There is plenty of fundamental research on it, probably a paper about it is published every week or so. The problem is that there is no general solution to the question, and everybody disagrees about how "human intelligence" should be defined in that context. The answers people give depend too much on untestable "philosophical stances."
Personally, I believe that AI is possible (hard AI thesis) and that computationalism with multiple realizability is right, since none of the philosophical arguments against hard AI and computationalism have convinced me so far. But there are as many opinions on that as there are people working on it.