How did Jack London contribute to literature?

Snarky answer: by writing a number of books, short stories and essays.

Incisive answer: Jack London was one of the leading proponents of a style of writing called naturalism, in which nature itself (human nature, at times) is sort of a character in the story. Nature is portrayed as neither kind nor cruel, but absolutely and utterly indifferent to the struggles of the human (and animal) characters in the story. Read the story "To Build a Fire," and imagine the cold as if it were a character. That's Jack London's genius.