How do scientists explain the origin of life on earth?

The Big Bang and evolution basically.

The earliest know life-forms on earth are putative fossilized microorganisms, found in hydrothermal vent precipitates, that have lived as early as 4.28 billion years ago, relatively soon after the oceans formed 4.41 billion years ago, and not long after the formation of the earth 4.54 billion years ago.