Early Earth

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Early Earth the Origin of Life
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Early Earth and the Origin of Life

• Most scientists agree that life on Earth came
  about through the following sequence
• 1. Synthesis of small organic molecules
• 2. These molecules became polymers
• 3. Self-replicating molecules emerged that made
  inheritance possible
• 4. All these molecules were packaged into
  membrane-containing droplets, whose internal
  chemistry differed from that of the external
  environment
• It is hypothesized that RNA was the first genetic
  material
• Protobionts may have existed before cells. These
  would have been aggregates of molecules that were
  produced abiotically with consistent internal
  environments and some other properties associated
  with life

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Laboratory versions of protobionts
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Fossil Record

• Life on Earth originated 3.8-3.9 billion years
  ago
• For the first ¾ of Earths history, all organisms
  were microscopic and primarily unicellular
• The earliest organisms were prokaryotes
• Eukaryotes appeared about 2.1 billion years ago,
  and multicellular eukaryotes evolved about 1.2
  billion years ago
• Plants, fungi and animals began to appear on
  Earth about 500 million years ago

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Clock analogy for some key events in Earths
history
Ceno-zoic
Meso-zoic
Humans
Paleozoic
Land plants
Origin of solar system and Earth
Animals
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1
Archaean Eon
Proterozoic Eon
Billions of years ago
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3
Multicellular eukaryotes
Prokaryotes
Single-celled eukaryotes
Atmospheric oxygen
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Oxygen and the Atmosphere

• Oxygen began to accumulate in Earths atmosphere
  about 2.7 billion years ago. What is the
  evidence?
• When the oceans first formed, the waters
  dissolved enormous quantities of iron ions
• These ions were the consequences of millions of
  years of rock weathering in an anaerobic
  (oxygen-free) environment
• The first oxygen produced in the oceans by the
  early prokaryotic cells would have quickly
  reacted with the dissolved iron
• This oceanic oxidization reaction produces ferric
  oxide that would have been deposited in ocean
  floor sediments
• The earliest evidence of this process dates back
  to the Banded Iron Formations
• These formations were created as sediments in
  ancient oceans and are found in rocks in the
  range 2 - 3.5 billion years old

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Banded iron formations evidence of oxygenic
photosynthesis
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Oxygen

• The most important change in Earth's environment
  since life began was the buildup of free oxygen
  in the air. 
• Before life became established on Earth, and even
  for some one or two billion years afterwards, the
  air contained no free oxygen.
• The atmosphere today is 21 oxygen. Where did it
  come from?
• The significant buildup probably began about two
  billion years ago, with the appearance of
  Cyanobacteria
• When the oxygen began to accumulate, it
  constituted a wholesale pollution of the
  environment, a challenge for all living things on
  Earth. Too much oxygen in living systems can lead
  to oxidation which is biochemically toxic
  (anti-oxidants today help protect)
• Some bacteria however were able to endure the
  oxygen atmosphere.  A symbiosis between bacteria
  and the formerly free-living mitochondria enabled
  eukaryotes to evolve in response to the crisis.
  Oxygen-based metabolism came into being. The
  environment changed and so did life.