Some science fiction writers imagine incredible adventures in bizarre or far-fetched environments. One personal favorite of mine is the Star Trek: The Next Generation episode “Where No One Has Gone Before.” A character referred to as “The Traveler” uses his unusual powers to transport the Enterprise to the outer rim of the universe where the line between thought and reality blurs. It was a fascinating episode (except for the presence of Wesley Crusher). Increasingly, science fiction writers imagine creatures that depart from an essential component of all Earth life—being carbon-based.
Sticking with Star Trek, I’m reminded of the Crystalline Entity and Horta, which also feature silicon-based life. Largely mechanical beings like the Transformers, R2-D2, C-3PO, and the liquid metal T-1000 represent advanced (and cool) sentient cyborgs or androids. While non-carbon alien biology abounds in science fiction, must real live organisms be carbon-based?
Nobody disputes the incredible effectiveness of carbon for life’s chemistry, but a number of prominent scientists argue that perhaps other truly alien forms of life exist. Carl Sagan, one of the most prominent advocates of extraterrestrial life, coined the term “carbon chauvinism” to disparage the idea that only carbon forms an adequate basis for life. According to Sagan, “A carbon chauvinist holds that biological systems elsewhere in the universe will be constructed out of carbon compounds, as is life on this planet.”1 Sagan and many others basically propose that our assessment that carbon forms the only basis for life relies on a bias intrinsic to our conditions. They note that we are based on carbon and experience a small range of environments here on Earth. So, given our limited experience, why should we conclude that no other chemical basis for life exists?
Even with that reasoning, one must suggest some alternatives. The usual list of alternatives includes silicon, boron, sulfur, and even a combination of nitrogen and phosphorus. Let’s examine the most likely alternative to see the challenges that alternative biochemistries face.
What about Silicon?
The guiding principle Dmitri Mendeleev used to organize the elements in the periodic table is that various elements share similar chemical properties. Consequently, the obvious place to look for alternatives to carbon is those elements around carbon in the periodic table. Elements in the same row don’t have the chemical complexity because they form fewer bonds per atom. Silicon sits immediately under carbon and exhibits many of the same chemical properties, particularly the capacity to form four covalent bonds per atom. When investigating deeper, silicon also behaves very differently from carbon in some significant ways.
Silicon strongly reacts with oxygen. Because of its high reactivity with oxygen, silicon is not found in a free state on Earth. Instead, it forms silicon dioxide or other silicates. This fact is more remarkable considering that silicon ranks as the second most abundant element in Earth’s crust (27 percent), behind oxygen at 46 percent. In contrast, carbon exists in three free forms around Earth in spite of composing just a fraction of a percent of the crust.
Silicon dioxide is sand, not a gas. The final reaction of carbon compounds with oxygen is carbon dioxide. In this molecule, each carbon atom forms two double bonds with two oxygen atoms. Because of the covalent nature of the double bonds, the interactions between molecules are weak. Thus, carbon dioxide exists as a gas at liquid water room temperatures. Silicon atoms are larger than carbon atoms such that silicon can’t form stable double bonds with oxygen. In silicon dioxide, the large attraction of oxygen atoms for electrons results in a more ionic type of bond where the oxygen atoms form bridges between the silicon atoms. This bonding structure results in a crystalline solid with a high melting point. Silicon dioxide, also known as silica, is often a major part of sand and is the most abundant compound in Earth’s crust.
Silicon compounds offer little “handedness.” Consider your right and left hands. In an ideal world, both have the same composition and structure, but they cannot be converted from one to the other. They are mirror images, or in more technical terms, have a definite chirality (or handedness). A large number of the compounds formed by carbon come in mirror-image copies, but that is not true for silicon compounds. Many of the important biological molecules have a definite handedness. Almost all amino acids are left-handed, where all the sugars tend to be right-handed. This handedness plays a critical role in how these molecules react with other substances. For example, one handedness of naproxen (the active ingredient in Aleve) helps relieve arthritis pain, while the opposite handedness causes liver poisoning (while not relieving any pain)! From a biochemical perspective, the functioning of life seems to depend on organic molecules having a specific handedness.
Silicon is relatively inactive at normal temperatures. Even though carbon and silicon can form sufficiently long molecules, carbon reacts with a wealth of other atoms in the range of temperatures where water is liquid. Carbon reacts with oxygen, hydrogen, nitrogen, phosphorus, sulfur, and a host of metals to provide the chemical diversity that life requires. In contrast, silicon reacts with a small number of elements at room temperature, although higher temperatures induce reactions with a few more elements.
Silicon compounds are less abundant in the universe. As astronomers observe the heavens, they detect a large number of elements and molecules in space. A simple analysis of the molecules detected shows that carbon-based molecules vastly outnumber silicon-based ones. For example, one website catalogs all the known interstellar molecules and separates them by the number of atoms the molecules include.2 The list includes almost 200 molecules but only 10 or so that contain silicon. Only two of those silicon molecules contain more than four atoms (C4Si and SiH4). Of the 60 molecules detected beyond the Milky Way Galaxy, only one contains silicon (SiO). In contrast, every one of the over 60 molecules with more than five atoms contains carbon. Additionally, astronomers have detected more 5+-atom carbon molecules (18 of them) in distant galaxies than silicon molecules of any size in our galaxy.
Scientists have invested great energy trying to understand the chemistry of silicon. Chemists even offer the Frederic Stanley Kipping Award “to recognize distinguished contributions to the field of silicon chemistry and, by such example, to stimulate the creativity of others.” The research over the past few decades shows that silicon chemistry is richer than originally expected. Even so, it appears that silicon cannot replace the complex chemistry required for life that carbon so readily offers.
Other Possible Alternatives
Discussion and investigation of alternatives to carbon-based life often occur in the rather limited range of environments experienced on Earth’s surface. This entails a certain temperature range (roughly 0°F to 200°F), pressure range (0.3 atm, on top of Mount Everest, to 250 atm, at hydrothermal vents), oxygen content (none to 0.2 atm), salt concentration (0–30 percent), and others. Scientists suggest that our relatively small niche environment biases the conclusions we draw in assessing alternative biochemistries. While silicon shows significant deficiencies compared to carbon under Earth conditions, maybe it exhibits the chemical complexity necessary at temperatures associated with liquid nitrogen.
When considering environments different from conditions on Earth, one must also look at solvents besides water and chemistries other than carbon. Pure ammonia (another cosmically abundant molecule) or ammonia-water mixtures remain liquid to temperatures as low as -110°F at normal Earth pressures. At much higher pressures (60 atm), ammonia is liquid over much the same temperature as water (at normal pressure). Other proposed solvents include sulfuric acid, methane/ethane, hydrogen fluoride, and even sodium chloride or silicon dioxide. The latter two materials melt above temperatures of 1500°F and 2900°F, respectively. Obviously, these extreme temperatures would require molecules formed from something other than carbon—maybe silicon, oxygen, or aluminum.
Such alternative solvents and backbone elements provide a fascinating arena for new research, and the diverse nature of life found on Earth gives some basis for speculating that perhaps other environments could support a completely different form of life.
The Bottom Line
Carbon’s chemical complexity undergirds all (known) life on Earth in a fundamental way. Further, observations and experiments over the past few decades affirm the remarkable ability of carbon to anchor life while revealing shortcomings of proposed alternative biochemistries. Even so, theoretical work shows some potential environments where truly alien life (different solvent than water, different chemistry than carbon) might exist. Personally, I find this research both intriguing and fascinating and anticipate the results from its continuation. Only by gaining a deeper understanding of how life on Earth operates, by studying how life might be different, and by searching the heavens to see what other planets have to say about the matter will we truly know whether life must rely on carbon or not. With all that said, if I were a betting man, I would still stand with Carl Sagan’s sentiment that the “common chauvinism—one which, try as I might, I find I share—is carbon chauvinism.”3