 The Neanderthal Parallax

"The Neanderthal Parallax" is a science fiction trilogy written by Canadian author Robert J. Sawyer. The series includes three novels: "Hominids" (2002), "Humans" (2003), and "Hybrids" (2004). The story explores parallel universes, one inhabited by modern humans and the other by Neanderthals who evolved differently than humans did.