Deep sea vents have high temperatures and release poisonous gases. Both of these properties existed in the conditions of early earth. The atmosphere was not thick enough to block out sufficient amounts of radiation and neither was it rich enough in oxygen to sustain life as it is now. Instead, it had poisonous gases such as oxides of sulfur. Moreover, the volcanoes were also very active, just as the hydrothermal vents are deep in the sea.
