Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on fascinating discoveries, scientific advancements and more . CNN  —  An enigmatic skull recovered from the bottom of a well in northeastern China in 2018 sparked intrigue when it didn’t match any previously known species of prehistoric human. Now, scientists say they have found evidence of where the fossil fits, and it could be a key piece in another cryptic evolutionary puzzle. After several failed attempts,