How did life on Earth begin?

No one really knows yet. Life on Earth began more than 3.5 billion years ago.

There are many hypothesis on the subject so let's take a look at some.

Electric sparks can generate amino acids and sugars from an atmosphere loaded with water, methane, ammonia and hydrogen, as was shown in the famous Miller-Urey experiment reported in 1953. And no, it has not been debunked.

Cairns-Smith suggests that mineral crystals in clay could have arranged organic molecules into organized patterns. After a while, organic molecules took over this job and organized themselves.

The deep-sea vent theory suggests that life may have begun at submarine hydrothermal vents, spewing key hydrogen-rich molecules.

RNA, which can store information like DNA, can serve as an enzyme-like protein, and help create both DNA and proteins.
Later DNA and proteins succeeded this " RNA world ," because they are more efficient.

Life might have begun with smaller molecules interacting with each other in cycles of reactions.
These might have been contained in simple capsules akin to cell membranes, and over time more complex molecules that performed these reactions better than the smaller ones could have evolved, scenarios dubbed the " metabolism-first " model.

Perhaps life did not begin on Earth at all, but was brought here from elsewhere in space, a notion known as panspermia . However the question of how life began on Earth would then only change to how life began elsewhere in space.