There isn’t a theory that can explain how life began, but there’s a few ideas.

As said in the answer, science has not yet reached a general consensus on how life formed on the planet. Most scientists advocate for some kind of mechanism whereby a ‘primordial soup’ of complex molecules reacted in watery conditions over millions of years to form even more complex molecules that could self replicate. This is the most basic definition of life. This carried on until an entire cell was formed and the rest was history.

Some scientists, however, believe life began (somehow or another) on another planet, such as Mars. They believe that this life was kicked up off the planet by asteroid strikes and the rocks landed on earth and seeded it. This idea is called ‘Panspermia’ and isn’t too far fetched. Of course, there’s no way to definitively prove it unless we find evidence of past life on Mars.

But as a general rule, I’d say science still has a ways to go when it comes to how life was originally formed.