Many modern organisms derive their energy either directly or indirectly from sunlight. This may not always have been so, particularly in the earlier steps of evolution when the simpler molecules were being synthesised. An alternative therefore to photosynthesis is the derivation of energy from a thermal source. The ready supply of heat energy at hydrothermal vents, supplied by the hot water, could be this source. Thus creatures which can sense heat (light of a specific wave length) develop an advantage in the very hot to very cold gradients of hot springs.
For more details visit the web site http://www.pbs.org/wgbh/nova/abyss/life/extremes.html.
There is some debate about how hot vents should be to foster the development of life. It is perhaps unlikely that super-hot vents are the likely site, since the high temperatures (400°C) are likely to destroy organic molecules, rather than permit them to form. Cooler vents (~ 100°C), forming away from the main axial ridge of an oceanic spreading system are perhaps a more likely environment.