The Moon was long considered an essential stabilizing presence in the development of life on Earth. Without its satellite, the Earth would have tilted too much on its axis, making life impossible. But maybe the Moon wasn't needed after all.
The scientific consensus has held that, without the Moon to stabilize the tilt of Earth's axis - known as its obliquity - our planet would have varied in tilt from 0 degrees, in which the Sun shines directly on the equator, to an insane 85 degrees, where the Sun pretty much permanently shines down on one of the poles and not much else. The Earth's axial tilt is current 23.4 degrees, which allows it to maintain a variety of climates conducive to life. That obliquity has varied very slightly over the years, which even then has had dramatic consequences - shifts by just one or two degrees likely played a huge role in recent Ice Ages.
There are a few factors involved in how a planet revolves around its star. While the center of gravity remains constant, the direction of tilt and the planet's orbital plane will move over time - these are known as the rotational precession and the precession of the planet's orbit, respectively. If those two processes are synced up, then the planet's obliquity can fluctuate wildly. The trick is to have the rotational precession have a far different speed than that of the precession of the planet's orbit. Thanks to the Moon, that's the case for Earth, and it creates a more stable obliquity than would otherwise be possible.
But how much more stable? As we've discussed, the old models suggested an 85-degree range for Earth's obliquity in the absence of the Moon. Now, according to researchers at the University of Idaho, the Moon plays only a small part in the stabilization of Earth's obliquity. The other planets orbiting the Sun - especially Jupiter, as you might imagine - are actually the objects mostly responsible for determining Earth's axial tilt. Even if the Moon had never formed, Earth would likely only tilt about 10 to 20 degrees over half a billion years.
Obviously, that's still some pretty extreme tilt, considering that's ten times what's needed to start an Ice Age. But even that amount of variation in the planet's obliquity wouldn't preclude the evolution of intelligent life. If the Moon hadn't been around, we probably would have had a bumpier ride along our evolutionary path, but odds are that humanity still could have emerged. And, as it turns out, the Moon isn't even a guaranteed stabilizer - if Jupiter was positioned closer to Earth, its gravitational influence combined with the Moon's would have actually increased Earth's obliquity.
Their research also suggests another way Earth could have been more stable: if it had rotated in the opposite direction that the Sun rotates. Our solar system has two such retrograde planets: Venus and Uranus. The researchers believe that the direction a planet spins is essentially random, and that it's simply a matter of which way it's struck last in a cosmic collision before its spin stabilizes.
If all this is true - and yes, that's a reasonably big "if" - then the researchers estimate that as much as 75% of all rocky planets found in a star's habitable zone may end up being habitable. Those are some pretty great odds, but the researchers are quick to point out that there's plenty more research to be done before they can be certain.