Sylvia was the last surviving human in an alternate timeline during the 1960s. When the Eleventh Doctor and Amy Pond met her, she was in London, trying to evade the Daleks which had invaded Earth. She was found and exterminated in the Underground while trying to set a trap.
The entire invasion and all the events that followed were reversed when the Doctor jumped through the Eye of Time on Skaro. After the Doctor had returned the timeline to normal, he and Amy saw her alive. (GAME: City of the Daleks)