[Just another Joe]

Read it before and I think that it's combinations of

Planets With the Right Conditions are Rare
Planetary systems are rare
Habitable zones, proper distance from star for liquid water, are narrow
Galaxy is a dangerous place (gamma-ray bursters, asteroid impacts, etc)
Earth/Moon system is unique (large tides needed for molecular evolution)
Life Is Rare
Life's Genesis is rare
Intelligence/Tool-Making is rare
Language is unique to humans
Technology/Science is not inevitable

AND a third thing, we are still assuming that life is like terrestrial life.
If a being has no eyes, doesn't see the stars, can't imagine another world beyond the limits of his own, why would he try to go beyond it?
And that's an EASY example.

[Doc On The Bay]

ALL TRUE, But Remember One Other Factor; the "Universe" is supposedly "Infinite," which suggests that "Contact" is, at least, "Statistically Possible!!"

In an "Infinite Universe," our "Lack of Contact" may Simply be a Statistical Event!!

An "Infinite Universe" is FAR LARGER than we can imagine, & chances of "Contact" may be FAR LESS than we can Calculate.

Doc

[edwin hubble]

"Planets With the Right Conditions are Rare
Planetary systems are rare
Habitable zones, proper distance from star for liquid water, are narrow
Galaxy is a dangerous place (gamma-ray bursters, asteroid impacts, etc)
Earth/Moon system is unique (large tides needed for molecular evolution)
Life Is Rare
Life's Genesis is rare
Intelligence/Tool-Making is rare
Language is unique to humans
Technology/Science is not inevitable "


Here are two more:

1. Early microbial plant life gave off oxygen as a product of photosynthesis.
This eventually formed ozone in the upper atmosphere.
This, over a billion years or so, prevented the otherwise certain disassociation of water in the vapor phase in the upper atmosphere.
Without this ozone (from early life) all water would now be gone from the Earth's system.

2. Tectonic plate motion on the Earth
Among terrestrial planets only Earth has the constant slow motion of the mantle. (although Venus has volcanoes and activity we don't fully understand).
Theories hold that tectonic plate motions give us recirculation of metal elements - necessary for a technology-using civilization.