Final answer:
The first living things likely generated ATP through anaerobic processes due to the lack of oxygen in the early Earth's atmosphere, using environmental energy sources and eventually evolving photosynthesis and aerobic respiration.
Step-by-step explanation:
The first living things on Earth probably generated ATP (adenosine triphosphate) through anaerobic processes such as fermentation. This is due to the fact that the early Earth atmosphere lacked sufficient oxygen, making aerobic respiration impossible. Initially, these ancient organisms likely continued the chemical reactions from abundant energy-containing compounds in their environment, which formed from sources like UV radiation and lightning discharges from storms. As evolution progressed, some organisms developed the capability to harness energy directly from light through a process called photosynthesis. Ultimately, this led to an increase in oxygen in the atmosphere, which further paved the way for the evolution of aerobic respiration, a more efficient way to produce ATP in the presence of oxygen.