As Lindsay describes in her Saturday Night Jukebox entry, the Moon, formed by a chance collision between a Mars sized object and the Earth (what are the odds of that? - in the vastness of space!), exerts a gravitational pull that keeps the Earth spinning at the right angle and stability for complex life to have arisen.
A few other unlikely factors had to be in place for the emergence of life, and of human beings.
-We're within the small range of distance from the Sun for habitability. Five percent closer or fifteen percent further out and forget it.
-The Sun is big enough to radiate energy, but isn't one of the more common kinds of much larger star, which burns itself out comparatively quickly.
-The Earth's molten core created "outgassing" which helped form the atmosphere, which is kind of important to those of us who breathe, and provides the magnetic field that protects us from cosmic radiation.
-The planet's molten core is the reason we have plate tectonics, and therefore, mountains, hills and land above the surface of the ocean. If not for that, every inch of the planet would be submerged at a depth of four kilometres.
-We have the right chemical elements in the right amounts and proportions.
-The dinosaurs were wiped out by a meteor after roughly 100 million years as top species. If not for that extremely unlikely event, mammals probably wouldn't have evolved past forms that were too small to qualify as T-Rex chow.
As if that confluence of improbable circumstances isn't fortuitous enough, consider the odds of you yourself being here. As Bill Bryson puts it in A Short History of Nearly Everything - his endlessly entertaining and informative overview of the natural sciences from which the facts in this article are drawn:
Not only have you been lucky enough to be attached since time immemorial to a favoured evolutionary line, but you have also been extremely - make that miraculously - fortunate in your personal ancestry. Consider the fact that for 3.8 billion years, a period of time older than the Earth's mountains and rivers and oceans, every one of your forebears on both sides has been attractive enough to find a mate, healthy enough to reproduce, and sufficiently blessed by fate and circumstances to live long enough to do so. Not one of your pertinent ancestors was squashed, devoured, starved, stranded, stuck fast, untimely wounded, or otherwise deflected from its life's quest of delivering a tiny charge of genetic material to the right partner at the right moment in order to perpetuate the only possible sequence of hereditary combinations that would result - eventually, astoundingly, and all too briefly - in you.
Stack up all of the odds against your existence, and that number would be really big. Tantamount to making a trick pool shot, blindfolded, across a table the size and shape of Saskatchewan. Or of someone on public transit talking loudly on their cell phone and saying something interesting.