Kurzfassung (nur Englisch): Over the past 40 years our understanding of the diversity of life in the deep sea has been revolutionized by the discovery of dense communities of large animals living at chemosynthetic environments. These communities can be found at hydrothermal vents, methane seeps and organic-falls (sunken dead large marine animals and wood) where the energy source at the base of the ecosystem is chemical (chemosynthetic), rather than coming from the sunlight falling on the upper layers of the ocean (photosynthetic). These chemicals are reduced compounds, such as hydrogen sulfide and methane, which is then oxidized by microbial organisms to produce energy (chemosynthesis). Many of the animals that dominate the biomass at modern chemosynthetic environments have symbiotic relationships with these microbes (chemosymbiosis), and this allows these animals to grow very fast and to large sizes, compared to their relatives in other marine environments. These animals include bivalves, gastropods and tube worms. Fossil vent communities are found in two different rock types in the geological record: volcanogenic massive sulfides (VMS), which formed at high temperature vents, and jaspers (iron-silica rocks), which formed at low-temperature, sulfide-poor vents. Animal fossils can be found in VMS deposits from the Silurian period onwards, although there are some enigmatic structures from Cambrian vents, which might have had an animal origin. Microbial fossils have been discovered in VMS deposits all the way back to the Paleo-archaean era (3.235 billion years ago) and in jaspers to the Eo-archaean (3.770, or possibly 4.280 billion years ago), with the latter being the oldest organisms yet discovered on Earth. These very early dates help to corroborate ideas that terrestrial life may have started at hydrothermal vents. The evidence also suggests that life may have been possible on Mars during its equivalent aged warmer period, and that life may be present at putative hydrothermal sites on the icy moons with liquid oceans (e.g. Europa and Enceladus).