In the presence of catalysts such as metallic sulfides, these produce complex hydrocarbons, and together with ammonia also the 10 primordial amino-acids that are abundant in abiotic environments.
All the complex organic molecules require energy for their production and only elemental hydrogen (reacting with the abundant carbon dioxide) or the less abundant carbon monoxide (reacting with the abundant water) can provide that in the absence of living beings.
The original source of energy that produces the free elemental hydrogen is the internal heat of the planet, which creates underground minerals that are stable at high temperatures, but unstable at low temperatures, so when they are exposed by volcanic processes to the water at the surface they are oxidized by the water, so they release the dihydrogen from water.
Since life cannot have existed from the Big Bang, it must obviously have appeared at some point in time after the formation of the planets.
It cannot have appeared in any place that does not provide the energy required for the synthesis of organic molecules, so the place where life has appeared must have included a source of dihydrogen (and carbon dioxide, but this is ubiquitous). Additional requirements are the presence of sulfides of iron, cobalt and nickel, which are necessary as catalysts, and also of water, ammonia and potassium ions (the latter are required to neutralize the organic acids that form much of the living matter; all the cells of all living beings are filled with potassium ions for this purpose). All these conditions are encountered in submarine alkaline vents or volcanic vents.
Wherever there exists air dissolved in the water, including at great depths in the ocean or underground, the bacteria and any other living beings obtain energy by the easier path of using dioxygen (elemental oxygen from the air) to oxidize either organic substances or inorganic substances, like elemental hydrogen or hydrogen sulfide. All such living beings, even if they live in the dark, depend on the solar energy captured by algae and plants, which have produced the free elemental oxygen.
At higher depths underground or in the marine sediments, where there is no longer enough air dissolved in the water, there are only bacteria that can use the energy of the reaction between dihydrogen and carbon dioxide. In some places where there is no air there are also sulfate-reducing bacteria, which use sulfate instead of carbon dioxide, but those are also indirectly dependent on photosynthesis, because the sulfate comes from other bacteria which live where there is air and which create the sulfate by oxidizing reduced sulfur compounds with oxygen from the air.