Is silicon-based life a possible alternative for carbon-based life?

In a recent debate, atheist philosopher Alex Rosenberg responded to the cosmic fine-tuning argument presented by William Lane Craig by asserting that complex life could be other than it is. He specifically mentioned silicon-based life.

Let’s see what scientists think of his speculation, using this article from Scientific American.


Group IV of the Periodic Table of the Elements contains carbon (C), silicon (Si) and several heavy metals. Carbon, of course, is the building block of life as we know it. So is it possible that a planet exists in some other solar system where silicon substitutes for carbon? Several science fiction stories feature silicon-based life-forms–sentient crystals, gruesome golden grains of sand and even a creature whose spoor or scat was bricks of silica left behind. The novellas are good reading, but there are a few problems with the chemistry.

Indeed, carbon and silicon share many characteristics. Each has a so-called valence of four–meaning that individual atoms make four bonds with other elements in forming chemical compounds. Each element bonds to oxygen. Each forms long chains, called polymers, in which it alternates with oxygen. In the simplest case, carbon yields a polymer called poly-acetal, a plastic used in synthetic fibers and equipment. Silicon yields polymeric silicones, which we use to waterproof cloth or lubricate metal and plastic parts.

But when carbon oxidizes–or unites with oxygen say, during burning–it becomes the gas carbon dioxide; silicon oxidizes to the solid silicon dioxide, called silica. The fact that silicon oxidizes to a solid is one basic reason as to why it cannot support life. Silica, or sand is a solid because silicon likes oxygen all too well, and the silicon dioxide forms a lattice in which one silicon atom is surrounded by four oxygen atoms. Silicate compounds that have SiO4-4 units also exist in such minerals as feldspars, micas, zeolites or talcs. And these solid systems pose disposal problems for a living system.

So, first of all, it makes SAND. Second of all, it is so attracted to oxygen that it can’t easily join to make any other polymers that could be used in the chemistry of the minimal functions of a living system.


Also consider that a life-form needs some way to collect, store and utilize energy. The energy must come from the environment. Once absorbed or ingested, the energy must be released exactly where and when it is needed. Otherwise, all of the energy might liberate its heat at once, incinerating the life-form. In a carbon-based world, the basic storage element is a carbohydrate having the formula Cx(HOH)y. This carbohydrate oxidizes to water and carbon dioxide, which are then exchanged with the air; the carbons are connected by single bonds into a chain, a process called catenation. A carbon-based life-form “burns” this fuel in controlled steps using speed regulators called enzymes.

These large, complicated molecules do their job with great precision only because they have a property called “handedness.” When any one enzyme “mates” with compounds it is helping to react, the two molecular shapes fit together like a lock and key, or a shake of hands. In fact, many carbon-based molecules take advantage of right and left-hand forms. For instance, nature chose the same stable six-carbon carbohydrate to store energy both in our livers (in the form of the polymer called glycogen) and in trees (in the form of the polymer cellulose).

Glycogen and cellulose differ mainly in the handedness of a single carbon atom, which forms when the carbohydrate polymerizes, or forms a chain. Cellulose has the most stable form of the two possibilities; glycogen is the next most stable. Because humans don’t have enzymes to break cellulose down into its basic carbohydrate, we cannot utilize it as food. But many lower life-forms, such as bacteria, can.

In short, handedness is the characteristic that provides a variety of biomolecules with their ability to recognize and regulate sundry biological processes. And silicon doesn’t form many compounds having handedness. Thus, it would be difficult for a silicon-based life-form to achieve all of the wonderful regulating and recognition functions that carbon-based enzymes perform for us.

The troubling thing I find about atheists is that they seem to be under the impression that an alternative speculative explanation is a refutation of an argument that is based in evidence.

So it goes like this:

  • origin of the universe? I can speculate about a naturalistic alternative cosmology which is falsified by observations
  • cosmic fine-tuning? I can speculate about an untestable multiverse
  • origin of life? I can speculate about unobservable aliens who seeded the Earth with life
  • Cambrian explosion? I can speculate about intermediary fossils that have not yet been discovered
  • habitability? I can speculate that habitable planets exist just outside the boundary of the observable universe
  • resurrection of Jesus? I can speculate that Jesus had an unknown, identical twin brother who showed up when he died and took his place

I think that if we are going to make a worldview, we should ground it in the evidence we have today. We should not have faith in speculative theories that we heard about on Star Trek. Seriously.

7 thoughts on “Is silicon-based life a possible alternative for carbon-based life?”

  1. Good stuff.

    I have mentioned before my struggle with the “fine-tuning argument”,and this indeed goes right to some of those areas where I have been trying to reach more clarity.

    I guess,for me, it always came down to the question of what we mean when we say “life form” or “life as we now it”. And this post does address that to some extent. I think my issue would push it a bit further and ask if we are not anthropomorphizing our definitions of what a “life form” can be.
    When I was young (8-10 years) I just intuitively felt that whatever the conditions extant on a given planet would determine what could arise in that place (I loved science fiction at that age and read tons of it). If there is some natural reason that “life” might emerge, then what that life looked like would probably be far outside anything we could recognize or imagine. That was always my problem with the idea of fine-tuning as an argument for God.

    Having since spent my life in science, including research in molecular biology, I have given up on much of these concerns. Yet, I still sense that my earliest intuition had some merit, and I am having a hard time ridding myself of it entirely.

    It would essentially make the ever increasing list of fine-tuned parameters (such as Hugh Ross makes) for life (as we know it), to be irrelevant to “life” on another planet, which would just form as dictated by the parameters of that location (ie: not as we know it).

    Anyway, I’ve also always had a sense that my view could be refuted somehow. But I have not really heard that refutation clearly. I also believe that I could be suffering some intellectual stumbling block and just in need of a more clearly articulated refutation.

  2. Well, not really IMO. I consider ‘Star Trek’ fairly lacking in imaginative appeal, at least compared to what we can learn about the reality of our world in it’s almost limitless and unimaginable beauty (at all levels from the quantum to the molecular biology of the cell to our realm to the astrophysical cosmos), it is diminutive and banal. Beyond just it’s silliness, I always thought it also morally repugnant, at least when I was a youngster. I have not seen the show in over 30 years.
    This is just my curmudgeonly opinion obviously, and not anything I actually consider of any importance, so please take it in that vein.

    But more relevant and important…..

    Wintery, I read Denton’s “The Place of Life and Man in Nature: Defending the Anthropocentric Thesis” (from the journal “Bio-complexity”), early this AM. I think it is helping me see a bit more clearly some answers to my concerns expressed above. It’s a fantastic read, BTW. Just beautiful stuff really…….

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