Biochemist Fuz Rana discusses a 2020 review article published in the journal Life in which a team of astrobiologists from MIT present a detailed evaluation of silicon’s life-support capacity.
The first scientific proposal for silicon-based life extends back to 1891 and the ideas of German astrophysicist Julius Scheiner. Since that time scientists have debated the prospects of silicon-based life—with some embracing its plausibility and others dismissing it. But as a team of astrobiologists from MIT have recently pointed out, to date no one has systematically and comprehensively assessed the capacity of silicon to support life in both a terrestrial environment and plausible nonterrestrial settings. They tackle this problem in a 2020 review article published in the journal Life in which they present a detailed evaluation of silicon’s life-support capacity.1
This work has obvious implications for astrobiology and origin-of-life models. But the implications extend beyond science. The research also highlights the design of the universe, our solar system, and the biochemical systems that constitute life.
Life’s Requirements
Before it’s possible to assess the usefulness of silicon as a chemical framework for life, it’s necessary to identify the general chemical requirements for life. The team from MIT notes that any life-supporting chemical element must display sufficient chemical diversity. This chemical diversity is required to produce the chemical complexity necessary to generate the diverse collection of molecular structures and chemical operations required to originate and sustain living systems.
There are two facets to this diversity: the capacity for an atom to produce molecules with a variety of shapes and the capacity of the atom to form compounds with a range of functional diversity. Both features are illustrated by carbon-based (organic) compounds.
This discussion of carbon’s diverse features leads to the question: Are there other atoms that display chemical diversity on par with carbon that could serve as the molecular basis for life?
Silicon Chemistry
Based on its position in the periodic table, at first blush silicon is expected to have the best chance of any other chemical element to rival carbon as a life-support system. Silicon has similar chemistry to carbon. It has a valence of four and forms Si-Si and Si-H bonds. It also forms bonds with heteroatoms such as oxygen.
But make no mistake, silicon chemistry only superficially resembles carbon’s chemistry. In many respects, silicon displays fundamentally distinct chemistry from carbon. This difference, I’ll explain, undermines silicon’s capacity to support life, at least in an aqueous environment.
As the authors of the Life paper state, “Silicon and carbon are ‘false twins.’ Their similarities are superficial and insufficient to mitigate their crucial differences. Chemistry that is stable and normal for carbon is unstable and exotic for silicon, and, similarly, chemistry that is unstable and impossible for carbon is stable and routine for silicon. Silicon’s distinct chemical characteristics and reactivity make it a challenging choice for life.”
In short, the fiction that permeates our culture about the prospects of silicon-based life—when carefully considered—leads us to a remarkable reality: A Mind must be responsible for the design of the universe. This Mind has a purpose for the universe—the advent of life.