Most lifeforms can only eat other lifeforms. Some have particular abilities to extract nutrition (energy) from the sun (green plants) or from heat and/or minerals (bacteria). All other lifeforms must consume other lifeforms for their nutrition. The energy all comes from the green plants and bacteria originally though, which is taken from the environment. Thus life can feed upon itself and still grow bigger. The things you consume are broken down and metabolised into chemicals providing energy specific to you.
