Question

Which statement accurately describes the amount of oxygen in Earth's early atmosphere 2.5 to 3.8 billion years ago?

a. The atmosphere contained a high concentration of oxygen, supporting early aerobic life forms.
b. Oxygen levels were similar to current levels, allowing for the development of complex organisms.
c. Oxygen was absent or present in minimal amounts in the early atmosphere.
d. Oxygen levels were fluctuating, with periodic increases and decreases over the specified period.

Answer 1

Oxygen was absent or minimal in Earth's early atmosphere, and the first organisms were anaerobic. Cyanobacteria evolved from phototrophs and started the oxygenation of the atmosphere, leading to the eventual colonization of land and the advent of aerobic life forms.

Step-by-step explanation:

The correct statement regarding the amount of oxygen in Earth's early atmosphere 2.5 to 3.8 billion years ago is that oxygen was absent or present in minimal amounts in the early atmosphere. During the first two billion years of Earth's existence, the atmosphere was anoxic and did not contain molecular oxygen, meaning the first organisms were anaerobic.

Phototrophic organisms appeared within one billion years of Earth's formation and evolved into cyanobacteria, which in turn began the process of oxygenation of the atmosphere. This led to a gradual increase in oxygen levels, enabling the development of aerobic life and the formation of Earth's ozone layer, which provides protection from harmful ultraviolet radiation, allowing life to eventually venture onto land.