What saved the Earth?
The Sun.
The "Solar Constant" isn't constant. Over Billions of years the Sun, like most stars, gradually gets hotter as it "burns" its nuclear fuel supply. Over time, Gaia must dial-down the amount of Greenhouse gas in the atmosphere to keep the temperature steady.
That's the straight answer.
The funny answer is - the Earth doesn't care what type of bugs live on its surface. Maybe we're like fleas, and if we cause enough itching, Gaia will go and get a bug rinse...