Why do most scientists believe that all life on earth evolved from a single cell?

Not all scientist believe this, because some scientist can think and know that the chances of the beauty of the design we see in the world all around us, wither man made or of nature, is absolutely impossible.

They are more chances that a Rolls-Royce on any showroom floor came from the back of a cement mixer full of all the right parts, than there are that a "single-celled ancestor" evolved into the complex world around us.

because cells only come from pre-existing cells A+