Ch 20 Origin of Life
-Chemical Evolution: the decrease of the
entropy of molecules (energy required)
-Biological Evolution: formation and
adaptation of cells
Chemical evolution is the increase in
complexity of chemicals leading to the first
cells.
Biogenesis (1865): life only comes from
life, but the first cells had to arise from
an increased complexity of chemicals.
The earth came into being about 4.6
billion years ago.
The earth's size provides a
gravitational field strong enough to hold
gas (atmosphere).
Earth's primitive atmosphere differed from
current atmosphere, consisting of:
water vapor (H2O),
nitrogen (N2),
carbon dioxide (CO2),
small amounts of hydrogen (H2), and
carbon monoxide (CO).
Primitive atmosphere was formed by
volcanic out-gassing characteristic of
the young earth.
The primitive atmosphere contained little
free oxygen (O2) and was a reducing
atmosphere as opposed to the oxidizing
atmosphere of today.
A reducing (e-?) atmosphere lacks free
O2 and allows formation of complex
organic molecules.
An oxidizing (e-?) atmosphere contains
free O2 and inhibits formation of complex
organic molecules
The earth was so hot that H2O only existed as
a vapor in dense, thick clouds.
As the earth cooled, H2O vapor condensed to
form liquid H2O, and rain collected in ponds,
etc.
The earth's distance from the sun allows H2O
to exist in all phases: solid, liquid, and gas.
Monomers Evolve
Aleksandr Oparin's 1938 Hypothesis:
Suggested organic molecules could be
formed in the presence of outside
energy sources:
1) methane (CH4)
2) ammonia (NH3)
3) hydrogen (H2)
4) water (H2O)
Miller’s Experiment (1953)
Used electricity and
convection to show
these gasses in the
primitive atmosphere
could react with one
another to produce
small organic molecules
like:
- Amino acids
- Nucleotides
- Simple sugars
Other ideas:
Ammonia may have been scarce in the
atmosphere; undersea thermal vents,
however, produce much ammonia and
additional natural reactions form peptides.
Meteorites have been found that contain
extraterrestrial amino acids (different from
any others used by life on our planet).
Polymers Evolve: Newly formed simple organic
molecules polymerized to produce larger molecules
and macromolecules
Gunther Wachtershauser
and Claudia Huber have
formed peptides using ironnickel sulfides as inorganic
catalysts under vent-like
conditions (high temp &
press).
Impact force of meteorites
have joined amino acids into
peptides
Protein-first Hypothesis: DNA genes came
after protein enzymes; DNA replication needs
protein enzymes.
Sidney Fox demonstrated
amino acids polymerize
abiotically if exposed to dry
heat
Amino acids collected in
shallow puddles along the
rocky shore; heat of the sun
caused them to form
proteinoids (i.e., small
polypeptides that have some
catalytic properties).
When proteinoids are
returned to water, they form
cell-like microspheres
composed of proteins.
The Clay Hypothesis:
amino acids polymerize
in clay, with
radioactivity providing
energy
If RNA nucleotides and
amino acids became
associated so polypeptides
were ordered by and helped
synthesize RNA, then
polypeptides and RNA arose
RNA-first Hypothesis: Only
the macromolecule RNA was
needed at the beginning to
lead to the first cell.
RNA can be both a substrate
and an enzyme, so RNA
could carry out processes of
life associated with DNA (in
genes) and protein enzymes
at the same time.
Polymerization of monomers to produce proteins
and nucleotides is the next step toward the first cell.
Biological Evolution
Protocell: would have with a lipidprotein membrane and carry on energy
metabolism.
Before the first cell arose, there would
have been a simpler intermediate stage.
Oparin(1940)
demonstrated a protocell
could have developed from
coacervate droplets:
complex spherical units
made up macromolecules,
that spontaneously form
when concentrated mixtures
of macromolecules are held
in the right temperature,
ionic composition and pH.
- engulf various substances
from the surrounding
solution.
In a liquid environment,
phospholipid molecules
spontaneously form
liposomes: spheres
surrounded by a layer
of phospholipids; this
supports a semipermeable-type
membrane.
A protocell could have
contained only RNA to
function as both genetic
material and enzymes.
Two ideas on how early cells obtained
energy.
If a protocell was a
heterotrophic fermenter
living on the organic
molecules in the organic
soup that was its
environment; this suggests
heterotrophs proceeded
autotrophs.
heterotroph is an organism
that cannot synthesize
organic compounds from
inorganic substances and
therefore must take in
preformed organic
compounds.
If the protocell evolved at
hydrothermal vents, it would
be chemosynthetic and
autotrophs would have
preceded heterotrophs
An autotroph is an
organism that makes organic
molecules from inorganic
nutrients
Photosynthesis comes next (3.6 bya)
Atmosphere’s CO2 increases
due to chemosynthesis and
fermentation.
Cyanobacteria adapt a use
for this CO2. (Stromatolites)
First release of O2 in our
atmosphere.
- Significance: aerobic
respiration now possible,
O2 + O  O3; protection from
UV rays = life on surface.
Evolution of the Eukaryotic cell(1.5
bya): The endosymbiosis hypothesis
Refers to a cell that
lives w/in another,
host cell.
Mitochondria and
chloroplasts were
formerly small
prokaryotes that
began living w/in
other heterotrophic
cells.
Evidence for endosymbiosis
Mitochondria and chloroplasts both:
Double membrane bound.
Have their own separate DNA (circular
chromososme)
Have their own ribosomes = own
proteins.
Divide independently of cell.