The interesting thing about this is that it could be a double whammy. The collision that formed the Moon not only made Earth smaller, it also ejected a lot of material away from the orbit. This made Earth even smaller than it would otherwise have been, had the two bodies merged. And the Moon also took up some of that mass. The Moon also causes the tides which are theorized to have a significant beneficial effect on evolving more complex forms of life.

So just being small might not be enough and having a big moon might also not be enough, but Earth was lucky enough to have both. And that’s just some of the things in a long list of things that have to go right to get complex life on a planet.

My feeling is that life is pretty rare, but given there are so many star systems in our galaxy there might be a lot of it still. But most of it is probably very simple stuff. Getting to where Earth is, might be a once every couple of millions of years event within our entire galaxy. So there really might be nothing intelligent out there at this moment in time, there might have been earlier and there might be in the future, but for right now we are it.