Some theorists have proposed that the earliest living things used reactions that resemble those modern organisms use to ferment sugar, glycolysis.[1] Yeast use a version of that reaction to get energy by turning glucose into alcohol.[2]
This makes sense because there wasn’t much oxygen in the early Earth and glycolysis doesn’t require oxygen. Some or maybe all of the chemical reactions involved can happen without biological enzymes. Heating a mixture of water, sugar, and minerals may be enough.[3]
But there might not have been enough sugar in the primordial soup to make this an important source of energy.
Some scientists have tried to reconstruct the biochemical reactions the earliest organisms used by looking at chemical reactions that are widely used in modern organisms and making guesses about the environment in which early organisms might have lived.
Here’s one reconstruction. In this scenario, a set of reactions called the Wood–Ljungdahl pathway use hydrogen as an electron donor and carbon dioxide as an electron acceptor. Hydrogen, CO2, and H2S could have provided the nutrients and some of the energy needed to power life. This model assumes that life arose at an undersea hydrothermal vent having an alkaline interior and surrounded by mildly acid water. That difference could have provided energy to make ATP[4]
You can learn more from these publications:
The physiology and habitat of the last universal common ancestor.
The last universal common ancestor between ancient Earth chemistry and the onset of genetics
Footnotes
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