Carbon-based life forms comprise (so far as we know) 100% of life on the planet Earth. There have been speculations that silicon can also be used to create lifeforms, but in order to be able to move, such life would have to exist in an incredibly hot world, and it's supremely doubtful such life could ever get as complex as what we see here on Earth.

Though carbon-based, lifeforms need far more than just carbon in order to function. Accumulatively, carbon, oxygen, nitrogen, iron, neon, magnesium, silicon and sulfur comprise the bulk of all heavy elements in the universe, and not including hydrogen and helium, these heavy elements are the most common. Most carbon-based lifeforms have a liquid oriented system coursing through their bodies. We call it blood and in humans it's composed of a combination of iron, carbon, hydrogen and oxygen. The bone structure of most animals includes calcium as well as carbon and other elements.

Though there is over six-hundred times more silicon on Earth than carbon, due to the cool temperate atmosphere of this planet, carbon was the more logical choice. From a physics standpoint, in this atmosphere, such things as carbon dioxide and H2O aka water are plentiful and make chemical reactions better. In a world where over 2/3s of the planet's surface is water, carbon-based life can thrive.

The human body is like a natural efficient machine, with chemical reactions happening constantly, even as you read this. When you consume food, your body digests that food and converts it into energy and nutrients that your body needs to function. When you read these words, your body's neurological system is transferring electro-chemical impulses from your eyes and into your brain, and responses of your fingers and other body parts are all operated from your central nervous system, which uses the energy converted from food in order to send those impulses back and forth, allowing you to think and function, telling your body what to do and when. This is even happening when you sleep, which is why you dream.

Humans are not the only creatures comprised of carbon. All mammals, reptiles, amphibians, birds and even insects are carbon based, as is all plantlife on Earth. All life as we know it is comprised of carbon.

Added August 25th 2002:
Professor Pi says re silicon-based life forms: "such life would have to exist in an incredibly hot world" - Why is that? And why would Si based lifeforms be less complex? It can form the same covalent bonds as C. Do you have any sources to this?

To be honest, my 'source' was a Physical Science textbook I read way back in 7th grade. A page was devoted to the theory of silicon based life forms, suggesting they'd exist on a volcanic planet, and would perhaps not be life as we know it. Interesting read at the time and quite a curious thing to see in the middle of a public school textbook, though I can't for the life of me recall the exact name of the text or where you might find it. I mean, it was some time ago. =)

The heat factor is also a matter of common sense. Silicon has a higher heat tolerance than carbon. In order for silicon to be used as something other than a crystaline structure, the temperature for a lifeform based on silion would need to be high. Just as we carbon based lifeforms get frost bite and sometimes die in subzero temperatures, silicon based lifeforms would freeze up and face death at what we take for granted as room temperature. They'd probably feel comfortable in temperatures of over two hundred degrees, and would start crystalizing at temperatures that we find comfortable.

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