Transcript Origin of life on Earth

Origin of life on Earth
Geology 103
How and when did life begin?
• Any “origin of terrestrial life” theory must be
consistent with and explain existing
observations
First observation
• All living things
share the same
basic molecules:
lipids (fats),
carbohydrates
(sugars), proteins
and nucleic acids
Stereochemistry
• Stereoisomers are two molecules that have the same
number and kind of atoms, and the same bonds
between those atoms, but the molecules have
different properties due to the orientation of the
atoms in space
• The simplest example are two molecules that are
mirror images of each other
Second observation
• All life on Earth uses
L-amino acids to
make proteins, and
D-sugars to make
carbohydrates
• Implies common
origin for life
First theory
• Cambridge University
geneticist JBS Haldane
(1929) proposed
“primordal soup” –
biologically-interesting
molecules created in
the ocean from
ultraviolet (UV) light
interacting with
methane, water and
ammonia.
“Secret of life” - next theory
• University of Chicago
scientists Harold Urey
and Stanley Miller
(1953) performed a
series of experiments in
which “early Earth”
atmosphere gases were
sealed in a bulb and
subjected to repeated
electrical discharges
Urey-Miller experiments
• Different atmospheric
compositions tested: the most
famous was the 1953 experiment
in which a
methane/hydrogen/ammonia and
water mixture generated amino
acids, the building blocks of
proteins
• But it was the 1958 experiment
in which hydrogen sulfide was
added that resulted in critical
sulfur-containing amino acids
being synthesized
Necessary conditions for life
• Significant amounts of carbon (the raw
material)
• Plentiful liquid water (the reaction
medium)
• Energy source (the driving force for
chemical reactions)
Carbon and water
• For liquid water, temperatures need to be
between 0° and 100°C at 1 atmosphere
pressure
• To freeze carbon in place, temps need to be
less than – 77°C
Earth carbon sources
Meteorite carbon
Energy sources
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Solar UV radiation
Lightning
Volcanoes
Thermal vents
Radioactive decay
Thermal vents
• Hot springs found on
the abyssal plain (about
4 km below the ocean’s
surface) eject extremely
hot (300°C) water,
along with metals
• JPL astrobiologist
Michael Russell (2001)
proposed the
“chemiosmosis model”
of utilizing this energy not direct use of thermal
energy
Radioactive beach
• Higher tides due to a
closer Moon during
Hadean may have
allowed the sorting of
radioactive elements on
a beach to generate
fission reactions (UW
astrobiologist Zachary
Adam, 2008)
Molecules to life
• Even assuming
all classes of
biomolecules
could be
synthesized, it’s
still a long way
to a cell!
• A lipid bilayer
(top) is not a cell
(bottom)
Life functions
• Self-replication –
make copies of itself
(reproduce)
• Metabolism – use
energy to perform
life functions
• Self-organization –
use molecules to
make structures
Clay minerals to replicate proteins
• University of Glasgow
chemist Graham
Cairns-Smith (1985)
proposed that the
surface of clay minerals
(which are silicates)
have atomic spacing
that can mimic the
spacing that amino
acids need in order to
bond to each other to
form proteins
• Problem: How to
replicate the same
protein over and over
Metabolism mimics reactions in
primordal soup
RNA connects organization with
replication
• Ribonucleic acid
(RNA) is used
primarily to code for
proteins, but some
enzymes are partly
made of RNA
• Perhaps RNA was
the first replication
and organization
molecule
RNA world to DNA world
• Columbia University
scientist Walter Gilbert
(1986) proposed RNA
world as a precursor to
DNA world – RNA
would be a template for
proteins, not clay
• Deoxyribonucleic acid
(DNA) is far more stable
– better to store genetic
information
Molecular evolution
Or maybe…panspermia
• “Life down here originated out there.”
• Cambridge University astronomers Fred
Hoyle and Chandra Wickramasinghe
(1970s) proposed that ice particles and
comets could contain micro-organisms
that “seeded” the early Earth with life.
This is based on evidence of complex
molecules (not living things) found
spectroscopically in deep space
Timeline: about 200 to 300 Ma for life
to happen after oceans form (length
of time multi-cellular creatures have
been around: 600 Ma
Conclusion
• Presence of
biochemicals pretty
well understood
• Biomolecules to life
transition poorly
understood
• First organisms to
more complex
organisms well
understood