Roughly 2 billion years ago Earths atmosphere contained 0% oxygen then a miracle occurred. Cyanobacteria that lived in the sea evolved to make food for itself in a new way using the light energy from the sun rather than just absorbing nutrients present in the water, photosynthesis was born and from this point onwards the photosynthetic bacteria started to release free oxygen.
This oxygen however was all absorbed by the sea for a very long time, the reason for this was the huge amounts of elements dissolved in the water that react with oxygen (mostly iron). The iron that was dissolved in the oxygen free water began to oxidise when it came in contact with the oxygen being released, the whole Earth literally rusted for millions of years.
When the iron (and other oxidisable elements) were all oxidised, the oxygen started to fill the atmosphere which as you probably know was key to the evoloution of complex life. Plants and trees have perfected photosynthesis and are so much more important than many of us realise. What is actually happening in photosynthesis? Leaves absorb carbon dioxode (co2) and combine the carbon from this with the water (h2o) to make glucose/carbohydrate (ch2o) then release the the left over oxygen (o2) into the atmosphere.
Absolutely anything that burns (combusts) only burns because of the presence of oxygen in the atmosphere (only present because photosynthesis put it there) and the presence of a carbon atom which is only present because photosynthesis took it from the atmosphere and locked it there, burning something turns that carbon back into carbon dioxide in the chemical reaction that is combustion where the oxygen from the atmosphere combines with the carbon from the wood, gas, food or whatever other type of fuel you burn. 
We do a hell of a lot of burning/carbon dioxide producing and need photosynthesis to keep reversing the process. The whole of the food chain also starts with photosynthesis. Its painful for me to watch as much of Earths beautiful rainforests and other natural habitats are being destroyed but what can i do? I plant a tree.
Really enjoyed reading this blog @tomthomas, photosynthesis is greatly under-appreciated. There was actually an Indian man, name Prahlad Jani, who survived for 8 years using only the sun and water to survive, it is quite remarkable!
Thanks yea photosynthesis really is a miracle and yea that is quite remarkable.