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The toughest life known is the invisible life: microorganisms and bacteria. Living beings capable of surviving in extreme conditions are called extremophiles. They survive in conditions that would be lethal to any other way of life. They resist extreme temperatures, above the degree of boiling water and below freezing, acidity conditions, lack of sunlight and oxygen, pressure, salinity ... They can remain dormant for thousands of years and return to revive on contact with water.
The only thing that extremophiles need is: organic matter, water and a source of energy. Organic matter abounds throughout the Cosmos. They can use a source of energy other than sunlight. In fact, at the beginning of the 90s, a bacterium was discovered that lived underground, 7 km deep, and fed on oil. What extremophilic life does need is water in a liquid state. Or, at least, we believe so. Until today, there is no evidence that any form of life can survive without liquid water. But we can be wrong.
Until now, Earth is the only place in the universe where the existence of water in a liquid state is confirmed. But in the Solar System itself there are planets and satellites with ice water. If it were demonstrated that extremophiles can survive with ice water, new possibilities in the search for extraterrestrial life would open up.
Microorganisms have been found in ice walls of some caves, but we still don't know if they would survive there permanently. Extremophiles can survive without liquid water for a long time, in their state of lethargy, but not eternally. The need for liquid water seems its only insurmountable obstacle.
Probably, the extremophiles were the first living beings on Earth. And they will be the last to disappear. They could even be earlier than stromatolites, which are officially the first living being on the surface of the Earth. It is possible that some microorganisms lived underground. There they protected themselves from the shelling of meteorites and the intense volcanic activity that ravaged the surface. The lack of sunlight and high temperatures are not a problem for them.
Within a billion years the Earth will heat up, the oceans will begin to evaporate and become increasingly salty. All life forms will be extinguished. Only extremophiles will resist for some time. They will be the last survivors of the Earth. Even when all the water has evaporated, they will live dormant inside salt crystals for several thousand more years. They will develop red pigments that will protect them from solar radiation. The last color of life on Earth will be red. It would be distinguished from space, if there was someone to see it.
The only thing that extremophiles have failed to do is evolve into advanced life forms. When life evolves, it specializes in a habitat and that makes it more fragile and vulnerable. Perhaps it is an inevitable consequence in every evolution of life.
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