I personally am not surprised this happened - the definition between concrete species is terribly fuzzy depending on how far back in time you go. There's differ

I personally am not surprised this happened - the definition between concrete species is terribly fuzzy depending on how far back in time you go. There's different ways the evolutionary tree can fork, from geographic to functional to reproductive isolation. Usually the environment selects for random mutations that differentiate two similar groups of organisms. What takes longer is for prezygotic (functional) and postzygotic (cellular biological) reproductive barriers to form to prevent direct genetic mixing. Their absence would allow human and Neanderthal populations to mix DNA pools and merge even with some biological differences, especially if the human population arrives in overwhelming numbers.