system
Given the following post, respond as if you're commenting on Reddit:
human
What would the first living cells have been feeding on?

Could they have survived on dirt and air?
gpt
Phosphates and hydrogen. 

Well, chemicals in general, but since those two are still used to power our cells today it's quite likely it was, if not the first, then the most successful energy source. 

Nitrates are popular, too, as is sulphur compounds found in deep sea volcanic vents. There are even bacteria that eats iron and poops rust (or at least a reddish iron oxide).

Beyond that the first living cells were powered by what's generally called chemosynthesis: taking chemicals in their environment and catalysing a reaction, gaining some usable energy in the process.