Explain why we can infer that the universal common ancestor of all life on earth was unicellular heterotrophic

Early life was too simple to fully fix the sun or use higher functions to provide it's own energy. They were probably more like archaea that are seen around vents in extreme regions, such as the floor of the ocean or in high temperature pools in Yellowstone. These items "eat" the sulfur and other energy to create their needs. This would develop by necessity into mitochondria and chloroplast like bacteria as they ventured out from the vents to other areas and competed for heat and energy