Mirror, Mirror, On the Wall

The concept of chirality – the existence of molecules that are chemically identical but mirror images of each other – is often taught in introductory chemistry courses using hands as an example. We have a left hand and a right hand that are exact copies of each other, yet can never be perfectly aligned unless flipped, as in a mirror. Chiral opposites, called enantiomers, function similarly, and scientists even refer to them as “right-handed” and “left-handed” forms of a molecule. Just like your left glove doesn’t fit properly on your right hand, a left-handed molecule won’t fit into an enzyme that normally catalyzes its mirror image. And chiral molecules aren’t just a novelty to fill space in a chemistry textbook. No, they’re everywhere: in medicine, food, and in the very molecules that give us life. RNA Polymerase specifically recognizes only our chiral DNA, transcribes it into chiral RNA, which is then translated into a string of amino acids that are, you guessed it, chiral.

Our bodies utilize exactly one enantiomer of these building blocks each: right-handed sugars in DNA and RNA, and left-handed amino acids in proteins. As far as we can tell, all life that has ever existed on Earth has used the same enantiomers, with no exceptions. When a trait is conserved across multiple species, it’s usually a sign that it’s highly beneficial for survival. A trait that is conserved between every species to have ever existed, then, must be entirely essential for functional life on this planet. Interestingly, though, the “unchangeable trait” here likely has nothing to do with which enantiomer we use. Rather, it’s probably just important that we picked only one and stuck with it. That means, in theory, life could exist that functions perfectly using the exact mirror image of our building blocks: so-called mirror life, based on left-handed nucleic acids and right-handed amino acids.

Of course, mirror life does not exist, and it would be highly unlikely (bordering on impossible1) for it to arise through the evolution of “normal” life. Because an organism requires all its building blocks to share the same handedness, every single one would need to flip simultaneously to yield viable life. Rapid, drastic change all at once wasn’t included in Darwin’s theory of evolution. This means that the only way we could encounter such life is if we made it ourselves, but that, too, would be a monumental biological feat, unlike anything done before. 

If mirror life is so unlikely to evolve, and making it synthetically would be herculean, why am I wasting your precious doom scrolling time?

While the concept isn’t new, it’s been in the news recently due to a letter and technical report released by a consortium of scientists warning of potential “unprecedented and irreversible harm” to normal life if mirror life is developed. The letter calls for halting any research that aims to create such organisms until their potential effects are better understood.

Oh. That’s why. 

But how could simply flipping the orientation of our molecules cause widespread destruction? These experts aren’t warning about the dangers of our hypothetical evil mirror people, either. They’re talking about mirror bacteria. Teeny, tiny, potentially world-altering mirror bacteria. 

The argument goes like this: mirror bacteria would act as an invasive species with no natural predators – anywhere on earth. Our immune systems detect microbial invaders by recognizing surface proteins; if those proteins are all mirror images, they might go undetected. Bacteriophages, the viruses that prey on bacteria, also wouldn’t be able to infiltrate past the cell wall and couldn’t commandeer the mirror bacteria’s replication machinery if they did. Even antibiotics aren’t guaranteed to work against mirror bacteria, since many of them rely on specific binding to non-mirrored proteins. So, not great for us.

Owing, in part, to the necessarily theoretical nature of the topic, the letter drew a number of critical responses. Some argued that aspects of mirror life that might make it less dangerous weren’t given enough weight; others stopped just short of chastising the authors for fearmongering. David Perrin, a Professor at the University of British Columbia Chemistry Department, brings up a number of reasons he believes we should be far more concerned about the pathogens we currently have than a hypothetical mirror bacterium2. For instance, mirror virulence factors may not actually be harmful to humans, and mirrored life would also need mirror nutrients which don’t occur naturally, turning our global pandemic into a mere localized fizzle.

So how dangerous would this hypothetical species really be then? That’s kind of the point: we have no idea. And we probably shouldn’t go around trying to make it until we better understand how it would interact with normal life. Particularly, how our immune systems would handle these novel invaders, and how well mirror life could proliferate using achiral (neither left- nor right-handed) nutrient sources3.

We’re still a way off from mirror life being feasible to create in a lab, but synthetic biology has advanced leaps and bounds in the last decade. And although there isn’t a ton of active work specifically to create mirror organisms4, there are efforts to synthesize mirror nucleic acid strands and proteins5. Once an entirely synthetic cell has been constructed, and we have a wider knowledge of synthesizing mirror proteins, combining the two could be comparatively easy. 

The good news is that there doesn’t seem to be much current appetite for generating mirror lifeforms outside of pure scientific curiosity, and we can make useful mirror molecules without building mirror organisms. The bad news is that it’s easy to imagine a future where 1) synthetic cells are cheap to make and 2) mirror biologics are approved but expensive, creating pressure to use mirror life as a cost-effective manufacturing system.

Despite fears of alarmism, this feels like the scientific community working as it should: a group of experts flagging potential risks and calling for caution before there’s a real problem. Raising the concern doesn’t mean it needs to keep my mother awake at night6, it just means the right people are thinking about it. I don’t worry about the structural integrity of every building I enter, but I’m glad someone does.

Discussions about hypothetical dangers always tend to feel outlandish and too early – until suddenly they’re not.