From: Rare Earth: Why Complex Life is Uncommon in the Universe,
Peter D. Wood & Donald Brownlee, 2000
prohibit or hinder bacteria from evolving into complex life, but first, the
Timeline of Evolution:
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
Why Complex Life is Uncommon in the Universe
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.
Ocean
Not too much.
Not too little.
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
time.
Wild Cards
Snowball Earth. Cambrian explosion.
Inertial interchange event.
By Dennis Overbye, New York Times, 11/11/03
arrive there and become established. Then the new colony and the original civilization each send a
spaceship on a similar voyage, and so forth, so that a wave of new colonies is formed, doubling the total
number every million years.
After 10 million years, there will be 1023 alien settlements, plus the original. After 20 million years, there will
be amillion. After 40 million years, if they keep it up, there would be a trillion -- more than there are stars in
the galaxy.
By now, after 10 billion years, if there were more than one spacefaring civilization in the galaxy, they would
be tripping over one another or one another's artifacts [and raidowaves]. But in fact there seems to be
nothing.
New York Times, 12/09/03
with radiation: Americans receive an annual average dose of 200 millirems of radiation. A chest x-ray is 8
to 50 mrems. A person in Kerala, India (where there's lots of sun) receives 1300 mrems/year. A person on
the space station for 84days receives 17,800 mrems but a one year trip to and from Mars would entail a
fatal 80,000 mrems.
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