Originally they may have depended on hydrothermal vents for energy and chemical "food". The development of photosynthesis gradually liberated them from the 'hydrothermal ghetto' by proving a more widely-available energy source, sunlight, although initially the photosynthesizing mats still depended on the diffusion of chemicals emitted by hydrothermal vents. The final and most significant stage of this liberation was the development of oxygen-producing photosynthesis, since the main chemical inputs for this are carbon dioxide and water.