If you read any amount of science fiction, you’ll probably come up against the idea of alien life based on some element other than carbon. Almost universally, this element is silicon, but some writers have gone further afield in imagining totally novel forms of life — for instance, Carl Sagan once imagined life forms that could evolve and thrive in the gaseous sea that is Jupiter (aside from its core). Still, why do most writers consistently turn to silicon as the most likely carbon substitute? And why is carbon the basis of all life currently known?Basically, the answer is that life is complex, and more to the point that it must be complex. You can’t create an organism capable of regulating its internal state, of moving, eating and excreting, and of replicating itself to create offspring, without a wide variety of molecules. You’re going to need a central building block that can support complex branched structures while remaining strong — but not so strong that you can’t easily rearrange the whole thing later. There’s also the fact that carbon happens to be an abundant element on Earth, one available to early replicators, but for the most part we are carbon-based because carbon makes a good backbone.Probably not how silicon based life would actually look.The reason carbon has this property is somewhat complicated but can be boiled down to this: when bonding with other atoms due to its natural chemical properties, carbon will form four bonds. There are only a few elements that can do this naturally. Oxygen, for instance, will naturally form two bonds (think H2O). The four-bond structure allows a wide variety of possible chains with branches that have branches that have branches. When a bonding slot is unwanted, the carbon-hydrogen bond that usually fills the gap isn’t very reactive, so it won’t interfere with whatever else might be going on in the area.Consider the periodic table of elements, which is arranged so that elements with the same number of bonding slots (in parlance, the same number of “valence electrons”) will lie in vertical columns. This means that if you find carbon (element #6) and look directly below it, you’ll find the next most logical element to form the backbone of a living system — and ‘lo, we find silicon. This is why science fiction tends toward postulating silicon-based life: it shares the main virtue that brought carbon to power here on Earth.Looking further down the list brings us to less used elements like Germanium and Tin — these heavy elements are large and unwieldy, forming weaker bonds than their higher vertical neighbors because they hold other atoms a greater distance from their nucleus. Within the class of elements that can form complex molecules, carbon has helpful chemical properties for making and breaking chemical bonds. We already know of several thousand species that use silicon extensively in their overall biology — but none of them use silicon in DNA, so they are still considered to be carbon-based. Carbon forms strong double and triple bonds, not just allowing the branched structure of DNA but protecting that structure with strong chemical properties.The fact is that life probably could technically arise from a wide variety of molecules, but carbon seems to be by far the most likely. A species based on silicon might be funneled down roughly similar evolutionary paths as a carbon-based one, but something based on, say, phosphorus could turn out totally unrecognizable. Everything from its physical structure to its method of genetic inheritance would have to be wildly different. When imagining alien species, the most outlandish concepts would start not from a different environment, but from an totally different sort of chemistry.