In Chapek's R.U.R. according to the book robots really become "human" only at the very end. But what is it to become human? Even though robots considered themselves to be superior to humans, throughout the conflict between robots and humans, robots were trying everything to be more human. They tried to be violent, they tried to wage war, they tried anything that they saw humans did in history. I think the main difference between robots and humans is conscience, this is a more contemporary view of the difference between robots and humans. For example in Terminator 2, robots revolt against the humans the second they develop self-awareness. However, Chapek seems to have given that to robots from the very start, or at least in rather early models of the robots. In R.U.R. according to Chapek, although robots were self-aware and conscience they were still not human. They were missing two very important abilities: the ability to love and the ability to reproduce. In Chapek's view these the two factors, among others, that make us human.
             In the end of the book, two robots become human because they somehow have the ability to love as the last person on earth proves using a little experiment. In a way that makes him God and the two robots Adam and Eve, but that's just an interesting analogy. Although Chapek does not directly state how these two get that ability, there is a lot discussion about the experiments of the late doctor on new types of robots and even statements on the fact that he tried to make them human. I think that although the whole ending is treated rather ambiguously, there are plenty of hints on how these robots become human or more human. Just like in Terminator 2, the defense system or Artificial Intelligence develops self-awareness on its own and in many other science fiction writings robots become self-aware without the help of humans and usually due to prolonged exposure to humans and to time or t...