Evolutionary History
Chapter 20
Before life…
Chemical evolution:
The formation of small organic
molecules preceded larger ones
Larger, more complex
molecules formed
The truth is – we just don’t know
exactly how this occurred
After life started, we have the fossil
record to help us out
Chemical Evolution
 Earth’s age is estimated at 4.6 billion
years
 Early atmosphere very different than
today:
 Carbon dioxide, water vapor, carbon
monoxide, hydrogen, nitrogen, ammonia,
hydrogen sulfide, and methane
 No free oxygen, lots of energy
 Earliest traces of life are
approximately 3.8 billion years old
(we think)
Stage 1 – abiotic synthesis of
organic monomers
 Monomer examples: amino acids, nucleotides
 Primordial soup hypothesis:
 Miller- Urey experiment in 1953  amino acids
produced
 Iron-sulfur world hypothesis:
 Thermal vents similar to what we have deep
in the ocean provided energy and chemicals
for synthesis
 Extraterrestrial origins hypothesis:
 Comets and meteorites provided needed
chemicals
Stage 2 – evolution of
polymers
 Where did the enzymes come from to catalyze
the reactions?
 Iron-sulfur world hypothesis
 We know that amino acids form peptide
bonds under conditions found at thermal
vents
 Protein-first hypothesis
 Maybe heat from the sun caused amino
acids to bind together
 RNA-first hypothesis
 Some viruses have RNA as their genetic
material
Stage 3 – evolution of
protocells
 Membrane-first hypothesis
In water fatty acids arrange
themselves into spheres
The hydrophobic tails are to the inside
Perhaps these micelles organized
themselves into a bilayer
Stage 4 – evolution of a selfreplicating system
 RNA-first hypothesis
RNA, then RNA reverse transcriptase,
then DNA
 Today’s process in most cells goes:
DNA  RNA  protein
The first cells
 Microfossils: the oldest fossil cells that are
widely accepted are 2 billion years old
 Stomatolites – rock columns of
prokaryotic cells
 Earliest cells were prokaryotes,
heterotrophs, and anaerobes
 Later cells developed that were
photosynthetic autotrophs
 Photosynthetically produced oxygen
was necessary for aerobic respiration to
evolve
Eukaryotic Cells
 Endosymbiont theory:
Organelles such as mitochondria and
chloroplasts arose from symbiotic
relationships between 2 prokaryotic cells
Now these organelles are obligated to
remain within the cells
These organelles are close in size to
bacteria, contain their own DNA, divide
independently from the cells they inhabit,
have folded internal membranes, and some
protein production abilities