Plants; probably the most ignored organism on this planet. People walk past plant and cut them down but nobody ever thinks about what they do for us, what the world would be if there was never any plants or photosynthetic organisms. If the world never had these organisms the world would be a desolate place. There would be no oxygen, atmosphere, or food.
We inhale air unconsciously every day. By taking in air, we absorb oxygen and discharge carbon dioxide in the process of generating energy. It is a physiological phenomenon, called respiration. Most organisms on earth breathe. Then, how is oxygen generated and where does carbon dioxide go? The answer to this is plants. When we breath out carbon dioxide the photosynthetic organisms (plants) "breath" in the CO2 and give off our much needed air. In general if the plants never existed there would be no organism to convert the useless CO2 to oxygen, and without oxygen we and all other living organisms (excluding plants) would not be able to survive.
Our atmosphere is made of 78% nitrogen, 21% oxygen, 1% argon, 0.03% carbon dioxide plus small amounts of water vapour. In the beginning of the earth there was no such thing as the atmosphere primitive plants absorbed the carbon in the air and transferred it to oxygen. This oxygen was the first of many layers of the atmosphere. The thin layer of oxygen aloud for water to condense and for lakes, rivers, and oceans. When this water evaporated it made a small adjustment to the atmosphere it now protected the earth for harmful rays. Finally the nitrogen attached it self to the water and oxygen molecules creating the current day atmosphere the permits things to grow and live.