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AP Biology
N. Cunningham
Evolution Note packet
The Origin of Life
(a.k.a.The Heterotroph Hypothesis)
A. The question arises, how have
the first forms of life ever evolved?
According to scientific theory, the
following steps have been
hypothesized that supports the view
that the first form of life was a
primitive, heterotrophic cell that
subsequently led to more complex
forms of life.
1. The earth and its atmosphere
formed.
Primordial gases?
Gases came from molten interior of
the planet.
Consisted of CO, CO2, H2, N2, H2O
(steam), S, HCl
What gas was missing from the
primitive atmosphere?
OXYGEN!
2. The primordial seas formed.
As the earth cooled, gases
condensed to form the first seas.
3. Complex molecules were
synthesized.
a. Organic “soup” formed
Organic molecules (have carbon
with hydrogen) were formed from
the inorganic molecules.
b. Energy sources? UV, lightning,
radioactivity, energy from
volcanoes, heat were abundant
c. What kinds of molecules
formed?
Monomers like amino acids that
would later serve as building
blocks for the synthesis of larger
polymers.
d. Oparin and Haldane
independently theorized that
organic molecules could only
form in the absence of oxygen.
Why?
Oxygen is a very reactive
molecule. Had it been present,
oxygen would have prevented
other molecules from forming by
replacing them with most
reactants in chemical reactions.
e. Stanley Miller’s experiment?
He tested the theories of Oparin
and Haldane by simulating
primordial conditions in the lab.
He applied electric sparks to
simple gases (but no oxygen) in a
flask. After 1 week, the water
contained various organic
molecules like amino acids.
4. Polymers and self-replicating
molecules were synthesized.
Monomers combined to form
polymers by dehydration
synthesis.
5. Organic molecules were
concentrated and isolated into
protobionts (precursors of cells)
Protobionts were able to carry out
chemical reactions within a border
across which materials could be
exchanged.
6. PRIMITIVE HETEROTROPHIC
PROKARYOTES FORMED!!!
The first cells formed that were able
to consume organics molecules.
Food source? Organic molecules
like proteins, lipids, glucose from
the sea
Reproduction? Asexual (binary
fission)
Type of respiration? Anaerobic
7. Primitive autotrophic
prokaryotes formed.
First organisms to make their own
food!
Who were these autotrophs – at
first?
1st chemosynthesis- autotrophic
bacteria that make food via
hydrogen sulfide from underwater
volcanic gases
Next- photosynthetic bacteria
(cyanobacteria) use light for food
production (cyclic
photophosphoylation)
Eventually – eukaryotic
photosynthetic organisms –
eventually non-cyclic
photophosphorylations
Type of autotrophic nutrition?
First  chemosynthesis – in
prokaryotes
Then  photosynthesis – cyclic
photophosphorylation
Finally  photosynthesis –non
cyclic photophosphorylation
How did this change the world?
Oxygen was released into the
atmosphere!
8. Oxygen and the ozone layer
formed
Ozone (03) formed as a protective
layer against UV- allowed for
terrestrial existence.
Presence of oxygen allowed for
aerobic respiration to evolve (more
energy production meant organisms
could get larger)
9. Eukaryotes formed
(endosymbiotic theory)
Mitochondria and chloroplasts –
once prokaryotes- took up
residence inside of other cells to
become first eukaryotic cells
This brought about changes in…
Respiration  Aerobic
More energy means larger
organisms can evolve as well.
Reproduction Sexual (more
diversity means greater chance for
survival)
http://www.pbs.org/wgbh/nova/evol
ution/where-did-we-comefrom.html