We intuitively think that the more inhospitable an environment, the less likely life will be found. We are wrong. The deep ocean is a place where life not only exists, it flourishes.
1000 meters or 3,300 feet below the ocean surface is the beginning of the “no sunlight” zone. This is a dark, cold and bleak place, the sort of place where Hell is a better description than its dull scientific name, the Bathypelagic. Amazingly, there exists “hot spots” scattered across the vast Bathypelagic zone (which spans the continents) that support a teeming variety of life. These are hydrothermal vents – underwater geysers that spew lava and super-heated, chemical-rich water from the Earth’s molten core. They appear as columns of seawater hot enough to melt lead (750 degrees). The water doesn’t turn into steam like it would on the surface because of the tremendous pressure at these depths, making hydrothermal vents a life-sustaining pressure cooker.
Ocean researchers were stunned when the first deep-sea images of hydrothermal vents were captured in the late 1970s. Far from looking like a desert, there was life down there, tons of it: scrimps, crabs, mussels, tube worms, lobsters, fishes. This was just the beginning. Discoveries later showed that the deep pelagic realms – those waters from about 13,000 to 35,000 feet, housed the largest animal communities, the greatest number of individuals, and the broadest animal biodiversity not only in the oceans but any place on Earth.
How is that possible? Until 1977, it was thought that all life required sunlight. Trees and plants eat carbon dioxide and excrete oxygen and sugar complexes that serve as nutrients for organisms that live on the Earth’s surface. The discovery of life in hydrothermal zones rewrote the textbooks. Life there depended on an entirely new biological process, one fueled by chemicals. Scientists called it chemosynthetic life.
The discovery of chemosynthesis also led to a rethink about the “soup” theory of evolution which argues that around 4 billion years ago, chemicals in the primordial sea (the “soup”), with input from energy sources like lightning, reacted to form the first organic compounds. These compounds eventually turned into more complicated structures, which then morphed into early life forms. Chemosynthesis, however, offers a radically different story, one in which life began in the oceans, not on the Earth’s surface, whereby complex compounds emerge when hydrothermal gases mixed with iron and nickel sulfides on the surface of rocks under high pressure. If this is true [see note 1], the iron-sulfur theory not only suggests that life could have started in hydrothermal zones but that it had to start there. Science aside, there’s a poetic element to this story: the sea is where all life began, and it will be where all living things will eventually return.
 In April 2000, researchers at the Carnegie Institute of Washington’s Geophysical Laboratory tested this hypothesis in the lab by simulating the pressures of hydrothermal chemical reactions in the deep ocean. The pressurized mixture produced pyruvate, a molecule made up of three linked carbon atoms and a key component of living cells. Years later, further tests of the iron-sulfur theory produced more startling revelations. In an article published in the 2003 issue of the Philosophical Transactions of the Royal Society, researchers Michael Russell and William Martin argued that certain structures of hydrothermal vents made perfect incubators for organic molecules. Click this link for more on this “Water World” theory of the origin of life: https://www.jpl.nasa.gov/news/news.php?feature=4109