Re: Tidal forces required in early evolution
A moon stabilizes the axis of rotation of a planet. Without one, the axis of rotation is unstable, and sooner or later will pass through the plane of the planet's orbit (with a timescale of under a million years). That's too fast for the evolution of all but mono-cellular life to adjust to a planet that "suddenly" no longer has a day-night cycle. (ie, one where the whole planetary surface is like our poles: half a year of night, then half a year of day).
Don't know how large a moon is needed, but no moon at all is no good at all.
Can Kepler identify how many planets have moons?