The theory, still unproven, is that life evolves from dead matter. We’re still searching for life on other planets to prove this wet-dream of scientists, but still we have no proof of concept – just a concept – to the chagrin of science. We are frankly disappointed we haven’t found life elsewhere than Earth.
Evolution starts with creation of stars that create elements like the ones we have on Earth. We think when conditions are right that the chemicals develop amino acids (this is proven), which are the basic building blocks of life.
From there, eventually by combination and permutation it develops into something that could be called life.
Again it’s not proven but widely accepted. We don’t know for sure, hence my first sentence.