How are humans "Carbon based" lifeforms?

Ok, I know I’m probably missing something obvious, but how exactly are we carbon based? What determines this? We’re about 60% water (thats Hydrogen and oxygen), bones are Calcium, theres lots of Iron in blood, so why do we say we are “carbon based?”

Because, chemically speaking, all organic matter on earth is based on carbon atoms. Usually, one only hears the term “carbon-based” when it is being compared to some other proposed type of life, such as “silicon-based.”

Hope this helps.

Maybe this will help, too:

I had to explain this to someone recently - why do we talk to silicon-based as an alternative, theoretical alien life form? I figure I might as well toss it into this thread before it gets asked.

The reason, in very simple terms, is that silicon is similar to carbon in it’s bonding capacity and affinity for other atoms. It is located directly under carbon on the periodic table, and so has the same number of valence electrons available for bonding. Since we understand life to work the way it does with carbon, it is fairly easy to hypothesize that just changing from one atom to another might still allow things to function in a similar manner. Although, to be honest, I don’t think there is anything precluding any other atom from being a biological backbone, its just that silicon has the most similar characteristics to life as we know it. Whether it would ever work, though…that’s a whole other story!

ok, let me get it straight (and thanks for the cite Kneadtoknow), its not so much the PERCENTAGE of chemical composition so much as WHAT the actual chemicals do that quantifies it…
That makes sense… thanks.

One of the big problems to silcon-based life is that carbon+oxygen–>gas (CO[sub]2[/sub], but silicon+oxygen–>sand.

We could say that humans are “water based”, since water is the solvent used by all life on earth. But all the important chemistry that makes up living things is done by organic molecules, and that means molecules that have carbon atoms. Carbon is a very unique molecule. Hydrogen typically forms single bonds, oxygen forms double bonds, nitrogen forms triple bonds, but carbon forms quadruple bonds. Having the capability to form four bonds makes the number of possible molecules that can be made with carbon practically infinite.

Silicon can also form four bonds, but it has some trouble. It isn’t as reactive as carbon. CO2 breaks down pretty easily, but SiO2 (silica) is one of the most non-reactive molecules there is. That’s why we can do all our chemistry experiments in glass test tubes and beakers, since glass is inert during the vast vast majority of chemical reactions.

So we have no real examples of Silicon forming very complex molecules like carbon does. Sure you can get some chains, but compared to the riot of carbon containing molecules silicon is kind of a dud.


  1. Carbon is an element, or in the singular form, an atom. Unless we’re talking fullerenes here, it’s not a molecule.

  2. Carbon does notform quadruple bonds. It tends to form four bonds. There is a difference, and a very important one. Likewise, nitrogen may form triple bonds, but more often it is found with three single bonds, or one single and one double. Moreover, nitrogen is very good at forming four bonds, it just carries a positive charge in this form, so it is not as stable as carbon.

  3. CO2 does not break down very easily, and that’s a good thing for us, or some of our body chemistry might go a little nuts. Just because it exists as a gas does not mean it breaks down easily; it means that it has less attraction between molecules at the same temperature.

Now, done with that, the main thrust of your argument is perfectly correct; silicon has a few small differences that really make it not nearly so good, at least for life as we know it. I have heard some noise about Boron being a possibility, since you can make boron cages with all sorts of stuff attached, but any real advance in that is a long way off.

Originally, the notion of silicon-based life was used in the way that mnenosyne described. More recently, the phrase has been co-opted to mean something entirely different; namely life based on computer chips, intelligent machines and the like. Chips don’t have to be made of silicon, of course. Germanium is faster, but although often predicted as the next generation it has never quite come to pass. Doubtless the reasons are a combination of economic and technical. And IBM spent some billions of dollars on Josephson junctions based on the speed that a superconductor stops superconducting in a (sufficiently strong) magnetic field before abandoning it.

What carbon does with incredible facility is form very long chain polymers. And that is what our life on earth is based on. Obviously, oxygen, hydrogen, nitrogen, sulfur, and various trace materials are also necessary.

To expand further on this point, a “double bond” is a bond in which two atoms share two electrons between them, and a “tripple bond” is a bond where three electrons are shared. Myrr21 is correct in saying that what makes Carbon special is that it forms up to four “single” bonds, meaning four seperate atoms can be attached to it simultaneously. It’s just a matter of semantics, but an important one.

It takes more energy to break a double bond than a single bond, and more still to break a tripple bond, etc.

Are you kidding? Ever hear of silicone? Silicones are molecules based on a chain of alternating Si-O atoms. These are quite stable and can form polymers thousands of units long. Each silicon atom in the chain has two spare binding sites available and just about any covalent atom or molecule can be attached.

A direct one-to-one replacement of silicon to carbon atoms would not yield useful compounds because silicon does not form extended Si-Si links. But Si-O compounds can be both very large and very complex- what you’d need to create something as complex as a living cell.

Yes, it may be very large but it is still essentially just Si and O linked together. The amount of information that could be stored in that kind of molecule is very small compared to, say a DNA molecule

I think the crux has been addressed, but if you’d like more info do a search on carbon’s main trick, tetrahedral bonding.

i think my orgo teacher once referred to a star trek episode on this… he said in one episode kirk and spock meet a species of silicon based life forms… they made a big deal about how they couldn’t live without oxygen… then in a later scene, you see spock and kirk walking in some cave somewhere, without any maaks or anything :slight_smile: