The Origin of Life

1
Chapter 26

• The Origin of Life

2
Origin of Life

• Earth formed 4.5 billion years ago and life began
  a few hundred million years later
• Organisms were only microscopic and unicellular
  for the first ¾ of evolutionary history
• Stromatolites are rocks made of banded domes of
  sediment in which are formed the most ancient
  forms of life prokaryotes dating back as far as
  3.5 billion years

3
Major Episodes in the History of Life

• the production of oxygen by early photosynthetic
  prokaryotes created an aerobic atmosphere
  aerobic life evolved
• oldest eukaryotic fossils are about 1.7 billion
  years old
• eukaryotic cells probably evolved from a
  symbiotic community of prokaryotes
• plants, fungi, and animals came from unicellular
  eukaryotes during the Precambrian period
• plants evolved from green algae

4
Major Episodes in the History of Life

• fungi and animals evolved from heterotrophic
  unicellular organisms
• the oldest animal fossils were from about 700
  million years ago
• life on Earth was aquatic for 90 of its
  existence
• when plants colonized land, they changed the
  landscape and created more opportunities for life
  on land

5
Prebiotic Chemical Evolution

• the first organisms were products of a chemical
  evolution in four stages
• abiotic synthesis and accumulation of small
  organic molecules
• the joining of these monomers into polymers
• the aggregation of abiotically produced molecules
  into droplets called protobionts
• the origin of heredity

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Abiotic Synthesis of Organic Monomers

• The conditions of early Earth favored the
  chemical reactions that synthesized organic
  compounds from inorganic materials.
• Primitive atmosphere consisted of reducing
  (electron adding) gases, instead of oxygen-rich
  atmosphere, enhancing the creation of complex
  molecules.
• Lightning and UV radiation from the sun probably
  provided the energy needed to catalyze the
  reactions.

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Abiotic Synthesis of Organic Monomers

• Stanley Miller and Harold Urey recreated early
  Earth conditions in a lab and have synthesized
  several organic compounds including all 20 amino
  acids, lipids, several sugars, and the bases
  found in DNA and RNA

8
Production of Organic Polymers

• Dilute solutions of organic monomers dripped onto
  hot sand, clay, or rocks causes polymerization.
• Clay was especially important because monomers
  bind to charged sites on the clay particles
• Pyrite, also called fools good, could have been
  the substratum or organic synthesis because it
  has a charged surface and it yields electrons
  that support bonding between organic materials

9
Protobionts

• Living cells may have been preceded by
  protobionts, aggregates of abiotically produced
  molecules.
• These protobiont droplets show some
  characteristics of life metabolism and
  excitability.