The question 'but how did life start' is not answerable by evolution, that biological theory can only describe how life changes. Evolution only describes how life changes, not how life started.
There are theories on how life began. The leading is called the RNA world hypothesis which claims that before DNA, the world was full of replicating RNA molecules. Since RNA is made of amino acids and the universe is plentiful in Amino-acids, the idea looks possible at least.