The Goldilocks Problem

Remember the equilibrium temperature of a planet is linked to its distance from the Sun (D):

And remember that the Earth's atmosphere and biosphere is very sensitive to temperature -- a few degrees makes a lot of difference!

So if the Earth was a bit closer or a bit further from the Sun, we might have radically different (inhospitable) conditions.

It gets worse: the sun has been increasing in luminosity over time. Early in the Earth's history (several billion years ago), the Sun had only 70% of its current luminosity. But liquid oceans have existed on Earth for nearly 4 billion years. Why?

The Earth's temperature is regulated both by solar input and by the greenhouse effect. If the solar input has changed over time, the greenhouse effect must have counterbalanced it to very good regularity. How would that happen?

The Carbonate-Silicate Cycle

(also known as the Urey Reaction)
In order to regulate the temperature to such high stability, the carbonate-silicate cycle must be self-regulating. That is, it must act to increase the greenhouse effect if the temperature drops, and it must decrease the greenhouse effect if temperature rises.

So let's presume that Venus, Earth, and Mars all started out roughly with roughly similar compositions when they formed. Why did the carbonate-silicate cycle make Earth "just right" while Venus and Mars became so unfriendly?