Rare Earth Factors
From: Rare Earth: Why Complex Life is Uncommon in the Universe,
Peter D. Wood & Donald Brownlee, 2000
Astrobiologists, Wood & Brownlee postulate that the following 18 factors
prohibit or hinder bacteria from evolving into complex life, but first, the
Timeline of Evolution:
4.5 Billion Years Ago: Earth formed
4 BYA: "Life" (bacteria/archaea)
1 BYA: Multicelluar algae and seaweed
i.e., "complex life" took 3.5 billion years!
600 Million YA: Sponges/Worms
475 MYA Plants/Fungi
450-365 MYA Non-flying insects, arthropods and fish
300 MYA: Reptiles on land
250 MYA: Mass extinction wipes out 95% of animal species
220 MYA: First mammals
200 MYA: Birds
65 MYA: 50% of animal species die from meteor cooling planet
1800 Thousand YA: Homo erectus
130 TYA: Neanderthals (Homo Neanderthenis)
100 TYA: Homo sapiens
"intelligent life"
30 TYA: Man migrates to North America
27 TYA: Neanderthals extinct
4 TYA: Humans record history
Rare Earth Factors:
Why Complex Life is Uncommon in the Universe
Right distance from star
Habitat for complex life.
Liquid water near surface.
Far enough to avoid tidal lock.

Right planetary mass
Retain atmosphere and ocean.
Enough heat for plate tectonics.
Solid/molten core.

Plate tectonics
CO2-silicate thermostat.
Build up land mass.
Enjance biotic diversity.
Enable magnetic field.

The right tilt
Seasons not too severe.

Atmospheric properties
Maintenance of adequate temprature,
composition and pressure for plants
and animals.

Right kind of galaxy
Enough heavy elements.
Not small, elliptical, or irregular.

Right mass of star (sun)
Long enough lifetime.
Not too much ultraviolet.

Jupiter-like neighbor
Clear out comets and asteroids.
Not too close, not too far.

Not too much.
Not too little.
Giant impacts
Few giant impacts.
No global sterilizing impacts after an
initial period.

Biological evolution
Successful evolutionary pathway to
complex plants and animals.

Right position in galaxy
Not in center, edge or halo.

Stable planetary orbits
Giant planets do not create chaos.

A Mars
Small neighbor as possible life source
to seed earth-like planet, if needed.

Large Moon
Right distance.
Stabilizes tilt.

Right amount of carbon
Enough for life.
Not enough for Runaway greenhouse.

Evolution of oxygen
Invention of photosynthesis. Not too
much or too little. Evolves at the right

Wild Cards
Snowball Earth. Cambrian explosion.
Inertial interchange event.
Another proof: Aliens don't exist
By Dennis Overbye, New York Times, 11/11/03
The Radiation Factor
New York Times, 12/09/03
Design and the Anthropic Principle: The unlikely-hood of intelligent life elsewhere in the universe
http://en.wikipedia.org/wiki/Rare_Earth_hypothesis: This article is based on the Rare Earth book, cited above.