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 self-replicating 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
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