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 do not directly enable life on land.
• Option 2: The formation of the moon due to a collision has no direct relation to life on land.
• Option 3: When single - celled organisms (like cyanobacteria) photosynthesized, they released oxygen. Oxygen led to the formation of the ozone layer (\(O_3\)) which protects organisms from harmful ultraviolet (UV) radiation. This protection was crucial for life to move from water to land.
• Option 4: Absorbing harmful gases is not the key factor for life to exist on land. The presence of a protective atmosphere (due to oxygen - based ozone layer) is more important.

Answer:

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