From the secular view, it came from a puddle of chemicals called the Primordial Soup that came from comets that bombarded the earth billions of years ago. How they come up with this - I don't know. Is it scientific? Absolutely not. According to this view the comets contained amino acids, which somehow came together in the oceans to form proteins, and then these proteins came together to form a living cell.
From the creationist view, "In the beginning, God..."