By 3 bya, Cyanobacteria (Eubacteria) were
       conducting photosynthesis and producing
       stromatolites.
II. The first eukaryotes evolved after
prokaryotes developed.
1.  Fossils date them to 1.2-1.5 bya, but chemical evidence
    suggests 2.7 bya.
2. Lynn Margulis developed the endosymbiosis theory to
    explain how eukaryotes developed.
3. Since mitochondria and chloroplasts share many
    characteristics with bacteria, they may have evolved
    from similar prokaryotes.
   a) Aerobic bacteria helped form mitochondria.
   b) Cyanobacteria helped form chloroplasts.
4. The cells that housed these primitive organelles were
    descendents from the same line that brought the
    divergence of the Archaens.
Lynn Margulis’ Endosymbiosis Theory

Modern comparisons of the DNA present in
 mitochondria and chloroplast with bacteria
      yield the answer to our question-
Mitochondria and Chloroplasts exist today as
 intact, but helpless bacterial passengers in
                   our cells!
III. The other 4 Eukaryotic kingdoms are
composed of eukaryotic organisms that are for
the most part multicellular.
1. Protists evolved as both producers and
    consumers.
2. Protists displayed the first forms of
    multicellularity by 1.8 bya.
3. Advantages of multicellularity:
   a) protection
   b) sensing prey
   c) locomotion
   d) etc.
III./4. Protists evolved into the other three
eukaryotic kingdoms.

a) Fungal, Plant and Animal
   representatives show up after
   multicellular protists had evolved.
b) By approximately 500 mya, these
   kingdoms had fully developed.
c) Aside from bacterial crusts on land,
   life still existed exclusively in the
   oceans.
        Summary question:
Explain why we believe that Eukaryotic cells
   are the descendants of an endosymbiotic
  relationship between ancient Prokaryotic
                    cells?
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
  Essential question 4.1:
  Why did life increase in diversity so
suddenly during the Cambrian explosion?
                             Ediacarans:

                              The extent
                              of animal
                             life prior to
                                  the
                               cambrian
                              explosion
I. The ancestors of today’s organisms originated
during the Cambrian period- the Cambrian
explosion!
 1.  The organisms we see today have ancestors that
     developed between 535 mya and 505 mya.
 2. Paleontologists uncovered many strange fossils in a
     mountainside in Canada called the Burgess Shale (and
     other rocks of similar age).
    a) Many of these fossils are unlike anything we see
        today.
    b) The survivors branched out, forming today’s major
        groupings (phyla).
I./3. Three explanations have been made for why
this sudden explosion occurred.

a) The gradual increase in atmospheric oxygen eventually
   hit a lower limit that suddenly released animals from
   metabolic constraints (Environmental).
b) The appearance of master control genes allowed minor
   genetic mutations (such as duplications) to produce
   large scale changes in anatomy (Genomic).
c) The onset of predator-prey relationships and the
   competition that resulted may have driven this
   explosion. (Ecological)
     • Vision had to develop prior to true hunting and
          escaping pressures.
Burgess Shale
    Essential question 4.2:
How do mass
  extinctions
    aid the
 evolutionary
   process?
II. Living things branched out and grew quickly
over the next 500 million years.
1.  Mass extinctions have played a major part in shaping
    the organisms we see today.
2. A mass extinction is the death of all members of many
    different species, as a result of a severe ecological
    change or disaster.
3. There were 6 mass extinctions in Earth’s history.
   a) (3rd) The worst mass extinction occurred 250 mya at
       the end of the Permian period (96% mortality).
   b) (5th) The most famous occurred about 65 mya at the
       end of the Cretaceous period, brought about the end
       of the dinosaurs and allowed for the evolution of
       mammals.
Mass extinctions
Mass Extinction at the end of the
      Cretaceous period.
   65 million years ago, a massive meteorite
   struck the Earth off the coast of Mexico’s
 Yucatan peninsula and a left a 200 mile wide
 crater. A thin layer of iridium rich sediment
   marking the end of the Cretaceous period,
    was laid down as ash from the meteorite
  rained down for a very long time. This ash
  blocked the Sun’s rays, and dropped global
     temperature by a considerable degree.
III. Many scientists believe that we are
entering another period of mass extinction.
a) It will be caused by the large scale
   destruction of habitat that humans have
   brought about.
b) Some estimates call for ½ of the world’s
   species to become extinct in the next 100
   years.
c) Countless insects, birds and other animals
   will die off as their habitats are reduced,
   partitioned or destroyed.
IV. Global rise in carbon dioxide levels may
contribute to global warming and add to the
pressures on life.
V. The groups who survive a mass extinction
undergo a major increase in biodiversity.

1. The Permian extinction (250 mya)
   allowed for the divergence of surviving
   reptiles into dinosaurs.
2. The Cretaceous extinction allowed for
   the divergence of surviving rodent-like
   mammals into all major forms seen today
   (carnivores, primates, marine, etc.).
Question: Is a mass extinction a bad thing?
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
 Essential question 5.1:
What needed to occur before tetrapod
  animals could colonize the land?
I. A newly formed ozone layer allowed life to
move out of the protection of the oceans.

1. UV light from the Sun made survival on land
    impossible during the Cambrian period (500
    mya).
2. However, since cyanobacteria had been
    producing O2 for billions of years, an ozone
    (O3 ) layer had began to form.
   a) O2 began combining to form O3, forming a
      thin layer in the stratosphere.
   b) Ozone blocks UV light from reaching
      Earth’s surface and protects living things.
II. By 440 mya, Algae and Fungi began living
together symbiotically on land.
1.  A symbiosis exists when two organisms live in
    extremely close proximity to one another.
   a) Algae can make their own food.
   b) Fungi can harvest resources from barren rock.
2. A symbiosis where both benefit is called
    mutualism.
3. A lichen is symbiotic relationship between algae
    and fungi that is capable of colonizing barren rock
    and producing simple soil.
4. Gradually, green plants evolved from these algae
    and took advantage of the soils that had been
    produced
III. Succession is the gradual accumulation of organic
material on a barren surface, resulting in the eventual
production of a lush forest and diverse fauna.
Question: What’s the difference between sand
and soil?
IV. During the 100 million years (440-340 mya) after
their colonization, plants developed dense communities
that provided food and shelter for animals.
1.  Arthropods were the first animals to colonize.
   a) Colonization had to wait for the presence of a
       protective ozone layer and plant flora to provide a
       suitable habitat.
   b) Arthropoda means “jointed legs”.
   c) Usually covered by a hard outer covering.
   d) Examples: crustaceans, insects, spiders
2. Because of these 2 advantages, insects quickly
    became the most diverse group of animals:
   a) Flight
   b) Fast rates of reproduction
    Essential question 5.2:
How did the body plans change during the
progression from fish-tetrapod-amphibian-
             reptile-mammal?
I. Although vertebrates with hard backbones had
already conquered the sea, modifications were
necessary for terrestrial survival.
1. Jawless fish evolved about 500 mya.
2. Jawed fish evolved about 430 mya and
   quickly became efficient predators.
3. Fish are the most successful vertebrates on
   Earth, making up almost half the living
   vertebrate species.
4. Major changes in the body plan of fish were
   needed to succeed on land.
A 375 million year old transitional form
between fish and Tetrapod amphibians.
II. About 360 mya, slimy skinned amphibians
rose from the oceans.

1. Today’s examples are frogs, toads, and
   salamanders.
2. Early amphibians might have been lured ashore
   by the presence of insects that had already
   invaded.
3. Two limitations caused their sprawl to be very
   limited.
   a) Amphibians must return to the water to reproduce.
   b) Their bodies must stay moist in order to absorb
      oxygen.
Amphibian evolution
III. Reptiles soon established their dominance
with water tight skin and eggs (350 mya).
1.  Modern examples: dinosaurs, snakes, turtles
2.  Their water tight skin and eggs prevent them from
    drying out.
   a) This allows some reptiles to live their entire life
       in dry desert conditions.
   b) Others, like turtles, have returned to the water.
3. By 240 million years, dinosaurs became the
    dominant land vertebrate
4. Since land conditions were extremely dry,
    dinosaurs were the dominant land vertebrate, and
    stayed that way until 65 mya.
IV. When the age of dinosaurs ended, birds and
mammals exploited the abundance of resources
(65 mya).
1.    Earth’s conditions had become wetter, and reptile advantages were
      no longer a necessity.
2.    Based upon structural homologies (such as skeletal structure and
      biochemical clues like the keratin molecules that form the structure
      of hair, feathers and scale) it’s clear that birds and mammals
      diverge from reptile ancestors.
3.    Birds bodies are highly adapted for flight.
     a) Hollow bones
     b) Reduced body systems (example: female birds have ½ of a
          mammalian reproductive system)
4.    The intelligence of mammals has allowed them to rise to the top of
      the food chain.
     a) By 5-7 mya, our ancestors diverged from the apes and began to
          walk upright.
     b) By 30,000 ya, Homo sapiens walked the Earth as the sole
          hominid (upright) species.
Archaeopteryx is thought to be the missing
    link between dinosaurs and birds.
Cynodont reptiles evolved into
     modern mammals.
Question: Is intelligence necessary for
survival?
            Unit 9: Concepts
1.   Historical geology (I)
2.   Abiogenesis (C)
3.   Prokaryotes to Protists (I)
4.   Boom and Bust (I)
5.   Life invades the land (I)
6.   Conclusions (E)
  Essential question 6.1:
Why does the pattern of appearance of
 Earth's life forms cause scientists to
  conclude that evolution occurred?
I. The order and way in which these changes
occurred is indicative of the evolutionary
process.
1. The first organisms to appear were the
   simplest, followed by increasingly complex
   forms.
2. Large scale changes occurred with
   intermediary steps bridging the gaps.
3. Major changes in Earth’s life forms
   coincided with major geologic and
   atmospheric fluctuations.
Eukaryotic evolution from
        Protists
II. Geologic time periods are named for major
geologic and biologic events.
     Essential question 6.1:
How might the actions of humans contribute
       to another mass extinction?


               Discussion