Short version: Growing plants use the sun’s energy and simple chemicals to make more plants, and animals “burn” the plants to get that stored energy from the sun. Almost everything that grows is burned, but in special cases some plants are buried without oxygen, escaping burning. Time and heat turn these buried plants into fossil fuels.
Friendlier but longer version: Recall that energy is the ability to do things. And, living requires doing things—fighting against randomness to put particular chemicals in particular places to make cells and cell walls, to protect oneself and reproduce.
Living things on Earth could tap into many energy sources. Heat flows out of the Earth beneath our feet, for example. But, the energy from the sun reaching the Earth’s surface exceeds that from inside the planet by more than 2000 times, so it is clear that harnessing the sun gives greater opportunities for living things. (This is also why you will never hear a weather forecaster worrying about the effects of the Earth’s heat!)
Please watch the following video:
Photosynthesis and Respiration (:51)
DR. RICHARD ALLEY: This is the simplest version of photosynthesis and respiration that we can come up with. This is a plant. It stands in for all the green plants.
And it takes CO2 from the air, and it takes water, and it puts those together. But that requires energy from the sun to make the chemical bonds which give us plant, shown here as CH2O. They're just sort of the formula of plant. And it releases oxygen to the air.
Now, animals and bacteria and fungi and all these other things take plant, and they take oxygen, and they burn them to get energy so they can run around and do things. And that releases the CO2 and H2O. Fires also do this, but they don't do it in quite so controlled a manner.
Photosynthesis is the process by which plants grow more of themselves, using simple chemicals and the sun’s energy to make more-complex chemicals that store energy. Respiration is the process by which animals, fungi, etc. run photosynthesis backward, “burning” plants to release the stored energy for use by the animals, and releasing simple chemicals, ready to be used by plants again.
You probably have seen the equations for photosynthesis, the process by which plants harness the sun. The simplest statement of the commonest type of photosynthesis goes something like this: water + carbon dioxide + solar energy → plants + oxygen. Or, if you prefer chemical symbols saying the same thing: H20 + CO2 + hν → CH2O + O2
(Don’t worry if you had a class sometime in which this equation was written with 6 waters plus 6 carbon dioxides making 6 oxygens plus the chemical glucose, C6H12O6; that’s the same story simplified in a slightly different way, and either way you write it is close enough for our purposes.)
Almost all of the biological activity on the planet depends on this pathway to capture the sun’s energy. When the sun isn’t shining, plants run this backward, and animals and bacteria and fungi all run it backward, combining oxygen with plants to release water, carbon dioxide, and the sun’s energy that the plants stored chemically. Fires do this, too. Depending on whether it happens in a fox or a fire, you may see this energy release called respiration, or burning, or oxidation, or combustion, or perhaps other words, but all serve to combine oxygen with plant material to release carbon dioxide, water and energy.
Please watch the following video:
Averaged around the planet and over a year, roughly 0.1% of the energy from the sun that reaches Earth is stored as chemical energy by plants. (This is called net primary production, if you want the technical term.) Clearly, plants capture more of the sun’s energy in some places and times than in others, and agricultural experts have worked hard to find ways to make plants especially productive for us in our gardens and farms, but plants are still not very efficient. Even so, the world’s plants capture about 10 times as much energy as humans use.
If plants would jump into our fuel tanks and liquefy, we would have far more energy than we needed, but things don’t work that way. And, because everything alive on Earth wants to burn plants for energy, we face large difficulties in harvesting plants and burning them for our use before something else beats us to it. Almost all plant material is burned rather quickly after it grows, sometimes being eaten by caterpillars or cows while still alive, other times by fungi or bacteria after dying.
But, it is a very large and very old world, so even a very small difference between what grows and what is burned will eventually add up to a very large store of energy. And, that is what fossil fuels are.