Scientists believe the original atmosphere of the primal Earth escaped into space. Compared to today’s breathable air, this primary atmosphere was toxic—rich in ammonia, neon, water vapor, and methane. The air was without breathable oxygen. Nonetheless, unicellular organisms arose and then breathed out oxygen into the air, which brought about a revolutionary change in the Earth’s atmosphere. Over many millennia, this atmosphere then evolved into the air that we breathe today.