a) Most of the oxygen in the atmosphere originally came from the activities of photosynthetic organisms in the ocean. This accumulation of oxygen in Earth’s atmosphere was necessary for life to develop and be sustained on land.
b) The first life is thought to have started in the ocean. The earliest evidence of life is found in the ocean.
c) The ocean provided and continues to provide water, oxygen and nutrients, and moderates the climate needed for life to exist on Earth ( Essential Principles 1,3, and 5).