All life on Earth is supported by Photosynthesis.

Because photosynthesis is the only process by which the sun’s energy ENTERS the living world. Green plants are the PRODUCERS at the base of every food chain, and all other organisms depend directly or indirectly on the food they make. Photosynthesis is also the source of all the free OXYGEN in the atmosphere, without which aerobic respiration would be impossible.

Give BOTH reasons — FOOD and OXYGEN. Either alone is half the answer, and the oxygen point is the one candidates forget.

THE FOOD-CHAIN ARGUMENT: a tiger eating a deer is ultimately living on the grass the deer ate, and therefore on photosynthesis. Every consumer, however far up the chain, is living on sunlight.

THE OXYGEN ARGUMENT: the earth’s early atmosphere had no free oxygen at all. It accumulated as a by-product of photosynthesis, and it made aerobic life — and the ozone layer — possible.