I heard from a friend that a form of life was found here on Earth that was sulfur-based instead of carbon-based, implying life formed at least twice. Is this true?
No. There are however some kinds of bacteria (carbon-based, like all other life on Earth) that are able to metabolize sulphur.
In addition to bacteria there are also certain invertebrates that have a sulfide-utilizing anaerobic metabolism, like Echiuran worms on mud flats or some of the thermal rift critters ( my old Animal Phys. professor was big on this ).
- Tamerlane
Is it possible to have life based on any element? Could I have an iron-based life form? Calcium? Neon?
No. The reason life is carbon-based is that carbon is able to form bonds with 4 different atoms, so that it is able to form extremely complex compounds. Only elements that can form 4 bonds, and which are also fairly common, are likely to be able to form the basis for life.
Silicon can also form 4 bonds, and it has sometimes been speculated that it could potentially provide the basis for life forms. But so far this is just science fiction.
not quite true colibri. Depending on the pH, silicon is quite stable in the form of soluble silicates, and could be easy transported or excreted. Sodium silicate solutions used to be used for preserving eggs and waterproofing. However, make the pH too acidic and you will get lots of insoluble silica.
It’s been a while since I took chemistry, but wouldn’t silicon have weaker bonds than carbon due to its larger atomic radius?
OK, none of the drawbacks to silicon absolutely rule it out, they just make it a much more unlikely basis for life than carbon. Silicon has lot of other drawbacks, including lack of chirality (handedness) in its compounds that give them less specifity in reactions than carbon compounds have. This site has more info on silicon’s drawbacks.
Here’s an article that discusses some other alternatives to carbon-based life: Carbon chauvinism.
Yes. This site has more info on silicon’s chemistry:
[quote]
. . . Si is not as versatile as C. It does not readily form double and triple bonds (since the atom is larger, the bonds must “bend” or “stretch” more to reach - this takes more energy and makes the bond less stable).
[ul]
[li]Si-Si bonds are half the strength of C-C bonds, so long Si chains are weaker than long chains of C. [/li]
[li]Si-H and Si-O bonds are stronger than Si-Si bonds, while C-H and C-O bonds are about as strong as C-C bonds. [/li]
[li]Si-O chains (silicones) are very stable, and not highly reactive. They make good lubricants. [/li]
[li]SiO2 (quartz) is very stable and insoluable, unlike CO2. It is the major constituent of sand. [/li]
[li]Si does not form rings (aromatic compounds, like benzene C6H6), unlike C.[/ul][/li][/quote]
I think there is a sulfur-based lifeform living in my dog’s intestines.
Really? You sure it isn’t living in YOUR intestines? I know how blaming it on the dog goes. 
We already have silicon-based life forms. Not yet sentient, however. But it’s coming.
If you’re talking about computers Hari, I’m going to disagree that they can be considered lifeforms.
There was quite a buzz a few years ago when it was discovered that some undersea vent creatures use elemental sulfur, rather than oxygen, as the final electron acceptor. In other words, they use sulfur to burn their metabolic fuels. IIRC all these creatures were bacteria. However the tube worm’s lifestyle surprised everyone:
What Sauroposeidon’s friend said was probably a version of this story that had been mangled by too many tellings.
:smack: Here’s the link for the tubeworm quote from The Wild World of Chemoautotrophs.
It’s a wild, Wild World !
The reason that you would expect any alien species to be carbon-based is just pure probabiltiy, I imagine that carbon based life, being more efficient, would have many more times the chance of suriving than silicon based life.