Question

which event in earths history most directly allowed life to exist on land?
earths temperatures 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:

• Option 1: Increased temperatures and evaporation don't directly enable land - life existence.
• Option 2: Moon - formation collision has no direct relation to land - life.
• Option 3: Single - celled organisms (like cyanobacteria) released oxygen through photosynthesis. Oxygen led to the formation of the ozone layer ($O_3$). The ozone layer protects organisms from harmful ultraviolet (UV) radiation, which is crucial for life to exist on land.
• Option 4: Absorbing harmful gases is not the key event for land - life emergence.

Answer:

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