Reddit mentions of Hominids: The Neanderthal Parallax, Book 1

For those who are interested, there is an excellent 3-book series by Robert Sawyer called "The Neanderthal Parallax."

It assumes a parallel quantum reality where Homo sapiens went extinct, and Neanderthals thrived. Through a quantum experiment gone awry in our world (Homo sapien-dominated), a scientist from the Neanderthal world appears in our world. It gets interesting from there.

• Hominids by Robert Sawyer which won the Hugo Award for Best novel in 2003. It deals with parallel dimensions, where an accident with quantum computing creates a hole between two alternate realities on Earth. One reality is ours, the other is where Neanderthals evolved rather than homo sapiens. Very easy read and it was a page-turner for me. It's a standalone book. I haven't read the sequels.
  
  
• The Many-colored Land by Julian May is the first book in the Saga of Pliocene Exile series. This is definitely a mix of fantasy and sci-fi. People in the future can get a one-way ticket to the old Earth, back to the Pliocene era six millions years B.C. What these people discover is that they aren't alone but an humanoid alien race lives there and has crashed landed on earth and set up a feudal-like government (they live in castles, ride horses, etc). What's more is that these humanoid aliens have super-powers by wearing a torc, a piece of jewelry. What's even better is that the humans find out that they can use the torcs as well to unlock their own superpowers. If you read all the books, it's also an epic time-travel story. Highly recommended.