(S01E08) If your mother has told you once, she's told you a thousand times: Don't mess with the time stream! Apparently, Rose (Billie Piper) never listened to her mum, or watched an episode of Star Trek (original or Next Generation) or any film in the Back to the Future series. Because, if she did, she would have realized that messing with the timeline can cause some serious consequences. In this case, by saving her father from a premature death, she opened the planet up to alien beings who cleanse Earth of all corrupted objects; i.e. humans.
See? Always listen to your mother!