Question

which event in earths history most directly allowed life to exist on land?
earths temperature increased, causing widespread evaporation.
earth collided with a large object, forming the moon.
single - celled organisms released oxygen into the air when they photosynthesized.
single - celled organisms absorbed harmful gases from the air when they first formed.

Explanation:

Oxygen is crucial for most land - based life forms. When single - celled organisms photosynthesized and released oxygen into the air, it gradually built up the oxygen - rich atmosphere that made it possible for more complex life to evolve and exist on land. Earth's temperature increase and evaporation did not directly enable life on land. The collision to form the moon is mainly related to the moon's origin and had no direct impact on life on land. Absorbing harmful gases by single - celled organisms when they first formed did not directly contribute to life existing on land as much as oxygen production.

Answer:

Single - celled organisms released oxygen into the air when they photosynthesized.