The Moon

The moon, by itself, isn't that important to life in general, it's that it's important to two key areas of our life in general - life on land, and civilization.  And it all started 4.5 billion years ago.

What happened was this:  In a Lagrangian point^[1] of Earth another planetoid formed, roughly the size of Mars.  It's been called Orpheus (who, in Greek mythology, invented song), Theia (who, in Greek mythology, gave birth to the moon goddess Selene), and Hephaestus (who, in Greek mythology, was the god of technology, blacksmiths, craftsmen, artisans, sculptors, metals and metallurgy, and fire).  These names are dumb, so I'm going to call it MichaelBay, because when it hit the Earth it would have been loud, flashy, and deadly to brain cells.

Anyway, as MichaelBay (ok, it's Theia from now on) grew to roughly the size of Mars, it became to large to exist in a Trojan Orbit fairly stable around the Earth, until fluctuations in it's angular momentum^[2] caused it to crash into the Earth at about 15 kilometers a second.

Fortunately, it wasn't a head on crash; Theia glanced the side of the Earth, giving it what scientists call "English' (but only when playing pool).  In a nutshell, the impact debris was flung into a circular orbit; this, along with the impact angle, changed the Earth's equatorial spin to a position more or less the same as today.  All but 2% of the debris crashed back onto the Earth a short time later; half of what remained flew off into space, and the remaining percent collapsed into the Moon in a matter of decades.

So why is such a big, loud, and destructive event important to life on land and stable civilization?

First, when Theia collided, a good chunk of the Earth's mantle and crust was thrown into space.  This is important for two reasons.  One, if all of that  was still on the surface of the Earth, Ocean depth would average about 3 kilometers higher than today.  So at best, we'd have about 5% of our surface at land.  But if you take erosion into account, it would be much, much less; probably just a few volcanic plumes here and there.  And without land, there would be no land-based life.  Without land-based life, there would be no technology (it would be rather difficult to smelt metals, build an internal combustion engine, and plug a toaster into an open electrical socket in a conductive saline environment.  Not to mention barnacles on the railroad tracks).

Second, when the mantle and crust were flung into space, it left gaps in Earth's surface.  These gaps are important for plate tectonics, and without tectonics, heat and pressure from the core of the Earth would have no way to escape.  So, like on our neighboring planet Venus, every few hundred million years the pressure under the crust would reach a critical point and cause a worldwide volcanic outpouring, literally resurfacing the entire planet in red hot magma.  Which sounds more intimidating if spoken in Dr. Evil's voice.

Third, the pull of the Moon on the Earth's surface and ocean's causes tides.  Earlier, when the Moon was much closer to the Earth, the tides were much larger, causing massive coastal erosion - this caused massive amounts of organic compounds to slosh around in the oceans, giving the early ocean-based microbes stuff to eat that was easier to get to than the food sources locked up in the Earth's surface.  That, and the availability of raw material for consumption and reproduction, allowed early life to spread all over the globe, and away from the deep sea vents life most likely started around.

Fourth, as the Moon tugs on our planet, it steals some of it's angular momentum - this slows the rotation of the Earth.  At the time of the impact, it's theorized the Earth had a five or six hour day.  Without the moon, our days would be about 8 hours, and we actually would work all day long.  But more importantly, biological rhythms would be completely different, everything from when to eat, when to sleep, when to make with the nook-nook.  Plus, the greater friction between the surface and the atmosphere (as well as faster daytime to nighttime temperature gradients) would cause near-constant hurricane force winds.  Good luck building a mud and twig hut in that environment.

Fifth, and this should be fairly obvious just looking at the crater-scarred surface of the moon, our lil' planetary sidekick has taken a great many asteroid and comet impacts that likely would've hit the Earth, causing major climate changing and extinction causing events, along with altering the course of evolution.

Sixth, the tug of the Moon keeps the Earth in a relatively stable orbital eccentricity.  Without a moon, our axis of rotation around the sun could vary from zero to ninety degrees, as evidence has shown has happened on Mars.  And if Earth wobbled like that, our seasons, ocean levels, and vegetation cover would be very unstable.  Sustainable agriculture would be impossible, because the regions we farm now would swing from freezing cold to unmercifully hot and dry - a one degree variation in our axis of rotation is what transformed Saharan Africa from a tropical rainforest to a parched desert.  And without sustainable agriculture, early man (if we had ever evolved) would have never been able to end it's nomadic lifestyle.  And if you spend long parts of the day wandering around, praying to Oog-The-God-of-Furry-Critters for a squirrel to eat, you don't have much time for thinking, discussing, inventing, or mining; you'd live in easy-to-construct shelters not made of concrete or steel; there would likely be no domesticated animals.  In short, life would suck, you'd die young and diseased, and we'd still think of the stars as guideposts to our future.

Ok, that last sentence still holds true for some people, but evolution and education can only do so much.

[1] - http://en.wikipedia.org/wiki/Lagrangian_point

[2] - http://en.wikipedia.org/wiki/Angular_momentum