A very interesting new study from NASA on the importance of a planet's axial tilt on its ability to sustain complex life. This may explain the "Cambrian explosion" of complex life on Earth. Evidence suggests that, possibly due to the ancient planetary collision that formed the Moon, primordial Earth had a severe axial tilt. This somehow changed around 540 million years ago, which finally stabilized Earth's seasonal climates and allowed complex life to flourish.
Of course, that's a big "somehow". If a massive collision with a Mars-sized planet caused the original change in Earth's axial tilt, what could have caused it to change back?