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On a Whim by Mat in Vadnais Heights.

Sometimes I don’t know where these articles are going to wind up. I sat down this month with a vague idea to write something about the phenomenon of vision. It’s a pretty wild subject if you think about it—an organ that uses the light bouncing around your environment to build a map of the world, including things too far away for you to interact with directly. Then I stumbled across a page about scallop eyes (I didn’t even know scallops had eyes) and the fact that instead of lenses they incorporate teeny little mirrors made of guanine crystals to focus light. Unbelievable! And before I know it, three or four Internet rabbit-holes later, I am off on a completely different arc altogether.

It was the guanine that did it. See, I only know of guanine from one other place. It is one of the four nucleotides found in DNA. Oh, and that its name derives from the fact that it was first isolated from guano. A lowly beginning. But anyway, the fact that there were bodies out there incorporating guanine crystals kinda threw me for a bit. But then it occurred to me that, well, my skeleton contains lots of calcium crystals in the form of hydroxyapatite (yeah, I looked up the name—can’t memorize everything) and that got me to wondering what else pops up in living things that you might not expect to find there.

The bulk of your basic biomass is made up of just four elements: carbon, hydrogen, oxygen, and nitrogen. Nearly 97%, in fact. The remaining trace elements are things like the iron in your blood, the calcium in your aforementioned bones, and various other elements found frequently in the enzymes that catalyze your body’s chemical reactions.

As a side note, did you ever wonder why radium is so alarmingly dangerous? Well, apart from the fact that it is fiercely radioactive, it also appears in the same column of the periodic table as calcium. And if you remember high school chemistry, elements in the same column of the table have very similar chemical properties. That means that if you accidentally ingest some radium, your body will mistake it for calcium and happily incorporate it into your bones. That means a) your own skeleton will start irradiating you from the inside and b) your bones will get extremely brittle, because radium does not have the same structural strength that calcium has.

If you count them up you will find that 36 of the chemical elements, well over a third, are known to be incorporated into some form of life somewhere. A few of the more surprising are arsenic, which is vital in small quantities to the human nervous system; strontium, which some radiolarians use instead of calcium to build their skeletons; tungsten, which appears to be necessary for ATP metabolism in some thermophilic archaea; and even uranium, which some bacteria use as a “terminal electron receptor,” whatever that is.

Looping back around to the major players—carbon, hydrogen, oxygen, and nitrogen—you might wonder why life-as-we-know-it is invariably referred to as “carbon-based,” when in fact it is oxygen, at 62%, that makes up the bulk of living matter. The reason has to do with the way carbon bonds with other atoms. Atoms bond with each other via their electrons, and carbon has sort of the perfect storm of properties for connecting its electrons up to those of other atoms and to itself; it is neither electropositive nor electronegative, so it is more likely to share electrons than gain or lose them; it has a maximal number of electrons, four, available to bond with other atoms; and it tends to form bonds in geometric configurations that are very stable. That means you can build up quite large, intricate, and stable molecules on a framework of carbon. And large, intricate, stable molecules are exactly what living systems need.

Are there any other elements that fit the bill? Well, maybe. It has been suggested many times that silicon might do. It also forms four covalent bonds with other atoms, just like carbon, and it is ubiquitous in Earth’s crust. The main impediment seems to be that the bonds in question are not as strong as those of carbon, though they might endure through a wider range of temperatures. This has led lots of sci-fi writers to speculate about the possibility of silicon-based life, though the hacks among them always seem to wind up with a kind of lumbering rock monster. This makes about as much sense as claiming that a carbon-based lifeform would have to be a lumbering diamond monster. The whole point of silicon is that it makes lots of interesting molecules with wildly varying properties. One of those molecules is silicone, which is fer cryin’ out loud used as an industrial lubricant. That’s not something you build out of boulders! Use your imagination!

Okay, I have wandered rather far afield from where I started. As I recall, that was with the scallops and their wonderful eyes made of guanine-crystal mirrors. That sounds like pure sci-fi right there, but it is just one little example of the riotous variety of life on Earth. The point, I guess, is that there is virtually no end to the use-what’s-at-hand philosophy of evolution. Need a scaffold to hang your squashy bits on? Grab some calcium. No calcium handy? Meh, strontium might do. Need to focus light within your eyeballs? Build a lens. No infrastructure compatible with lenses? Mirrors focus light, and hey, guanine forms nice, shiny convex crystals. Why not? I think that as long as we keep hunting around for new and interesting stuff within biology, sci-fi is going to have to work hard to keep up.