Download Chapter 17 History of Life

The History of Life
Chapter 17
The First Living Things
• Alexander Oparin and John Haldane
(1920s and 1930s)
– Noted that an oxygen-rich atmosphere would
not have permitted the spontaneous formation
of complex organic molecules
– Speculated that the atmosphere of early Earth
contained little oxygen
– Proposed that prebiotic chemical evolution
gave rise to life
The First Living Things
• Oparin and Haldane envisioned that
prebiotic chemical evolution occurred in
four stages
– Prebiotic synthesis and accumulation of small
organic molecules
– Small organic molecules combined to form
larger molecules
– Origin of self-replicating molecules
– Packaging of molecules within some kind of
enclosing membrane
Organic Molecules
• Stanley Miller and Harold Urey (1953)
– Noted that the atmosphere of early Earth
probably contained methane, ammonia,
hydrogen, and water vapor, but no oxygen
– Simulated early Earth’s atmosphere by mixing
the above gases in a flask and adding an
electrical discharge to simulate lightning
• Simple organic molecules appeared after a few
days
Organic Molecules
• Exact composition of “atmosphere” was
unimportant
– Must contain carbon, hydrogen, and nitrogen,
and exclude oxygen
• Type of energy source was unimportant
– Electrical discharge, UV light, and heat were
equally effective
Organic Molecules
Accumulate
• The lack of both life and oxygen gas on early
Earth allowed large quantities of organic
molecules to accumulate in areas protected from
UV radiation (beneath rock ledges, in oceans)
• UV radiation bombarded early Earth’s surface
because there was no ozone to block it
• UV radiation can break apart organic molecules
• Accumulated simple organic molecules
combined to form complex organic molecules
Organic Molecules
• Similar experiments by Miller and others
have produced amino acids, short proteins,
nucleotides, and ATP
• Complex organic molecules join together
to form clusters – coacervates or
aggregates
• Aggregates became more complex &
highly organized
RNA
• May have been the first self-reproducing
molecule
• Thomas Cech and Sidney Altman (1980s)
discovered an RNA molecule (ribozyme)
• Ribozymes could catalyze a chemical reaction
• Naturally-occurring ribozymes have been found
that catalyze reactions including
– Cutting other RNA molecules
– Splicing together different RNA fragments
– Attaching amino acids to growing proteins
RNA
• Since Cech and Altman’s initial discovery
researchers have synthesized ribozymes
that catalyze the replication of small RNA
molecules
• Discovery of ribozymes led to hypothesis
that RNA preceded the origin of DNA
• RNA served as
– The information-carrying genetic molecule
– The enzyme catalyst for its own replication
When Did Life Arise on Earth?
• Earth formed about 4.5 billion years ago
• Life arose 3.9 to 3.5 billion years ago
during the Precambrian era
– Oldest fossil organisms found to date are
estimated to be about 3.5 billion years old
Heterotroph Hypothesis
• The first cells to arise in Earth’s oceans were
prokaryotes, cells that lack a membrane-bound
nucleus
• First life forms were not able to synthesize their
own organic nutrients
• These primitive bacteria probably:
– Obtained nutrients and energy by absorbing organic
molecules from their environment
– Metabolized organic molecules anaerobically (early
Earth lacked oxygen gas)
Capturing the Sun’s Energy
• The first photosynthesizing organisms
(ancestors of cyanobacteria) appeared
about 3.5 billion years ago
• Photosynthesis requires sunlight, CO2,
and hydrogen
– Earliest source of hydrogen believed to be
hydrogen sulfide
– Eventually, water replaced hydrogen sulfide
as the source of hydrogen and photosynthesis
became water-based
Increased Oxygen in
Atmosphere
• Water-based photosynthesis resulted in
the release of oxygen gas as a by-product
• Initially, oxygen combined with iron in the
Earth’s crust to form iron oxide
• Subsequently, oxygen began accumulating
in the atmosphere
– Chemical analysis of rocks suggests that
significant levels of atmospheric oxygen first
appeared about 2.2 billion years ago
Aerobic Metabolism
• The accumulation of oxygen in Earth’s
atmosphere probably
– Exterminated many anaerobic organisms
– Provided the environmental pressure for the
evolution of aerobic metabolism
• The evolution of aerobic metabolism was
significant because aerobic organisms can
harvest more energy per food molecule
than anaerobic organisms
Membrane-Enclosed
Organelles
• The first eukaryotes (cells that possess
membrane-bound organelles) appeared
about 1.7 billion years ago
• Several organelles (mitochondria,
chloroplasts, centrioles) may have arisen
when primitive cells engulfed certain types
of bacteria (the endosymbiont
hypothesis)
Evolution of Mitochondria
• Anaerobic, predatory prokaryotic cell
engulfs an aerobic bacterium that it failed
to digest
• Predatory cell and bacterium gradually
enter into a symbiotic relationship
• Descendants of engulfed bacterium evolve
into mitochondria
Evolution of Chloroplasts
• Mitochondria-containing predatory
prokaryotic cell engulf a photosynthetic
bacterium
• Predatory cell and bacterium gradually
enter into a symbiotic relationship
• Descendants of engulfed bacterium evolve
into chloroplasts