Final answer:
Oxygen initially posed a toxic threat to early microbes that evolved in an oxygen-free environment. Some microbes developed metabolic pathways that didn't require oxygen due to its reactivity and scarcity, while others adapted to use oxygen as a new energy source through aerobic respiration. The correct option is A.
Step-by-step explanation:
The question relates to the evolution of microbial life in anoxic (oxygen-free) conditions. Initially, the Earth's atmosphere lacked oxygen, making it an anaerobic environment suitable for microbes that could not tolerate oxygen.
As photosynthetic organisms like cyanobacteria evolved, they began to release oxygen into the atmosphere, a process often referred to as the Great Oxidation Event. This posed a challenge to the existing anaerobic microbes because oxygen can be highly reactive and toxic to organisms that did not evolve mechanisms to handle it.
Over time, some microbes developed the ability not to use oxygen due to its toxicity and scarcity and evolved alternative metabolic strategies to generate energy, such as anaerobic respiration or fermentation.
Aerobic respiration evolved as a protective mechanism against oxygen, additionally allowing certain microbes to harness more energy from organic compounds. However, those organisms that couldn't evolve such mechanisms had to continue living in oxygen-free environments. This evolutionary adaptability illustrates how life on Earth can adjust to significant environmental changes.