In the early days of science fiction television, aliens on shows like Star Trek and Dr. Who typically looked like humans with slight feature changes. Some had green skin, some were given antennae, others had enlarged heads, and so forth—nothing too complex thanks to small production budgets. These days, digitally created aliens in Hollywood blockbusters can take on any shape or form, yet many are still presented as anatomically similar to humans, having two arms, two legs, and a head with two eyes, a mouth, and a nose. (Think of Guardians of the Galaxy or Avatar.)
It is difficult for writers to imagine life that is truly different from terrestrial life. The book that gets our award for having the most distinctly original alien life-form is The Visitors. It features benign aliens that look like floating black rectangular boxes that are several hundred feet long. Now that is weird life!
Life as we know it on Earth utilizes molecules built primarily out of carbon. Scientists, however, have contemplated the possibility of “weird life”—that is, alien life based on silicon, rather than carbon. Do the universal laws of chemistry support such a possibility? Before we can answer that, we need to consider why known life is carbon based. (The information presented here is summarized from our longer paper on this subject.)
Requirements for Life Chemistry
One of the hallmarks of life is its extreme chemical complexity. All life must be able to absorb nutrients and food, convert food to energy, remove waste, repair or replace body parts, reproduce, and so forth. Such tasks necessitate the existence of a large and diverse collection of complex molecular machines. We infer, then, that any element serving as a basis for life chemistry must be able to support a vast array of chemical structures.
Life chemistry must also be able to form large polymeric structures. This is important for at least two reasons. First, using just a few different building blocks one can generate an endless array of stable but variable and interchangeable molecular forms. For example in terrestrial organisms, proteins are polymers made from about twenty different amino acids. Proteins have to be enormous in order to carry out their precise catalytic functions while simultaneously being specific enough to not react with other molecules. Second, long polymer chains are critical for encoding genetic information. In Earth life, this role is handled by DNA, which is a polymer constructed from just four nucleobases (plus sugars and phosphates). Organisms require an enormous amount of genetic information for reproduction, so the ability to form strands of nearly unlimited length is absolutely vital. Thus, alien life—even weird life—almost certainly requires the ability to form long polymers even if the specific building blocks are different from those of Earth life.
The Life-Essential Properties of Carbon
Chemists have identified at least five major features of carbon that explain why it is uniquely qualified to serve as a basis for life chemistry. Three of these properties are shown in the context of the periodic table (see figure 1).
- Forms up to four single bonds. Carbon (and the other elements in the same column) generally forms four bonds, whereas other nearby elements form three or less. With the exception of hypervalent molecules (which we will discuss below), this represents the effective maximum in bonding, which contributes to carbon’s ability to form an exceptionally wide range of molecules.
- Forms stable double and triple bonds. Carbon can form strong multiple bonds, which greatly increases the number of possible molecules that carbon can form. In contrast, elements in the rows below carbon on the periodic table, such as silicon, generally do not form multiple bonds.
- Forms aromatic compounds. Aromatic molecules (in chemistry, “aromatic” does not refer to the aroma or odor of a molecule) are a special case of multiple bonds in ring systems that display exceptional chemical stability. Because of their unique chemical properties, aromatic molecules play an important role in many biological molecules—including four of the twenty main amino acids, all five nucleic acids, as well as hemoglobin and chlorophyll.
- Forms strong carbon-carbon bonds. The carbon-carbon single bond is the strongest among elements located near it on the periodic table. This has two important consequences for life. First, carbon-based biomolecules are very stable and can persist over long periods of time. Second, stable self-linking (carbon-carbon bonding) allows for rings, long chains, and branched chain structures that can serve as the structural backbone of a dizzying array of compounds.
- Can form indefinitely long chains. Carbon is unique among all the elements in its ability to form gigantic polymers needed for genetic information, without which life would be impossible.
Taken together, these properties allow carbon to form a wider array of possible chemical compounds than any other element—without exception. For perspective, carbon is known to form close to 10 million different compounds. In fact, the field of organic chemistry, which focuses exclusively on the chemistry of carbon, is far richer and more diverse than the chemistry of all other elements combined.
Figure 1: Upper-right corner of the periodic table showing some important trends in bonding. Image credit: John Millam
Stability of Carbon-Based Molecules
Carbon is limited to forming no more than four bonds, but its cousin silicon can, under certain conditions, form structures with five or even six bonds. These cases are referred to as being hypervalent. While this gives silicon a small advantage over carbon, it comes at a big price—silicon compounds are generally much more reactive than analogous carbon compounds. Because carbon cannot form hypervalent structures, its structures are suitably stable to serve as the basis for biochemistry. Chemists Michael Dewar and Eamonn Healy concluded that this is what makes life possible.1
The Verdict Is In—Carbon Is Special
In 1961, physicist Robert Dicke said it best when he declared, “It is well known that carbon is required to make physicists.”2 Clearly no other element can even come close to matching carbon’s chemical virtues. While those who promote the concept of extraterrestrial life frequently lambast the carbon-only view of life, a working alternative presented in any detail has yet to be presented.3 Biochemist Norman Pace went so far as to suggest that wherever life might be found, it would be subject to the universal nature of biochemistry.4 Or to put it more simply, life we may find elsewhere will most likely have to be carbon-based, and therefore chemically similar (though not identical) to Earth life.
We will continue exploring the possibility of weird life in future articles.
Dr. John Millam
Dr. John Millam received his PhD in theoretical chemistry from Rice University in 1997, and currently serves as a programmer for Semichem in Kansas City.
Mr. Ken Klos received his MS in environmental studies from the University of Florida in 1971, and worked as an environmental/civil engineer for the state of Florida.