Final answer:
Photosynthetic organisms like cyanobacteria evolved and produced oxygen through photosynthesis, which was essential for the subsequent evolution of aerobic organisms, including animals.
Step-by-step explanation:
Photosynthetic organisms had to exist before animals because the atmosphere of early Earth lacked oxygen. The first organisms were anaerobic and did not require oxygen for survival. However, photosynthetic organisms, such as cyanobacteria, evolved and started the process of converting sunlight, water, and carbon dioxide into glucose and oxygen. This process of photosynthesis altered the atmosphere, leading to an increase in oxygen levels, which was necessary for the evolution of aerobic life forms, including animals.
Over billions of years, photosynthetic organisms like cyanobacteria changed the Earth's atmosphere through the release of oxygen. This transformation allowed other life forms that depend on oxygen, such as animals, to evolve. Thus, without the earlier existence of photosynthetic organisms, the oxygen-dependent animals, including humans and dugongs, would not have been able to exist.