Papers which led to Charles Darwin's theory of evolution were previously only available to scholars at Cambridge University's library.
The draft notes are among 20,000 archive items created by the 19th Century naturalist during his lifetime.
Dr John van Wyhe, a Darwin specialist at Cambridge University, said: "He changed our understanding of nature."
The online archive about Charles Darwin is so vast it would take someone two months to view it all if they downloaded one image per minute.
"His papers reveal how immensely detailed his researches were. The family has always wanted Darwin's papers and manuscripts to be available to anyone who wants to read them," said Dr van Wyhe.
"The fact that everyone around the world can now see them on the web is simply fantastic.
"Charles Darwin is one of the most influential scientists in history. The collection of his papers now online is extremely important and therefore very exciting.
"This release makes his private papers, mountains of notes, experiments and research behind his world-changing publications available to the world for free."