Scientists don't know for sure. Perhaps the most popular theory says that, shortly after the Earth formed, millions of asteroids and comets, saturated in water, slammed into the planet, releasing their payloads to form Earth's oceans [source: CNN]. Scientists are working hard to understand more about what our planet was like billions of years ago, and each new piece of information moves us closer to understanding how Earth's oceans, lakes and rivers came to exist.
Almost all life-forms on Earth require water. In most cases, water is the primary ingredient in the body of a living thing -- for example, the human body is made up of about 60 percent water, and most terrestrial plants are closer to 90 or 95 percent water. Without water, almost none of the human body's basic functions would be possible; this goes for other animals, too. Animals need water to act as a solvent for the nutrients and minerals needed by their cells.
Plants rely on water mostly for the process of transpiration. After being collected from the ground via root structures, water rises in the inner tubes of a plant via capillary action. The water enters the plant's cells through the process of osmosis. When the water reaches the leaves of the plant, it exits through holes called stomata, allowing the plant to cool itself. Such processes are standard requirements for almost all forms of life; water allows creatures to process nutrition, regulate temperature and perform all kinds of essential tasks.
Nonetheless, there are some specialized life-forms that can go for long periods of time with little or no water:
- A tartigrade is a microscopic organism that replaces the water of its cells with sugar when conditions are particularly dry. This tactic, known as "anhydrobiosis," protects the tartigrade from extreme temperatures. It also lowers the organism's metabolism, so it can remain in a barely-alive state until water returns.
- Some plants, like cacti, use Crassulacean acid metabolism photosynthesis to store carbon dioxide in the form of acid inside their cells. These plants only open their stomata to allow water to evaporate at night, and they can keep their stomata closed altogether in extremely dry conditions.
- Some humans have claimed extraordinary feats of hydration refusal. Supporters have said that a boy in Nepal attained a state in which he needed no food or drink for two years while meditating. This has not been substantiated [source: BBC]. There are other reports of humans surviving up to two weeks without water.
Regardless, so much earthly life depends on water that without it -- however it got here -- we wouldn't even be alive.
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