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