The second organism to appear on Earth is not known with certainty, as the exact sequence of early life's evolution is still a subject of ongoing scientific research and debate. However, based on the current understanding of the evolutionary history of life on Earth, here are some possible candidates for the second organism:

1. Prokaryotes: After the first organisms, which are believed to be simple protocells or similar structures, the next major group to emerge were prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus or other membrane-bound organelles. Bacteria and archaea are the two main groups of prokaryotes.

2. Methanogens: Methanogens are a type of archaea that produce methane as a byproduct of their metabolism. They are believed to be among the earliest life forms on Earth and may have played a significant role in the development of Earth's atmosphere.

3. Phototrophic bacteria: Phototrophic bacteria are a type of bacteria that use sunlight to produce energy through photosynthesis. They are believed to have evolved after methanogens and played a crucial role in producing oxygen and transforming Earth's atmosphere.

4. Heterotrophic bacteria: Heterotrophic bacteria are a type of bacteria that obtains organic compounds from other organisms. They may have evolved after phototrophic bacteria and played a vital role in the decomposition of organic matter and nutrient cycling.

It's important to note that the exact order and timing of the emergence of these and other early organisms are still subjects of ongoing scientific research and may be revised as new evidence is discovered.