Early Earth and the Origin of Life

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Early Earth and the Origin of Life

• AP Biology Chapter 26

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Study of the Origin of Life is Speculative

• No fossil evidence of lifes origin exists.

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Still, origin of life is thought to have occurred
through 4 main stages

• Abiotic synthesis of small organic molecules
  (monomers)
• Amino acids
• nucleotides

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Still, origin of life is thought to have occurred
through 4 main stages

• Joining of monomers into polymers
• Proteins
• Nucleic acids

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Still, origin of life is thought to have occurred
through 4 main stages

• Packaging of polymers into protobionts
• Droplets with membranes that maintained an
  internal chemistry different from that of their
  surroundings.

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Still, origin of life is thought to have occurred
through 4 main stages

• Origin of self-replicating molecules that
  eventually made inheritance possible.

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When Did Life on Earth Arise?

• Earth formed about 4.6 billion years ago.
• First evidence of life on Earth is found in rocks
  that are 3.8 billion years old.

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Earliest Earth

• No seas
• Collisions of rocks in formation of the Earth
  generated to much heat
• Water would have vaporized

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Likely atmosphere of early Earth

• Thick with water vapor
• Compounds released by volcanic eruption
• Nitrogen and its oxides
• Carbon dioxide
• Methane
• Ammonia
• Hydrogen
• Hydrogen sulfide
• Notably missing - OXYGEN

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So, What Was Early Earth Like?

• Obviously, atmosphere different little O2
  present
• Lots more volcanic activity
• Lots more lightening
• Static from eruption particles
• Meteorite bombardment
• More intense UV radiation from the sun
• Such conditions might have not only made
  formation of life from nonlife possibleit might
  have been inevitable.

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As Earth cooled

• Water vapor condensed into oceans
• Much hydrogen escaped into space

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What Was the Origin of the First Cells?

• Most biologists subscribe to the hypothesis that
  life on Earth developed from nonliving materials.
• These nonliving materials became ordered into
  molecular aggregates that eventually became
  capable of both replication and metabolism.

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Doesnt the Cell Theory Say that Life Cannot Come
from Nonliving Things????

• YES! Certainly that is true today.
• Conditions on early Earth were much different
  from what they are today and MAY have been
  conducive to the formation of living things from
  nonliving materials.

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Oparin and Haldane

• 1920s
• Hypothesized that conditions on primitive Earth
  favored chemical reactions that synthesized
  organic compounds from inorganic precursors.
• This idea is often called chemical evolution.

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Oparin and Haldane

• Why dont we see such chemical evolution
  occurring today?
• Oparin and Haldane said this was because the
  oxygen rich atmosphere we have today is not
  conducive to spontaneous synthesis of complex
  molecules.
• Oxygen in atmosphere attacks chemical bonds
  (oxydizying)
• Before photosynthesis, the Earth had a much less
  oxidizing atmosphere derived mainly from volcanic
  vapors a reducing atmosphere.

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Oparin and Haldane

• Whats so great about a reducing atmosphere?
• reducing also means electron adding
• Would have enhanced the joining of simple
  molecules to form more complex ones.

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Oparin and Haldane

• Still, even with a reducing atmosphere, making
  organic compounds would require lots of energy
  (think photosynthesis)
• Oparin and Haldane said this could have been
  provided by the intense lightening and UV
  radiation that was abundant in the early Earths
  atmosphere.
• There was no protective ozone layer in early
  EarthOzone is made from oxygen and there wasnt
  much.

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Testing the Oparin-Haldane Hypothesis Miller
and Urey

• 1953
• Tested the Oparin-Haldane hypothesis
• Created conditions in the lab that were
  comparable to those of early Earth.
• Goal was to see if amino acids and/or other
  organic monomers would spontaneously form from
  inorganic compounds.

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Miller and Ureys Experiment

• Atmosphere water, hydrogen gas methane and
  ammonia - to mimic early Earth.
• A warmed flask of water simulated the primeval
  sea.
• Energy was provided by sparks that mimicked
  lightening.
• A condenser cooled the atmosphere and allowed
  water to rain back into a flask for testing.
• After a week the water became murky brown and was
  tested for organic compounds.
• Learn MORE Click HERE!

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Results of Miller and Ureys Experiment

• Their apparatus produced a variety of amino acids
  and other organic compounds found in living
  organisms today.

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HOWEVER

• Scientists now think that the early Earths
  atmosphere was probably not as reducing as the
  atmosphere in the Miller-Urey experiment.
• More evidence today suggests that nitrogen and
  carbon dioxide were primary components of
  atmosphere not reducing (or even oxidizing)
• Experiments have NOT been able to produce organic
  monomers in such an atmosphere

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Where else could life have originated?

• Pockets where reducing atmosphere may have
  existed near volcanoes
• Deep sea vents

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Extraterrestrial origin of organic compounds?

• Organic monomers have been found in meteorites
  that strike Earth
• Carbonaceous chondrites
• 80 amino acids found in one

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Life Elsewhere?

• Mars
• Europa
• One of Jupiters moons
• Outside our solar system?

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What about the Polymers?

• Miller and Urey suggested that organic monomers
  could form spontaneously from inorganic compounds
• Could organic polymers form spontaneously from
  these organic monomers?
• Remember that there would have been no enzymes to
  help these chemicals come together (DNA
  Polymerase, for example)

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Experiments on Abiotic Synthesis of Organic
Polymers (DNA, Proteins)

• Dilute solutions of organic monomers dripped onto
  hot sand, clay or rock.
• This process vaporizes water and concentrates the
  monomers on the sand/clay/rock.
• Results Polymerization occurs
• Clay (and some other materials) might have been
  important in bringing together organic monomers
  to make polymers due to its chemical nature.
• They are not like the proteins we have today, but
  could have served as a weak catalyst for early
  chemical reactions.

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So now weve got polymers Whats Next?

• Protobionts
• Aggregates of abiotically produced molecules
  surrounded by a membrane or membrane-like
  structure
• Made of mostly hydrophobic molecules surrounded
  by a shell of water molecules.
• Precursors of actual living cells?

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Properties of Protobionts that are associated
with life

• SIMPLE reproduction
• Maintain an internal chemical environment
  DIFFERENT from their surroundings
• Exhibit SOME qualities of life such as metabolism

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What experiments tell us

• Protobionts COULD have formed spontaneously from
  organic compounds.
• Example Liposomes

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Liposomes

• Membrane-bounded droplets that form spontaneously
  when lipids or other organic molecules are added
  to water
• Hydrophobic molecules organize into a bilayer at
  the surface of the droplet Like a bilayer cell
  membrane.
• Liposome bilayer selectively permeable
• Therefore, liposomes can undergo swellling or
  shrinking due to osmosis
• Some can even discharge energy across their
  membranes like a nerve cell can.

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Next step is to achieve heredity

• First genetic material was likely RNA
• WHY?

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We know that RNA has multiple functions

• Of course, it has a central role in protein
  synthesis
• mRNA, tRNA, rRNA, etc.
• But, RNA can ALSO act as a catalyst
• Ribozymes
• Catalytic capability may have been critical early
  on. Allowed RNA molecules to perhaps catalyze
  their OWN REPLICATION!!
• Something that the more stable DNA molecule could
  probably NOT do.

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What do experiments tell us?

• Natural selection can be observed acting on RNA
  molecules in the labHow?
• RNA molecules fold and make unique shapes based
  on their nucleotide sequences NOT like uniform
  structure of DNA
• RNA molecules with certain shapes are more
  adapted to certain environments than other RNA
  molecules
• These will reproduce most often, but will not
  give a single RNA species due to mutational
  changes
• RNA is less stable than DNA
• Natural selection will further screen these
  offspring molecules such that only those best
  suited for the environment will survive

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An RNA World

• The DNA world of today may have been preceded by
  an RNA world in which small RNA molecules
  carrying genetic information were able to
  replicate and store information about the
  protobionts that carried them

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Once RNA was established, further changes were
possible

• RNA could have provided the template on which DNA
  molecules were assembled.

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But why bother to have DNA if RNA works?

• DNA is a much more stable molecule.
• Once DNA appeared, RNA molecules would have begun
  to take on their modern roles as intermediates
  between DNA and protein.

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History of Life

• Two Eons
• Precambrian
• Oldest
• By FAR, the greatest amount of time
• Least detail least diversity of life
• Phanerozoic
• More recent
• Less time than Precambrian
• LOTS of life

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History of Life

• Phanerozoic Eon made up of 3 eras
• Paleozoic
• More ancient
• Mesozoic
• Middle life
• Age of reptiles
• Cenozoic
• More recent
• Age of mammals
• MASS EXTINCTIONS separate each of the 3 eras

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Mass Extinctions

• At the end of the Paleozoic Era (Permian Period)
• Wiped out 96 of marine animal species Many land
  species also affected (70)
• Occurred over a period less than 5 million years
  which is an instant in terms of geologic time
• Cause enormous volcanic eruptions in what is
  now Siberia
• Eruptions may have produced enough carbon dioxide
  to warm the global climatesound familiar??
• Reduced temperature differences between poles and
  equator reduce ocean water mixing.
• This affects oxygen available in oceans for life

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The Permian Mass Extinction

• Click HERE for an article on this mass extinction
  from Scientific American

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Mass Extinctions

• At the end of the Mesozoic era (Cretaceous
  period) 65 mya
• Evidence suggests asteroid or large comet
  collided with Earth
• Layer of iridium in rocks at 65 mya
• Not commonly found on Earth, but very commonly
  found in meteorites, etc.
• Cloud raised from the hit would have blocked
  sunlight and disturbed global climate for months

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Cretaceous Extinction

• Is there evidence for a strike?
• Yes. Chicxulub crater
• Off coast of Yucatan peninsula Mexico
• Right age, right size
• Object that caused this would have been about 10
  km in diameter.
• Also evidence for a spike in volcanism at this
  timeis it due to the Chixulub impact?

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Cretaceous Extinction
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Chixulub Crater
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What are Stromatolites?

• Banded domes of sedimentary rock
• Made of bacteria and sediment
• Very similar to layered mats formed today in salt
  marshes by colonies of bacteria.
• These are currently the oldest known fossils of
  living organisms.
• 3.5 by old
• Life is probably older3.9 by

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Stromatolites
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Present day stromatolite formation
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First life

• PROKARYOTIC

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Two major branches of prokaryotes arose early on

• Bacteria
• Archaea
• Both these lineages still thrive today
• These are at the DOMAIN level of classification

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The first prokaryotes

• Arose from the most successful protobiontsones
  that could
• Produce all their needed compounds from molecules
  in their environment
• These most successful protobionts diverseified
  into a rich variety
• Autotrophs
• Autotrophs probably led to emergence of
  heterotrophs
• These could have lived on products excreted by
  autotrophs
• OR on autotrophs themselves
• These autotrophs and heterotrophs were the first
  prokaryotes
• Prokaryotes were the sole inhabitants of Earth
  for a VERY LONG time 3.5 by ago to 2 by ago.

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Significance of Electron Transport Chains

• Common to all three domains
• Strong evidence that this electron transport
  mechanism originated in organisms that lived
  BEFORE the last common ancestor of all present
  day life.

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How did living things alter the Earth?

• Oxygen
• Cyanobacteria only living prokaryotes to
  generate oxygen by photosynthesis
• When the first bacteria began to make oxygen
• Seas and lakes first had to get saturated
• Then dissolved iron became oxidized and
  precipitated out. We see this as bands of rust
  in sedimentary rock layers
• Once all iron oxide had precipitated, then the
  oxygen finally began to gas out of seas and
  enter the atmosphere
• This change is seen in rust of terrestrial rocks
  at about 2.7 by ago

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Effects of the Oxygen Revolution on Life

• Doomed many prokaryotic groups
• The first mass extinction?
• Some prokaryotes survived in habitats that are
  still anaerobic today.
• Other survivors evolved a diversity of
  adaptations to the new atmosphere
• Cellular respiration

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What about Eukaryotes?

• Oldest eukaryotic fossils
• Date to 2.1 by ago
• Remember
• prokaryotes lack organelles
• where did these come from?

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What led to organelles like mitochondria and
chloroplasts?

• Endosymbiosis
• Mitochondria and plastids were formerly small
  prokaryotes living within larger cells
• Ancestors of mitochondria
• Aerobic heterotrophic prokaryotes
• Ancestors of plastids (chloroplasts)
• Photosynthetic prokaryotes

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How did the endosymbionts get in?

• Undigested prey of the host OR
• Parasite of the host
• Either suggests an earlier evolution in the host
  of
• endomembrane system
• Cytoskeleton
• Only these organelles would have made it possible
  for the host to
• engulf the smaller cells
• package them within vesicles.

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What would be gained by the participants in the
symbiosis?

• Host
• Use nutrients released from photosynthetic
  endosymbionts
• An anaerobic host could benefit from harboring an
  aerobic endosymbiont that could make use of the
  oxygen in the environment
• Endosymbiont
• protection

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Serial Endosymbiosis

• Since ALL cells have mitochondria, but NOT all
  cells have chloroplasts, it is hypothesized that
  mitochondria evolved first.
• Serial endosymbiosis a series of endosymbiotic
  steps

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Evidence for Endosymbiosis

• Overwhelming
• Double membrane
• Inner membrane (endosymbionts membrane) has
  membrane proteins homologous to other prokaryotes
• Mitochondria and chloroplasts each have their own
  DNA that is a single circular DNA molecule like
  bacteria
• Contain RNA, ribosomes, etc. needed to
  transcribe/translate DNA into protein
• Ribosomes are more like prokartyotic ribosomes
  than eukaryotic ribosomes

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Determining very ancient lineages

• Nucleotide sequence of the RNA of the small
  ribosomal subunit.
• It is present in all organisms, thus it can be
  used to study the deepest branches of the tree of
  life

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Endosymbiont genes

• Over time some of the genes of the endosymbionts
  were transferred to the nucleus
• Transposable elements may have been responsible
• Eukaryotic cells have one genome, but it is
  complemented by the genes that remain in the
  mitochondria and/or chloroplasts

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What about other organelles?

• Golgi apparatus and ER may have originated from
  infoldings of the plasma membrane of a prokaryote

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Example of a complex symbiosis

• Mixotricha paradoxa
• Protist
• Lives inside gut of termites digests wood
• 3 types of bacteria attached to surface to
  provide motility
• 4th type lives inside the protist to digest wood

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Evolution of Multicellular Eukaryotes

• Common ancestor of the multicellular eukartote
  dated to 1.5 by ago

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Multicellular eukaryotes relatively limited in
size, diversity, distribution until fairly
latewhy?

• Snowball Earth hypothesis
• Glaciers covered the planets landmasses from
  pole to pole
• Life confined to areas of deep sea vents, etc.
• Fossil record of first major burst of
  multicellular life occurs at the time when
  snowball Earth thawed.

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Multicellularity and colonies

• 1st multicellular organisms were colonies
• Colony collections of autonomously replicating
  cells
• Cells that each reproduce on their own but agree
  to hang out together
• Eventually some cells in some colonies became
  specialized for some particular function
• Division of function led to tissues, organs,
  organ systems

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Multicellularity evolved several times among
early eukaryotes

• This led to the rise of several multicellular
  eukaryotic lineages
• Plants
• Fungi
• Animals
• Also some multicellular algae

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The Cambrian Explosion

• Time when most of the major phyla of animals
  appear in the fossil record

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Movement to Land from Water

• Involved adaptations to
• Prevent dehydration
• Reproduce on land

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Land organisms

• Plants and symbiosis with fungi
• Widespread and diverse land animals are
  arthropods
• Humans did not diverge from other hominids until
  about 6-7 my ago

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Organizing The Tree of Life

• 2 kingdom system
• 1969 5 kingdom system
• Recognizes 2 cell types
• Prokaryote and eukaryote
• 5 kingdoms are
• Monera
• Protista
• Fungi
• Plantae
• Animalia

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3 kingdoms of multicellular eukaryotes

• Fungi, Plantae, Animalia
• How were these divided?
• Nutrition
• Plants autotrophs
• Fungi and animals heterotrophs
• Fungi decomposers
• Animals ingest and digest w/i a body cavity

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Kingdom Protista

• Not so great
• All eukartyotes that did not fit the definitions
  of plants, fungi or animals

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3 Domain System

• superkingdoms higher than the kingdom level
• Bacteria, Archea, Eukarya
• Though both prokaryotic, bacteria and archea
  differ in many significant ways
• 3 domains makes monera kingdom obsolete
• Protists have been divided into 5 or more new
  kingdoms

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