Here's why:

* Early Earth Conditions: The early Earth was a very different place from today. It had a very different atmosphere, with little to no free oxygen. It also had a lot of volcanic activity, releasing chemicals like hydrogen sulfide and methane.

* Energy Sources: Chemoautotrophs are organisms that obtain energy from inorganic chemicals, rather than sunlight like plants. These chemicals were abundant on early Earth.

* Fossil Evidence: While finding fossils of early bacteria is incredibly difficult, there's some evidence that points towards chemoautotrophs being the earliest life forms. For example, some of the oldest known fossils are found near hydrothermal vents, which are rich in the chemicals these bacteria need.

However, it's important to note:

* Theories are still being debated: While chemoautotrophs are a leading theory, other possibilities exist.

* Evidence is limited: The study of early life is incredibly challenging due to the difficulty of finding and interpreting fossils.

So, while chemoautotrophs are a strong contender for the first bacteria on Earth, the scientific community continues to explore and refine our understanding of the origins of life.