First off, this should be under biology.
Without photosynthesis there would be no oxygen in our atmosphere only carbon dioxide, meaning that every living thing on this planet would be dead within probably days considering how much we already pollute our atmosphere. Only very few bacteria organisms named anaerobes would be capable of surviving, and in several million years these bacteria could mutate into humans surviving without oxygen, trust me, we've done it once before. Also, all plant life would die because they would not be able to produce glucose as food to keep themselves alive.
If there was no photosynthesis, plants and animals likely could not exist. In addition the atmosphere would have very little oxygen because photosynthesis releases a large amount of oxygen into the air. Otherwise, the Earth would be a pretty barren lifeless place without photosynthesis.
