I first ran into the concept of
chirality when I was a fifteen-year-old Trekkie science fiction nerd.
I grew up watching the original
Star Trek, which impressed the hell out of me as a kid even though rewatching some of the episodes now generates painful full-body cringes at the blatant sexism and near-jingoistic chauvinism. Be that as it may, after going through the entire series I don't even know how many times, I started reading some of the fan fiction.
The fan fiction, of course, was more uneven than the show had been. Some of it was pretty good, some downright terrible. One that had elements of both, putting it somewhere in the "fair to middling" category, was
Spock Must Die by James Blish. Blish had gotten into the
Star Trek universe writing short-story adaptations of most of the original series episodes, but this one was entirely new.
Well,
mostly. It springboarded off an original series episode, "Errand of Mercy," in which the Federation and the Klingons are fighting over the planet Organia, which is populated by a peaceful, pastoral society. Kirk
et al. are trying to stop the Klingons from massacring the Organians, but much to Kirk's dismay, the Organians refuse Federation protection, insisting they don't need any help. And it turns out they don't -- in the end, you find out that the Organians are super-powerful aliens who only assumed human-ish form to communicate with the two humanoid invading forces, and are so far beyond both of them that they indeed had nothing to fear.
In
Spock Must Die, the crew of the
Enterprise is sent to investigate why Organia has suddenly gone radio-silent. It turns out that the Klingons have surrounded the entire planet with a force field. Spock volunteers to try to transport through it, which fails -- but after the attempt, suddenly there are
two Spocks in the transporter room, each claiming to be the real, original Vulcan.
[spoiler alert, if anyone is actually going to go back and read it...] What happened is that the transporter beam was reflected off the surface of the force field, and it duplicated Spock -- there was the original (who never left the transporter pad) and the duplicate (the reflection, recreated in place). Since both the original and the duplicate were identical down to the last neuron, each of them had the same memories, and each was convinced he was the real Spock.
The key turned out to be the fact that the duplicate had been reflected
all the way down to the molecular level.
Why this matters is that a number of molecules in our bodies -- amino acids and sugars being two common examples -- are
chiral, meaning they have a "handedness." Just like a glove, they exist in two possible forms, a "right-handed" and a "left-handed" one, which are mirror images of each other. And for reasons unknown, all of our amino acids are left-handed. No organism known manufactures right-handed amino acids. Further, if you synthesized right-handed amino acids -- which could be done in the laboratory -- and fed them to a terrestrial organism, the organism would starve.
But the reflected Spock, of course, is exactly the opposite. Kirk eventually figures out what's happened because one of the Spocks barricades himself in one of the science laboratories, claiming the other Spock wants to kill him. The truth was he had to have access to a lab in order to synthesize the right-handed amino acids without which he'd die.
Clever concept for a story, right there.
Chirality is quite a mystery. Like I said, the left-handedness of amino acids is shared by all known terrestrial organisms, so that bias must have happened
very early in the generation of life.
Why it happened is another matter entirely. A persistent question in scientific inquiries into the origin of life on Earth (and the possibility of life elsewhere) is how much of our own biochemistry and metabolism is
constrained. We code our genetic information as DNA; could it be done a different way elsewhere? Our primary energy driver is ATP. Are there other ways organisms might store and access chemical energy? The question of constraint goes all the way up the scale to macroscopic features, such as
cephalization -- the clustering of the sensory processing organs near the anterior end of the animal. Makes sense; you want your sensors facing (1) the direction you're traveling, and (2) what you're eating. But are there other equally sensible ways to put an animal together?
Some things we take for granted almost certainly
aren't constrained, like bilateral symmetry. So many animals are bilaterally symmetrical that the ones that aren't (like adult flounders) stand out as bizarre. Aficionados of H. P. Lovecraft might remember that amongst the innovative ideas he used was that the aliens in "At the Mountains of Madness" weren't bilateral, but had five-way symmetry -- something completely unknown on Earth. (You may be thinking, "wait... starfish?" Starfish have what I'd call pseudo-pentaradial symmetry. As larvae, they're clearly bilateral, and they lose a lot of bilateral features when they mature. But some characteristics -- like the position of the
sieve plate, their water-intake device -- give away that deep down, they are still basically bilateral.)
Anyhow, all this comes up because of a recent discovery by astrobiologists at NASA's Goddard Space Flight Center. In
a press release, we hear about a meteorite discovered in Antarctica called Asuka 12236, which is a
carbonaceous chondrite -- a peculiar type of meteorite that is rich in organic compounds. Asuka 12236 contained large quantities of amino acids, which isn't as bizarre as it sounds; amino acids have been shown to form relatively easily if there are raw materials and a source of energy.
What stands out is that all of the amino acids in Asuka 12236 are left-handed -- just like the ones on Earth.
The scientists studying the meteorite are up front that the first thing to do is rule out that the amino acids in the meteorite aren't contaminants absorbed after the rock crash-landed. Most of the experts, however, think this is unlikely, and that we're looking at a genuine sample of extraterrestrial amino acids. And the fact that they all show left-handed chirality is pretty remarkable -- suggesting that the chirality of our biochemicals might, in fact, be constrained, and that we could well find biochemistry similar to our own on other planets.
In that way, at least.
So that's one less thing to worry about if we ever go to an alien world. Unlike the right-handed reflected Mr. Spock, we'd be able to metabolize alien amino acids just fine.
Of course, how familiar-looking everything
else would be is still open to question.
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This week's
Skeptophilia book recommendation of the week is a brilliant retrospective of how we've come to our understanding of one of the fastest-moving scientific fields: genetics.
In Siddhartha Mukherjee's wonderful book
The Gene: An Intimate History, we're taken from the first bit of research that suggested how inheritance took place: Gregor Mendel's famous study of pea plants that established a "unit of heredity" (he called them "factors" rather than "genes" or "alleles," but he got the basic idea spot on). From there, he looks at how our understanding of heredity was refined -- how DNA was identified as the chemical that housed genetic information, to how that information is encoded and translated, to cutting-edge research in gene modification techniques like CRISPR-Cas9. Along each step, he paints a very human picture of researchers striving to understand, many of them with inadequate tools and resources, finally leading up to today's fine-grained picture of how heredity works.
It's wonderful reading for anyone interested in genetics and the history of science.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support
Skeptophilia!]
