If sunlight would have been the source of energy for humans too then there would have competition between plants and human beings for resources.If people would have been relied on photosynthesis to have energy then there would have been no use of plants for food.The existence of plants would have been in danger when it comes to humans.Humans would have been living the life of plant.There would have been more requirement of water , carbon dioxide and the life of individuals have been more easy as their needs would be fulfilled by them only.