Re: why is life based on carbon?

Hi Darrin,

It does seem that life requires carbon. Let's look at why, and then I'll give you an interesting exception.

First we need a good definition of life. Unfortunately, scientists have been arguing about the technical definition of life for some time. I prefer to use a broad definition that is as simple and clear as possible. Therefore let's define life as anything that can both duplicate itself and evolve. That is, it must be capable of reproducing, mutating, and reproducing its mutations. This excludes crystal growth and fire, but includes parasites such as bacteria (with DNA), viruses (with DNA or RNA), and possibly prions (a type of protein).

Prion proteins would be included as life only if they are capable of passing on mutations to subsequent generations of protein. Evolution has thus far not been demonstrated for any protein. However, a good argument could be made that a life form derived from amino acids (the building blocks of proteins) is possible, so we'll include them here to be sure we've covered everything. For more information on prions, see this site.

Common to DNA, RNA, and protein is their ability to involve themselves in a diverse number of complex chemical reactions. The diversity of these reactions is made possible by the molecule's possession of different functional groups. These functional groups allow for the interactions necessary for (among other things) replication. (This is important because replication is one of our two requirements for being life.)

An argument could be made that a life form could use functional groups other than those used by life forms on Earth. None-the-less, there still must be multiple numbers of these functional groups to allow for self-replication.

How do you stick many functional groups together into one molecule? That's where carbon comes in. DNA, RNA, and protein all use carbon in their "backbones" to link all the functional groups together. The functional groups (which may or may not contain carbon) are held together by a backbone (which must contain carbon).

So why does a backbone require carbon? Carbon is able to form bonds with an incredible number of other atoms. It is one of seven elements described as nonmetals based on bonding characteristics. (The nonmetals are the green elements on this periodic table.) Carbon, along with five of the other nonmetal elements (excluding Selenium) make up nearly all of the atoms in all organic molecules.

Of all the atoms carbon can form bonds with, it is its ability to make strong, stable bonds with other carbon atoms to form polymers (the backbones) that is most crucial for life. That is, it is the ability of one carbon atom to bond with two other carbon atoms and still have room to hold a wide range of functional groups that makes it so well qualified. No other atom has these qualifications.

Silicon has been proposed as a possible alternative to carbon for other life forms. This proposal is based on some similarities between the two elements. (Silicon is placed on the periodic table immediately below carbon.) However, polymers (backbones) of silicon are unstable, and are unable to form double bonds. (Double bond formation is another very important ability of carbon atoms.) It is these differences which explain why carbon is the basis of all known life forms even though silicon is far more abundant on Earth.

I highly recommend reading the chapter entitled "Origins of Life" from the book Biology and the Future of Man which was edited by Philip Handler and published in 1970 by Oxford University Press. It is easy to read and it covers what I've discussed here, plus many other great questions like it.

One neat exception to this rule applies. Some computer programs (Artificial Life/ Artificial Intelligence) can also meet our definition of life. These programs allow for replication and evolution within a computer system. Many computer chips are made from silicon. So in a very round-about sort of way, life can be based on an element other than carbon! Learn more about exciting advances in artificial life at the International Society for Artifical Life.

Remember, this is an amazing universe we live in. Anything is possible.


Thank you for your question!

John