Once a living cell has evolved the final stage in its survival is the ability to sustain itself. Nowadays many organisms require light energy to sustain them. This mechanism is known as phototrophy and includes the process of photosynthesis. Alternatively, and maybe more primitive is chemotrophy
'Systems of metabolism in which energy is derived from endogenous chemical reactions rather than from food or light-energy, e.g. in deep-sea hot-spring organisms.'
There are two types of chemotrophy. Autotrophy uses inorganic substances as building blocks, while heterotrophy uses organic molecules.
It is possible that the earliest organisms obtained their energy through chemotrophy, maybe utilising methane as an energy source.