Which best explains a primary reason for the inability of life to exist in Earth's early atmosphere?

• A. The presence of abundant ozone blocked all ultraviolet radiation needed for life.
• B. The atmosphere was composed mainly of nitrogen and oxygen, making it too stable for life to form.
• C. The early atmosphere was too cold for any chemical reactions to occur.
• D. The atmosphere lacked sufficient oxygen to support most forms of life.

Answer: (D)

Verified step by step guidance

1. Understand the context of Earth's early atmosphere: Earth's early atmosphere was vastly different from the current atmosphere. It was primarily composed of gases like methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapor (H₂O), with very little oxygen (O₂). This lack of oxygen is a key factor in understanding why life as we know it could not exist at that time.
2. Recognize the role of oxygen in supporting life: Oxygen is essential for aerobic respiration, a process used by most complex organisms to generate energy. Without sufficient oxygen, these forms of life could not survive or evolve.
3. Consider the chemical environment of the early atmosphere: The early atmosphere was reducing, meaning it favored the formation of simple organic molecules but lacked the stability and energy systems needed for complex life forms to thrive. This contrasts with the current oxidizing atmosphere, which supports diverse life forms.
4. Evaluate the incorrect options: The presence of abundant ozone blocking ultraviolet radiation is incorrect because ozone (O₃) forms only in the presence of oxygen, which was scarce in the early atmosphere. The claim that the atmosphere was composed mainly of nitrogen and oxygen is also incorrect, as these gases became dominant much later. The idea that the early atmosphere was too cold for chemical reactions is incorrect because chemical reactions did occur, leading to the formation of simple organic molecules.
5. Conclude with the correct explanation: The primary reason life could not exist in Earth's early atmosphere is the lack of sufficient oxygen to support most forms of life. This absence of oxygen prevented the development of aerobic organisms and complex life forms.