The Origin and History of Life on Earth

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The Origin and History of Life on Earth

• What is Life?
• Life is, to the best of our knowledge to date,
  unique to our planet Earth.
• There is no simple definition of Life, except
  that life forms are able to act on their own
  behalf to support their own existence, and to
  reproduce themselves.
• The field of science known as Biology is
  dedicated to the study of life, its component
  species, and the variations within species of
  life forms.

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The Origin and History of Life on Earth

• What Is a Species?
• Species Are Groups of Interbreeding Populations
• Appearance Can Be Misleading
• Allopatric and sympatric speciation
• Geographic Separation of a Population Can Lead to
  Allopatric Speciation
• Ecological Isolation of a Population Can Lead to
  Sympatric Speciation

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Earths Biosphere and Life Science

• Our Earth is, to the best of our current
  knowledge, the only planet in our Solar System
  (or beyond) which supports life of any kind.
• The time table and processes of the origin of
  life is poorly known at present, but there is
  evidence that the simplest forms of life came
  into being more than 3 billion years ago (out of
  the total of 4.6 billion years our Earth has been
  in existence).
• Earths original atmosphere contained little or
  no molecular oxygen, which is required by current
  animal and advanced plant life forms.

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Earths Biosphere and Life Science

• The oxygen currently in our atmosphere is
  produced by green plant photosynthesis, which
  began with reduction of carbon dioxide by
  anaerobic bacteria, also known as cyanobacteria
  or blue-green algae, inhabiting the oceans in
  Earths early history.
• Animal life, as we know it (which requires
  atmospheric oxygen), did not come into existence
  until about 600 million years ago.
• An important topic of the present day is that
  humans are returning CO2 to the atmosphere at a
  faster rate than plants can reduce it to form O2,
  which can cause global warming.

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Evolution of Oxygen Content of Earths Atmosphere
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How Do New Species Form?

• Changes in Chromosome Number Can Lead to
  Sympatric Speciation
• Speciation by polyploidy
• Change Over Time Within a Species Can Cause
  Apparent Speciation in the Fossil Record
• Under Some Conditions, Many New Species May Arise

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How Is Reproductive Isolation Between Species
Maintained?

• Premating Isolating Mechanisms Prevent Mating
  Between Species
• Members of Different Species May Be Prevented
  from Meeting
• Different Species May Occupy Different Habitats
  (Ecological isolation)
• Different Species May Breed at Different Times
  (Temporal Isolation)
• Different Species May Have Different Courtship
  Rituals
• Species Differing Sexual Organs May Foil Mating
  Attempts
• Postmating Isolating Mechanisms Limit Hybrid
  Offspring
• One Species Sperm May Fail To Fertilize Another
  Species Eggs
• Hybrid Offspring May Survive Poorly
• Hybrid Offspring May Be Infertile

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What Causes Extinction?

• Localized Distribution and Overspecialization
  make Species Vulnerable in Changing Environments
• Very localized distribution can endanger a
  species
• Extreme specialization places species at risk
• Interactions with Other Organisms May Drive a
  Species to Extinction
• Habitat Change and Destruction Are the Leading
  Causes of Extinction
• Natural events, of geological or extraterrestrial
  origin, can cause sudden, and extremely
  catastrophic, extinctions on a large, or even
  global, basis.

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How Did Life Begin?

• Experiments Refuted Spontaneous Generation
• Chemical Evolution Preceded and Gave Rise to
  Life
• Organic Molecules Can Form Spontaneously Under
  Prebiotic Conditions
• Organic Molecules Probably Accumulated Under
  Prebiotic Conditions
• Organic Molecules May Have Become Concentrated
  in Tidal Pools
• RNA May Have Been the First Self-Reproducing
  Molecule
• Membrane-Like Microspheres May Have Enclosed
  Ribozymes

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No growth (Broth cooled without exposure to room
air)
Growth (Broth cooled and exposed to room air)
Broth Heated to Boiling to Sterilize

• Louis Pasteur's experiment disproving the
  spontaneous generation of microorganisms in
  broth.

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Experimental Research on the Origin of Life

• One of the greatest unsolved mysteries of science
  is, how did life evolve on Earth?
• We have evidence that the oldest and simplest
  forms of life, anaerobic bacteria, may have
  formed more than 3.6 billion years ago (only 1
  billion years after Earth was created).
• These life forms, also called cyanobacteria or
  blue-green algae, did not require oxygen to
  breathe (as do nearly all organisms inhabiting
  Earth at the present time).

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Experimental Research on the Origin of Life

• However, the capability of these anaerobic
  bacteria to convert atmospheric carbon dioxide
  (CO2) into oxygen (O2) and carbohydrates made
  possible the evolution of more complex life
  forms, including all animal life, that breathe
  oxygen and feed upon plant carbohydrates.
• Since we have very few geologic records of the
  earliest periods of life on Earth, scientists
  have performed laboratory experiments with
  electric discharges (simulating lightning) in
  various mixtures of gases (simulating Earths
  earliest atmosphere) to see if the chemical
  substances required for life forms could have
  been produced by lightning early in Earths
  history.

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• The Experimental Apparatus of Stanley Miller
  and
• Harold Urey
• Life's earliest stages left no fossils, so
  evolutionary historians have pursued a strategy
  of re-creating in the laboratory the conditions
  that may have prevailed on early Earth.
• The mixture of gases in the spark chamber
  simulates lightning in Earth's early atmosphere.

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• Did Microspheres Enclose the Earliest Cells?
• Cell-like microspheres can be formed by agitating
  proteins and lipids in a liquid medium. Each
  microsphere in this photo is about 5 micrometers
  (µm) in diameter.

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What Were the Earliest Organisms Like?

• The First Organisms Were Anaerobic Prokaryotes
• Some Organisms Evolved the Ability to Capture the
  Suns Energy
• Photosynthesis Increased the Amount of Oxygen in
  the Atmosphere
• Aerobic Metabolism Arose in Response to the
  Oxygen Crisis
• Some Organisms Acquired Membrane-Enclosed
  Organelles
• Mitochondria and Chloroplasts May Have Arisen
  from Engulfed Bacteria
• Evidence for the Endosymbiont Hypothesis Is
  Strong

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• The Relationship between Time and the Decay of
  Radioactive 40K (yellow) to 40Ar (blue)

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• The Probable Origin of Mitochondria and
• Chloroplasts in Eukaryotic Cells

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• Symbiosis Within a Modern Cell
• The ancestors of the chloroplasts in today's
  plant cells may have resembled Chlorella, the
  green, photosynthetic, single-celled algae living
  symbiotically within the cytoplasm of the
  Paramecium pictured here.

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What Were the Earliest Multicellular Organisms
Like?

• Some Algae Became Multicellular
• Higher organisms with differentiated cells
  evolved more than 1 billion years ago
• Animal Diversity Arose in the Precambrian Era
• Original animal forms were primarily aquatic
  shelled invertebrates (ocean dwellers), dating
  back to the Proterozoic (pre-Cambrian) era, more
  than 600 million years ago.
• Life Invaded the Land beginning in the Cambrian
  Era
• Land plants began in the Cambrian era, about 500
  million years ago
• Oxygen content of the atmosphere reached
  near-current levels in the Devonian era,
  beginning nearly 400 million years ago
• Land animals date back to the Devonian era, about
  350 million years ago.

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• Diversity of Ocean Life during the Silurian
  Period
• (a) Life characteristic of the oceans during the
  Silurian period, 440 million to 410 million years
  ago. Among the most common fossils from that time
  are (b) trilobites and their predators, such as
  nautiloids and (c) ammonites. This (d) living
  Nautilus is very similar in structure to the
  Silurian nautiloids, showing that a successful
  body plan may exist virtually unchanged for
  hundreds of millions of years.

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How Did Life Invade the Land?

• Some Plants Became Adapted to Life on Dry Land
• Primitive Land Plants Retained Swimming Sperm and
  Required Water to Reproduce
• Seed Plants Encased Sperm in Pollen Grains
• Flowering Plants Enticed Animals to Carry Pollen
• Some Animals Became Adapted to Life on Dry Land
• Amphibians Evolved from Lobefin Fishes
• Reptiles Evolved from Amphibians
• Reptiles Gave Rise to Both Birds and Mammals

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• A Fish that Walks on Land
• Some modern fishes, such as this mudskipper, walk
  on land. Like the ancient lobefin fishes that
  gave rise to amphibians, mudskippers use their
  strong pectoral fins to move across dry areas in
  their swampy habitats.

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What Role Has Extinction Played in the History of
Life?

• Evolutionary History Has Been Marked by Periodic
  Mass Extinctions
• Climate Change Contributed to Mass Extinctions
• Catastrophic Events May Have Caused the Worst
  Mass Extinctions
• The extinction of the Dinosaurs has recently been
  determined to have been caused by an asteroid
  impact with the Earth about 65 million years ago
• There is evidence of an even more catastrophic
  extinction event further back in Earths history
  (about 300 million years ago)

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• Continental Drift from Plate Tectonics
• The continents are passengers on plates moving on
  Earth's surface as a result of plate tectonics.
• (a) About 340 million years ago, much of what is
  now North America was positioned at the equator.
• (b) All the plates eventually fused together into
  one gigantic landmass, which geologists call
  Pangaea.
• (c) Gradually Pangaea broke up into Laurasia and
  Gondwanaland, which itself eventually broke up
  into West and East Gondwana.
• (d) Further plate motion eventually resulted in
  the modern positions of the continents.

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Evolution of the Earth with Time Continental
Drift
200 Million Years Ago
50 Million Years Ago
150 Million Years Ago
100 Million Years Ago
Present
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How Did Humans Evolve?

• Some Early Primate Adaptations for Life in Trees
  Were Inherited by Humans
• Binocular Vision Provided Early Primates with
  Accurate Depth Perception
• Early Primates Had Grasping Hands
• A Large Brain Facilitated HandEye Coordination
  and Complex Social Interactions
• The Oldest Hominid Fossils Are from Africa

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How Did Humans Evolve?

• The Earliest Australopithecines Could Stand and
  Walk Upright
• Several Species of Australopithecus Emerged in
  Africa
• The Genus Homo Diverged from the
  Australopithecines 2.5 Million Years Ago
• The Evolution of Homo Was Accompanied by Advances
  in Tool Technology
• Neanderthals Had Large Brains and Excellent Tools

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How Did Humans Evolve?

• Modern Humans Emerged Only 150,000 Years Ago
• Cro-Magnons and Neanderthals Lived Side by Side
• Several Waves of Hominids Emigrated from
  AfricaThe Evolutionary Origin of Large Brains May
  Be Related to Meat Consumption
• The Evolutionary Origin of Human Behavior Is
  Highly Speculative
• The Cultural Evolution of Humans Now Far Outpaces
  Biological Evolution

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• Representative Primates
• The (a) tarsier, (b) lemur, and (c) lion-tail
  macaque monkey all have relatively flat faces,
  with forward-looking eyes providing binocular
  vision. All also have color vision and grasping
  hands. These features, retained from the earliest
  primates, are shared by humans.

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• The earliest hominid
• This nearly complete skull of Sahelanthropus
  tchadensis, which is more than 6 million years
  old, is the oldest hominid fossil yet found.

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• Representative hominid tools
• (a) Homo habilis produced only fairly crude
  chopping tools called hand axes, usually
  unchipped on one end to hold in the hand. (b)
  Homo ergaster manufactured tools that were
  typically sharp all the way around the stone at
  least some of these blades were probably tied to
  spears rather than held in the hand. (c)
  Neanderthal tools were works of art, with
  extremely sharp edges made by flaking off tiny
  bits of stone. In comparing these weapons, note
  the progressive increase in the number of flakes
  taken off the blades and the corresponding
  decrease in flake size. Smaller, more numerous
  flakes produce a sharper blade and suggest either
  more insight into tool making, more patience,
  finer control of hand movements, or perhaps all
  three.

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Paleolithic Burial

• This 24,000-year-old grave shows evidence that
  CroMagnon people ritualistically buried their
  dead.
• The body was covered with a dye known as red
  ocher, then buried wearing a headdress made of
  snail shells and with a flint tool in its hand.

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• Competing Hypotheses for the Evolution of Homo
  Sapiens
• The "multiregional" hypothesis suggests that
  populations of H. sapiens evolved in many regions
  simultaneously from the already widespread
  populations of H. erectus.