So, I always thought that the El Dorado sarcophagus was cursed, and that was what caused that Spanish to turn into descendants, but in an interview, Evan Wells gave a slighly more logical explanation for it:
"I don't know if we made the point clear enough in the first game. But the idea was that there was this virus trapped inside that sarcophagus, or that mummy, for thousands of years. It was released, and of course these people have no immunities built up to this virus and it has some really nasty effects.
People that survived the effects inbred and they created those creatures that we saw in the first game. That was the more scientific explanation as opposed to the mystical one, that they are just demons."
Your thoughts? Read the interview here: http://www.joystiq.com/2009/02/04/evan-wells-interview-page-3/