1. Ch. 15 Evolution Section 1: Origin of Life Continued

2. RNA’s Role in Early Life: RNA World Hypothesis
For DNA to replicate it needs enzymes like helicase and polymerase, but RNA does not need these enzymes (this property makes it an ideal candidate for early replication)
RNA can:
Carry its own genetic material
act as its own enzyme called a ribozyme

3. Protobionts: A Step towards Cell Membranes?
Not technically cells, but may have features that helped early cells evolve
Form spontaneously
Protect polymers from outside environment by forming primitive “cell membranes”

4. Prokaryotes make Oxygen
Early (~ 4bya) life forms (bacteria) were anaerobic and heterotrophic
Eventually over-production led to competition and some of these organisms had an advantage b/c they made their own food (autotrophs) instead of consuming other molecules (heterotrophs)
~3.5 bya some bacteria evolved the ability to photosynthesize

5. The photosynthesizing bacteria take in CO2 but releases O2 which caused a massive oxygen build up in the atmosphere called the Oxygen Catastrophe which occurred ~2 bya
(this caused anaerobic bacteria to be killed off in large numbers..the survivors were bacteria that were protected from the atmosphere…in mud, underground, protected inside cells)

6. Other Results
No Need to Write!
Caused the oxidation of sediments in the ocean (visible in iron-rich rocks that date back to that time)
Ozone layer (O3) formed in atmosphere and blocked out UV radiation; blocked production of future organic chemicals

7. Endosymbiotic Theory
Bacteria were the only organisms from ~ bya
Primitive eukaryotes evolved ~ 2 bya
Theory states that organelles (like chloroplasts and mitochondria) that are found inside eukaryotic cells were once independent prokaryotes
These organelles were not digested, but instead were kept alive and a symbiotic relationship evolved
Since natural selection favored eukaryotes that could make their own food or eukaryotes that could metabolize food more efficiently, these organisms survived better than other organisms

9. SUPPORT of THEORY PROBLEMS with THEORY
Both chloroplast and mitochondria have two membranes
One cell engulfing another does not guarantee that the parent cell will pass on to its offspring the ability to synthesize the new organelle
Both have their own circular loop of DNA which is a circular loop like prokaryotes
When you remove chloroplasts and mitochondria from a cell, they cannot survive on their own
Both can perform protein synthesis using their small ribosomes
Both can replicate themselves

10. Brought into cell via endocytosis which leads to being double-membraned