Origin of Life on Earth
What do we know about early life on Earth?
• Evidence from rocks called stromatolites
• Consist of banded domes of rock
• Similar to layered mats found in salt marshes today
Stromatolites
• Large mats found at Shark Bay, Western Australia are considered “living stromatolites”
Cross section of a stromatolite
• Notice the layers of microbes known as fossilized “mats” of bacteria
What do we know about early life on Earth?
• Evidence in individual fossilized cells suggest life as early as 3.2-3.5 billion years ago
• See Fig. 5.2 (text)
What do we know about early life on Earth?
• Evidence from analysis of ratios of carbon isotopes in ancient rocks (C13, C 12)
• Rocks older than 3.85 billion years may had organic life
If life existed 3.85 billion years ago…
• Would we find traces of it?
• No! Why?
Tree of life as evidence?
Evidence of early life on Earth
• Stromatolites
• Fossilized cells
• Use the tree of life to determine relationships
What was the early Earth like?
• Water, no land
• No oxygen present
• No protection from UV radiation
• Shallow ponds the home of first life
Early life needs better energy source
• deep sea vents• http://www.ocean.udel.edu/
deepsea/level-1/geology/smoker.mov
• Surface hot springs
Miller-Urey Experiment
• Used flasks to simulate chemical conditions present on Earth
• Generated organic molecules…but not enough for life
Where could organic compounds come from?
• Impacts (asteroids, comets, meteors)
• Chemical reactions near deep sea vents
What do we have so far?
• Organic compounds
• Environment that can produce these compounds
• What else do we need??
What type of molecule would be best at replication?
• DNA?
• RNA?
1980’s discover replicating RNA
• Cech and colleagues identify that RNA can self-replicate….acts as enzymes
• Called ribozymes
If it was an “RNA World”
If not “RNA World” then what?
• DNA…still have the “chicken and egg problem”
• Proteins were first
• Double origin, proteins and replication of nucleic acids
• Clay crystals serve as scaffolding
How can we get to a more “cell-like” model?
• Need to confine the organic compounds in membranes
• “pre-cells” formed from– Amino acids– Lipids mixed with water
Summary of early life…takes many steps
• Need materials to form “organic soup”
• Generate more complex molecules (polymers)
• Membranes formed
• Natural selection of RNA leads to DNA model
Is this the only idea behind early life on Earth?
• No
• What is panspermia?
Evidence to support Panspermia
• Organic molecules in meteorites
• Microbes can survive moderate time in space
When did bacteria evolve compared to other cells?
A look at early microbes
• General characterisitics
Deep Sea vents model early life for microbes
Photosynthesis changed the Earth
• Organisms could obtain energy from the sunlight
• Evolved slowly…first just pigments trap uv light
• Purple and green sulfur bacteria are much like early photosynthetic microbes
Photosynthesis using water
• Evolved 3.5 billion years ago
• Earliest fossils resemble modern cyanobacteria (blue-green algae)
Cyanobacteria in Lake Sammamish
• Sept 1997• Algal bloom of
Microcystis aeruginosa
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