h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The number of 'earth-like' planets, capable of supporting human life and communicating with us, if judged by the Earth's history, would likely be much smaller than any result that I've seen of the Drake Equation.

There are a number of very odd (if not unique to earth) events that got us where we are today.

That Earth formed as it is has came about through bizarre effects like asteroid bombardment - one was possibly large enough to create the moon - and then having a moon to keep tektonic plates active through its tidal forces.

Life may have become possible through semi-complex molecules arriving with asteroids.

What factor discusses our bombardment history?

Life was humming along just fine, evolution and all, until the dinosaurs died out. If we're correct, that this was another very rare type of exploding asteroid event - one we haven't seen on such a massive scale since - there may never have been a successful and intelligent mammalian species (and the dinosaurs did not appear to be moving towards using tools or communicating beyond that which modern day lizards can accomplish).

Without the moon, primitive man would not see so many natural forces. Without its tidal forces, there would be no tides. No tides, less wind and storms. Mankind, if mammals did evolve into a human-like race, would have no reason to look up. The Sun is too bright, and there would be nothing in the night sky with features.

Mankind may never have considered that there were other suns or, if Pangea was the one and only continent, may never have realized they were on a planet. Even if they did figure that out, they'd have no reason to believe that they were anything but alone.

Without any neighbors, why would they create a complex communication system to talk to the stars?

We may, or may not, be alone; but the unusual factors in Earth's development suggest that we will probably never have contact with an off-world species unless we go out and find them.