The next time you're flying through outer space, take a look at the eight planets and the smaller rocks that circle them.
You'll notice one stands out. That's right, our very own moon is conspicuous for its size. Not that it's the largest moon in the solar system, but it is very large indeed compared to the relatively small planet earth.
Now take another look around. Notice anything? The second conspicuous thing is that earth, with its sizable moon, is also the only planet we know of with life on it. Coincidence?
Maybe. The relationship between our unusually large moon and the abundance of life on earth is still hypothetical. However, there could well be a very direct connection, the stable rotation of the planet.
As we know from grade school, planets not only orbit the sun, they also spin. However, the planets are not perfect spheres, they bulge a little along their equators.
The sun, and the other planets, tug on this bulge slightly and, over time, cause the orientation of each planet to wobble. You can visualize this by thinking of the equator rocking up and down rather than remaining straight.
Such a wobble can mean significant climatic changes across time, causing intense periods of heat or cold on a planet's surface. Such climatic instability is not very hospitable to the evolution of life.
One solution? You could have a moon large enough that its own gravitational tug on the planet balances out that rotational wobble every day, which our own planet earth happens to have. It could just be that the steadying effect of our moon across the millennia has everything to do with our being here.