Which type of organism is believed to have been the first life on Earth?

The first life forms on Earth are generally believed to be chemoheterotrophs. Chemoheterotrophs obtain their energy from organic compounds, which aligns with the conditions of early Earth where energy sources such as volcanic activity and chemical reactions were prevalent. Since these organisms do not rely on sunlight for energy, their existence does not depend on photosynthesis, which likely developed later in Earth's history when conditions became suitable for photosynthetic organisms.

In this context, the early Earth lacked significant oxygen and sunlight, which do not favor the initial emergence of aerobic autotrophs or photoautotrophs. Aerobic autotrophs, which rely on oxygen for their metabolism, would not have been able to thrive in an anoxic atmosphere. Similarly, photoautotrophs require sunlight and a stable environment, which were not present in the early stages of Earth's development.

Facultative anaerobes, while capable of surviving in both aerobic and anaerobic environments, are part of a more evolved stage of life that came after the first chemoheterotrophic organisms had already established a biosphere.

Overall, chemoheterotrophs represent a critical evolutionary step as they adapted to utilizing available organic matter, paving the way for more complex life forms to evolve later.