Tough Times on Land

Life:

• Astronomical Definition: self-replicating information
• Biological Definition: replication via nucleic acids (DNA

Evolution and Chance:

Dispersal of Parental Units by a variety of mechanisms:

Passive: (slowly alters gene pool over time)

• Migratory Random Walks
• Continental Drift
• Mountain Building
• Climate Changes
• Milloins of years timescale

Active: (catastrophe --> "overnight" change in ecosystem)

• Asteroid Impacts
• Large Floods
• Volcano eruption
• massive earthquake

Survival of the fittest works in well-defined, slowly evolving ecosystems (like the ocean); On long time scales, the role of random catastrophe is more important

Mass Extinctions in Geological History:

• 500 million years ago (unclear)
• 230 million years ago --> definite
• 65 million years ago --> famous

Mass extinctions mean:

• 1/2 to 2/3 of all species (plant and animal) eliminated
• plankton in the ocean is extincted
• both land and water are affected

Timescale of extinction is crucial in identifying its cause:

• slow --> environmental degradation
• rapid --> nuked

Evidence from Fossil Record: 100-150 million year extinction periodicity ???

• Plate Tectonic Cycle ?
• Climate Cycle ?
• Solar Cycle ?
• Solar System Cycle ?
• Galactic Cycle ?

Since the orbital period of the Sun around the center of the Galaxy is about 250 million years then every 100-150 million years the earth is in a spiral arm which is a region of high supernova density. It is possible the earth could be subject to high radition from these supernova.

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