1. Earth’s Early History (Ch 19.3)
Unit 5: Evolution

2. The Mysteries of Life’s origins
Early Earth
Evidence proves cosmic debris collided with one another to form the earth
The earth was struck by huge objects (comets and asteroids)
Volcanic activity shook the earth
Atmosphere
Little to no oxygen
Major gases: Carbon dioxide, water vapor, nitrogen
Minor gases: carbon monoxide, hydrogen sulfide, hydrogen cyanide
4.2 BILLION YEARS AGO – Earth cooled and allowed water to condense (STABILITY)

3. The first organic molecules
Miller and Urey
Can organic molecules assemble in early earth?
They tested the theory by combining elements found on earth with water and stimulating lightning – then they let the water condense and cool
Results: 21 amino acids were formed! (building blocks of life)
Their results suggested how organic compounds necessary for life arose on a primitive earth
Formation of microspheres
Organic molecules for tiny bubbles called proteinoid microspheres
Microspheres are not cells but have some characteristics of living systems
They appeared on earth 3.8 billion years ago

4. Miller and urey Experiment

5. Evolution of RNA and DNA
Scientists have NOT found the answer yet, but have some hypotheses
Rna formed from simpler molecules
RNA existed before dna – DNA directed protein synthesis would result from the prescence of rna
Production of Free Oxygen
bacteria have been found that lived in the presence of little to no oxygen (3.5 billion years ago)
Photosynthetic bacteria became common 2.2 billion years ago
Oxygen combined with iron – iron oxide (rust)
No iron = blue green oceans
Oxygen accumulated in the atmosphere = formation of ozone
Some organisms went extinct because of the oxygen while others prospered or adapted

6. Origin of eukaryotic cells
Endosymbiotic theory
Ancient prokaryotes began evolving internal cell membranes (ancestor of eukaryotic organisms)
Prokaryotes entered the ancient eukaryotes, and instead of digesting the prokaryotes, the prokaryotes lived in the eukaryotes
Symbiotic relationships evolved over time between primitive eukaryotic cells and the prokaryotic cells within them
Two Hypotheses:
Mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate ATP (used in respiration)
Chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize (used in photosynthesis)

7. Endosymbiotic theory

8. Modern evidence for endosymbiotic theory
Marguilis gathered evidence to support endosymbiotic theory
Mitochondria and chloroplasts contain DNA similar to bacterial DNA
They have ribosomes that resemble those of bacteria
They reproduce by binary fission like bacteria when cells divide by mitosis
Sexual Reproduction and Multicellularity
After eukaryotic cells arose they reproduced sexually to speed up evolutionary change and produce genetic variation