Understanding how Earth has sustained surface liquid water throughout its history remains a key challenge, given that the Sun's luminosity was much lower in the past. Here we show that with an atmospheric composition consistent with the most recent constraints, the early Earth would have been significantly warmed by H-2-N-2 collision-induced absorption. With two to three times the present-day atmospheric mass of N-2 and a H-2 mixing ratio of 0.1, H-2-N-2 warming would be sufficient to raise global mean surface temperatures above 0 degrees C under 75% of present-day solar flux, with CO2 levels only 2 to 25 times the present-day values. Depending on their time of emergence and diversification, early methanogens may have caused global cooling via the conversion of H-2 and CO2 to CH4, with potentially observable consequences in the geological record.