Which of the following correctly explains why the first anaerobic organisms on earth are not classified as autotrophs? chemicals were not available for these organisms to produce their own food. oxygen was in abundance and it was more efficient to eat food rather than make their own. carbon dioxide was not present on earth at that time. the first organisms did not have sunlight available to make their own food.

The correct answer is - Oxygen was in abundance and it was more efficient to eat food rather than make their own.

The first organisms were a group of bacteria. They were probably heterotrophic anaerobes because there was virtually no oxygen in the atmosphere at that time. These bacteria did not breathe oxygen, thus they were anaerobic. They cannot make their own food. These early bacteria  probably consume amino acids which were present in nature . Amino acids, sugars, and other organic compounds formed spontaneously in the atmosphere.