According to a study out of the University of Montreal, the last common ancestor to all life lived around 3.8 billion years ago. The story, in eScience, notes:
The study changes ideas of early life on Earth. "It is generally believed that LUCA was a heat-loving or hyperthermophilic organism. A bit like one of those weird organisms living in the hot vents along the continental ridges deep in the oceans today (above 90 degrees Celsius)," says Nicolas Lartillot, the study's co-author and a bio-informatics professor at the Université de Montréal. "However, our data suggests that LUCA was actually sensitive to warmer temperatures and lived in a climate below 50 degrees."
The early world would have originally contained only organisms with RNA, rather than DNA and that, when DNA evolved, it allowed organisms to move into warmer environments.