why life has a bias to the left

April 9, 2009

As the faux conservative pundit Stephen Colbert warned President Bush, reality has a bias to the left. And interestingly enough, so does all life on Earth. We’ve know that all amino acids, the essential components for any terrestrial organism, have left-handed chiralities. Or to translate from science-speak, their molecules wind to the left. This is why genetic engineering experiments can splice genes from very different species in a lab. The molecules will line up and assemble correctly because they’re all wound in the same direction.

dna strand

But for many years, there’s been a nagging problem. Why does all life on our planet have these left-handed amino acids? You could very easily wind them to the right and they’d work just as well. In fact, you could get an identical biosphere with a different chirality. Of course you would have a very hard time finding a viable organism with both chiralities since the molecules it had to produce couldn’t match up. It would be like fitting together two different jigsaw puzzles and that means somewhere along the line, there had to be a process which resulted in our present day left-hand chirality gaining dominance over its rightward-wound counterparts. Could it be one the earliest cases of natural selection at work? Actually, it seems that primordial chemistry in the vacuum of space played a major role in what life could evolve on our world.

This is the conclusion of NASA astrobiologists Daniel Garvin and Jason Dworkin who examined meteorites rich in carbon to find the amino acid isovaline. Since isovaline isn’t actively used by living things on Earth and maintains the same chirality over billions of years, scientists can use it to take a look back at the dawn of the solar system and determine the ratio of left-handed to right-handed molecules without the threat of biological contamination. As it turns out, when a meteorite is rich with water, it has up to 18% more left-handed isovaline so when meteors with disproportionate amounts of left-handed amino acids fell to Earth and deposited their organic cargo, they would’ve created a bias towards organisms with our current chirality.

Once again, we’re finding evidence of life on our world coming from the depths of outer space as organic compounds ready to combine into functional organisms and just take it from there. And that raises some curious questions. Could life on other planets be very similar to ours? Is there a chance that alien and terrestrial hereditary mechanisms would be similar since they’re built from the same pool of left-handed organics? To be perfectly clear, being made of similar compounds doesn’t mean aliens would look anything like us. Evolution and the environment of their home world would shape their anatomy. Just look at the variety of life that evolved on our planet from the same 20 amino acids. Who knows what the conditions on another planet could do with the same basic mix of chemical compounds?

See: Glavin, D., & Dworkin, J. (2009). Enrichment of the amino acid L-isovaline by aqueous alteration on CI and CM meteorite parent bodies Proceedings of the National Academy of Sciences, 106 (14), 5487-5492 DOI: 10.1073/pnas.0811618106

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  • Hey awesome post! The question about chirality seems to be a big mystery. My sister studied the handedness of shell spirals in molluscs and I was engrossed by the implications of this. I think NASA has even carried out tests on the space station with this in mind to see if chirality in terrestrial creatures is influenced by microgravity conditions. I need to find that research to find out its conclusions.

    Cheers, Ian

  • Anonymous

    You mentioned that “This is why genetic engineering experiments can splice genes from very different species in a lab. The molecules will line up and assemble correctly because theyre all wound in the same direction.” I beg to differ with your interpretation and phrasing. The reason why genes from different species can be mixed and matched is because the base DNA is made from exactly the same components. In other words, you can put human DNA into bacteria unaltered, and the bacteria will know what to do with it. More to the point, the nucleotides (A, T, C, and G) have multiple chiralities, and it makes no sense to call these bases “left” or “right” handed. In fact, DNA molecules can have one of 3 local energy minimums or “conformations” of A, B, and Z. B is the left-handed spiral DNA is mostly commonly depicted as being; on the other hand, the Z conformation is a right-handed twist. Talking about the chirality of molecules only makes sense in terms of simple molecules like a particular mono-saccharide or amino acid that have precisely one chiral carbon. In fact, the for some of the more complicated amino acids, the chirality of that particular amino acid refers only to the alpha carbon (the one between the amino and carboxylate functional groups). I know it’s tempting to relate to science “poetically” as you’ve attempted to do in your metaphors, but I have to call this out as what Richard Dawkins calls “bad scientific poetry” because instead of using your metaphors to better convey the scientific facts, it has instead obscured the facts.

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  • Greg Fish

    “I have to call this out as what Richard Dawkins calls ‘bad scientific poetry’ because instead of using your metaphors to better convey the scientific facts, it has instead obscured the facts.”

    I don’t think that talking about the process in its most simple form is obscuring facts. I’m also well aware of the similarity of genetic code across all species and what you’ve done is added very detailed specifics to my one sentence simplification, and I’d like to thank you for contributing your expertise on the subject.

  • Daniel

    But you don’t say anything about why water makes one chirality of isovaline more common.

    Can it be traced to the weak interaction ?

    Also, I find it amazing that we can smell chirality!

    ( I wrote about this here)

  • john

    uhm, it’s to the right…

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  • It’s because the door is hinged on the right.

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  • neat!

  • Your post is wonderful. And it may indeed be true that the material aspects of life on planet earth favor the left. As a Republican, I’m admitting this very reluctantly.

    But how much of your right hemisphere did you use to gain these insights? Hmm?

    Much unconscious insight steals hidden into the left hemisphere from the ever watchful right. I think there’s some revision needed here as regards not merely the brain — but the mind.

    If you can read this, yes, indeed thank your left hemisphere. But for the insights, the right hemisphere gets to take a bow.

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  • Jerry Miller

    “Once again, were finding evidence of life on our world coming from the depths of outer space as organic compounds ready to combine into functional organisms and just take it from there.” Seriously? Its just that simple? How? I know, for evolutionists, time is the great hero. Given enough time anything can happen, right? Well, two problems. First, in the beginning we’re not talking about a lot of time, we’re talking about what could not have been a long time, but a relatively short time. The unlikely event of this stuff coming to life occurs and you’d think that would be it, it would be snuffed out. It was lucky to get as far as it did, right? But, no. It also (unintentionally) somehow develops a method of reproducing itself. What an unlikely event following another unlikely event. And all unintentional. Did this happen after repeated accidental “sort of” reproductions, like a spin-offs or something. Otherwise, how would it have survived long enough to develop? If it unintentionally duplicated in some way, even if the second one retained all the properties of the original, its essentially, for all intents and purposes, the very same organism. Its no more “developed” than the original and no closer to unintentionally developing a predictable, directed mechanism of reproducing itself. And if this happened, did it happen repeatedly? How come the material didn’t wear out? Or did this mechanism just fall into place pretty much overnight? Which is easier to believe? It seems to me that these unintentional coincidences are against so many odds–especially in succession–that it doesn’t really make any difference. Its like putting magic in a story. Once you do that, it really doesn’t matter how impossible something seems. And these successive, crucial, convenient theorized coincidences sound like they would have needed magic to have actually occurred. There is not only time to think about. What about the inordinate compression of coincidence we’re expected to swallow? These would have to happen in an unbroken chain. Stalled, delayed maybe, but unbroken, otherwise it would have just been starting over again and again.

    Evolutionists almost universally maintain that no intelligent designer guided evolution, and turn around and use very intentional language like “it adapted” or “this evolved so that”. I realize it sounds like semantics, but I think its actually more than that. I think that to actually speak of evolution consistently (maintaining language consistent with the notion of the supposed unintentional nature of it) would be to admit that its all a house of cards. I realize this is not a direct comment on the subject of this article, I’m just baffled by the simplicity of that statement that I quoted. I realize it is a pithy, cute summary, but the germ of the belief is still there, and I think it ignores the factor of the necessity of an inordinate compression of successive coincidences, in a very small area of the universe. Its magic.

  • Jerry Miller

    And another thing….For those who still look to mutations as the engine of creation. How did the progenitor of all life even develop a mechanism of reproduction, when it was that very mechanism which was to be its means of development? In other words, how did it evolve its evolver? Did it start with another one? Well, if so, how? My what means? I didn’t mean for this to sound so contentious, I just think we need to look at this. I don’t mean to insult anyone.

  • Greg Fish


    Well we’re off on a little Gish Gallop here. The problem with that is that rather than giving the person you’re asking a direct question, you’re flooding the debate with your assumptions and go off on many tangents.

    When you have organic chemicals that will interact with one another, they will when the conditions are right. And we’re talking about a minimum time frame of 500 million years. That’s a pretty long time even if it’s just a little part of the planet’s 4.5 billion year history, not a short stretch you’re implying.

    The idea of “this stuff coming to life” is somewhat vague because there’s really no clear, definitive line between dead and alive in biology. Viruses are often said to be more like a collection of biochemical shards than living things and yet, they’re also one of the most widespread organisms on Earth. Oh and by the way, they’re made from the same organic compounds that would’ve arrived on those meteors.

    All living things are spin offs. In fact, evolution can be thought of as 4 billion years of various spin offs, experiments and branches. Depending on the conditions, most die (over 99 out of 100) and others prosper (less than 1 in 100) so all life we see today is a fraction of a percent of all the biodiversity over the entire history of life. Those duplications you mention are very messy and create countless mutations, enough to start branching out into new species. Evolution is actually a very messy process.

    There was no “progenitor” but most likely, a whole series of chemical reactions which eventually lead to a large pool of potential Universal Common Ancestors. Whatever mutations they underwent and how natural selection played out dictated what life we have today.

    Finally, if you understand some basics of chemistry, it’s really not so difficult to believe that organic compounds which want to fuse into more complex molecules in an open system such as Earth, would eventually combine into life. In fact, it takes a lot more effort to think that there was an eternal magic being stringing carbon atoms to hydrogen atoms and so on, than just let chemical reactions play themselves out.

    The purposeful language about evolution is a very bad habit I’ve written about quite a bit and if you were to ask someone who actually studied evolution with a little but of depth, you wouldn’t get this “design jargon.” It’s a cultural phenomenon which permeates popular science.

  • nornerator

    Jerry, well written but you lack a basic understanding of chemistry, biology, and specifically natural selection.

    “I didnt mean for this to sound so contentious, I just think we need to look at this.” Your claim is dishonest because it implies 1) That we haven’t looked into these things 2) That you have actually studied natural selection, chemistry, biochemistry, biology, etc.

    I can see that this is a thinly veiled attempt to discuss creationism, although I have to admit it was a pleasure reading your comment, you write very well. Your questions display a clear lack of understanding of the basic premises of evolution and natural selection.

    The selfish gene is a good book to read on natural selection, if that is too advanced you may want to consider taking some biology and chemistry courses. Once you get it, you realize how elegantly simple natural selection is and exactly how it works

    Science is looking at the world and saying, “If god didn’t do it, how did it happen?” Lets not mix god with science, we tried that in the dark ages.

  • Very nice comments! I enjoyed reading this.

  • Anonymous

    Amino acids are the building blocks of life, correct. But there are other organic compounds in the human body that have right chiralities, and without them the human body cannot survive. What is the message from this article, that life has a bias to the left? That is not true. Also this article uses a picture of a DNA molecule….normal DNA in the human body is a RIGHT-HANDED helix….author, please get your facts straight before posting an article like this.

  • Avi

    Apart from any undesired lewd/ scatological overtones, I am reminded of a limerick, more as a reminder of the various aspects of Nature, and various alternatives thereto, as under :

    “I deeply grieve the death of my friend Dick,

    He went through life with a corkscrew prick,

    He spent his days on a fruitless whim,

    of trying to find a similar quim;

    Then, he found one; BUT he fell down dead,

    since the damn thing had a left hand thread.”

  • Greg Fish

    “But there are other organic compounds in the human body that have right chiralities, and without them the human body cannot survive.”

    Right, but the article is not about them. It’s about why amino acids wind to the left based on studies of isovaline in carbon rich meteorites. We’re not going into the level of detail that would apply to nucleotides or any other chemical processes of living things. We have a very narrow focus here.

    “What is the message from this article, that life has a bias to the left? That is not true”

    We’re talking about a bias to left-handed chirality in amino acids that are vital for life. Nowhere in the post does it say anything about every single molecule in every living thing being left-handed or anything about molecules other than amino acids. A bias means “skew” rather than “only.”

    However, if you’re trying to say that life actually doesn’t have left-handed amino acids and the NASA study is wrong, please let me know and point me to where I can find a better source. I just figured that NASA astrobiologists should have a good grip on this whole chirality of amino acids thing.

    “Also this article uses a picture of a DNA molecule.normal DNA in the human body is a RIGHT-HANDED helix”

    Actually, there are helixes with different handedness out there and this simple graphic seems to be like a Rorschach inkblot. Some people say that it’s wound to the right, you’re saying that it’s wound to the left. It all seems to depend on how it shows up on your monitor and from where you’re picking a starting point for analyzing the direction of the twist.

    “author, please get your facts straight before posting an article like this.”

    I’ve referenced an official, peer-reviewed NASA study on the subject. I can also provide the announcement page for the study which confirms the basis of the post. All I’ve done was a brief write-up and posit questions about what it could mean for alien life.

    Critiques about nucleotides, other molecules and in which exact direction is the DNA molecule in the image wound, are addressing things that were never in the article to begin with.

  • David

    This is a very interesting read. It’s a shame that the creationism argument has to always rear its head. I wish we could just appreciate the beauty and simplicity (not really) of the science behind it all.

  • Amadan

    The creationist 11th commandment:

    “Thou shalt not use the accursed paragraph.”

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  • Drew Reed

    So is there no indication as to why extra-terrestrial amino acids are biased to the left?

    I would like to suggest too, that instead of the higher prevalence of left handed molecules from space giving an advantage to left handed organisms, doesn’t it make more sense to say that life would simply be more likely to originate from left handed molecules? Maybe this is what you meant, but I do see a difference in the interpretation. Great article!