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Chapter 17
Origins of Life
Origins of Life
• Astrobiology is study of the
origin, evolution, distribution, &
destiny of life in the universe.
• It attempts to answer 3
fundamental questions:
– How did (does) life begin and
develop?
– Does life exist elsewhere in
the universe?
– What is life's future on Earth
and beyond?
Advances in the biological sciences, space exploration, and astronomy
make it possible for us to realistically attempt to answer them.
Here’s what we “know”:
• Free oxygen probably was not present in the early atmosphere.
– Compounds in early rocks do not contain oxygen…… significant
amounts of oxygen would not have formed until the process of
photosynthesis was under way.
• Some of the ideas about the origin of life cannot be stated in
the form of hypotheses or tests;
– therefore, science is limited in it’s ability to investigate such ideas.
• Mineral crystals have been proposed as an early information
molecule because they have a definite repeating structure.
• RNA is a more likely candidate for life formation than DNA
because DNA is more complex than RNA and requires proteins
for its replication.
– RNA can cataylze its own splicing.
• The conditions on early Earth were probably similar to
those on Venus and Mars today,
– that is, containing high levels of CO2, water vapor, and N2 and small
amounts of H2.
• It’s easier to understand the formation of the organic
compounds necessary for life than the formation of the first
cells.
• Chemical evolution can provide a mechanism by which life
could have originated from increasingly more complex
chemicals in the primitive seas.
– Darwinian evolution gives us a way by which organisms can change
over time, but that could not start until chemical evolution resulted
in the first life.
• And then there is the question of energy sources……….
– Early energy sources included lightning, UV radiation, and heat from
the radioactive decay within earth.
– These energy sources drove the chemical reactions that produced
the first life.
– Those organisms most efficient at using the energy had an
advantage over less efficient organisms.
Heterotroph Hypothesis
• Proposed that the first living cells were heterotrophs
– Therefore they had to obtain organic nutrients from the environment
• In the 1920’s, Oparin and Haldane independently proposed this
theory
– Early atmosphere had methane, ammonia, hydrogen, and
water vapor in it.
– An energy source (radioactivity, lightening, cosmic radiation,
and/or heat from volcanoes) reacted with those gases and
formed organic compounds.
• This hypothesis requires 3 major steps:
– A nonbiological processes had to supply some organic
molecules.
– Some process had to trigger the small molecules to form
polymers like nucleic acids and proteins
– Some other processes had to organize the polymers that could
replicate itself.
Urey & Miller’s Experiment- 1952
• Can organic compounds be generated under conditions
similar to those that existed on primeval earth?
Conclusions:
The organic building
blocks of life (amino
acids and all the
building blocks for
nucleic acids)
formed in the liquid.
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Early Organisms
• Early atmosphere lacked oxygen gas
– the first organisms must have been
anaerobic (probably methanogens)
• There is some evidence of
photoautotrophs dating back to 4
billion years ago.
• Oxygen gas was probably insignificant
until about 2 billion years ago.
– Aerobic respiration may have
evolved as a way to produce large
amounts of ATP from food sources.
Endosymbiotic Hypothesis
• The first microfossils of
eukaryotes are about 1.4
billion years old.
• This hypothesis proposes
that eukaryotes originated
from a symbiotic
relationship between large
anaerobic prokaryotes and
smaller aerobic or
photosynthetic prokaryotes
animation
• The question of whether life was
created by a supernatural being cannot
be investigated by the methods of
science because no controlled
experiments can be set up and run.