If Life Went Widdershins

In 1848, Louis Pasteur looked at tiny grains of a salt under a magnifying glass and was able to distinguish two crystal forms. They looked almost identical – but not quite. The two crystals had a symmetry of reflection: they were ‘in relation to each other what an image is, in a mirror, in relation to the real thing’.

It’s been suggested that Pasteur was alert to this subtle symmetry because of his experience as an artist. As a teenager he made lithographs, in which an image etched into a stone plate is transferred onto paper, creating a mirrored version. (At least once he used a mirror to make sure a portrait would be acceptable when inverted.) The visible difference in the crystals, Pasteur reasoned, arose from an invisible difference in their molecules.

Many molecules can exist in one of two reflected ways, each a strange twin of the other. The symmetry is similar to the way a right hand and a left hand resemble a mirror image of each other but can never be superimposed. That ‘handedness’ is known as a molecule’s chirality (from the Greek word for hand). Not all molecules are chiral: water and methanol, for example, are both symmetrical.

Many of the key molecules of life, however, exist only in one chirality and assemble into structures that reveal this distinction, just like Pasteur’s crystals, in one of the most compelling pieces of evidence for the shared origins of all life on earth. If we travelled through the looking glass into mirror world, we would be able to tell.

The double helix of DNA, for instance, is right-handed. (Imagine it as a spiral staircase where walking down the staircase means turning right.) In mirror world, it would be left-handed. The flipped version doesn’t occur in reality, although images of it crop up in everything from popular science books to corporate logos. A biologist at the National Institutes of Health in Maryland maintains a hall of fame for egregious images of impossible left-handed DNA, including a cover of Nature Biotechnology last year.

In mirror world, your mirrored self would have mirror cells containing mirror DNA, coiled up into left-handed helices. But we needn’t travel into mirror world to get mirror life. In principle, there seems to be nothing in the laws of physics or chemistry that would prevent us from constructing mirror organisms. We could apply Pasteur’s reasoning in reverse, starting from mirrored biomolecules to build up to creatures that worked widdershins.

It would be very difficult. But making bizarre bacteria has become a staple of biotechnology, from recoding their genomes to turning one species into another. From this perspective, a mirror bacterium is an attractive challenge. Indeed, it’s an idea that has already received funding: in 2019, the National Science Foundation in the US gave $4 million to a research group who hoped ‘to design, construct and safely deploy mirror cells’. Other donors have given money to similar efforts. None has yet succeeded, and the suite of technological developments it would require are probably decades away. But as an international consortium of scientists outlined in a technical report and an article published in Science last week, there are good reasons to pause before rushing through the looking-glass.

We know that mirror molecules lead to differences that aren’t simply visual. They can be trivial: one molecule smells like caraway while its mirror form smells like spearmint. But they can also be fatal: the drug thalidomide exists in two forms – one is a mild sedative but the other causes severe birth defects. We can’t be certain what would happen with a mirror bacterium, but it would probably be closer to the thalidomide end of the spectrum of mirror dangers.

A mirror bacterium wouldn’t need mirror food. It would be able to subsist on achiral molecules such as water and glycerol – and human bodies contain plenty of those. But the worrying asymmetry is that our immune system is chiral, because it has evolved to target real bacteria. As the report says, many immune mechanisms therefore ‘appear unlikely to function properly against mirror bacteria’. A mirror bacterium could slip through the net.

It would also be unaffected by current vaccines and most antibiotics. We could probably create new ones, but that would take time, and they in turn might interact problematically with our own cells. A mirror bacterium wouldn’t need to be actively pathogenic to kill us: it could do so simply by growing unchecked.

And not only us. While normal life must compete with a whole ecosystem, a mirror bacterium might behave like the only real thing in a world of phantom reflections. Normal organisms are kept in check by an ecological balance between their death and growth rate. But without any predators, mirror bacteria that escaped a laboratory might grow exponentially, even with a lower growth rate than normal bacteria.

The technical report is necessarily speculative about how an as yet uncreated entity would interact with the world we know. But, over two hundred pages, it goes into plausible and concerning detail, with chapters not only on human infection but on infections in animals and plants, as well as on the difficulty of containment. Before reading it, I had never considered many of these risks. Neither had many of the scientists who wrote it, who confess that ‘several authors of this report have long considered the possibility of creating mirror cells.’

It’s clear they have changed their minds. Now, they think that mirror organisms should never be created. ‘Although we were initially sceptical that mirror bacteria could pose major risks,’ they write, ‘we have become deeply concerned.’ These words echo those of the scientists on the MAUD Committee who wrote a report in 1941 on the possibility of an atomic bomb: ‘We entered this project with more scepticism than belief … as we proceeded we became more and more convinced.’

Creating mirror life may remain impossible for unknown reasons. But in the next few decades, it is at least conceivable that we could get there if we tried hard enough. The report is a compelling argument in favour of stopping those efforts. For more than four billion years, there has been no mirror life. The authors refer to its dangers as ‘unprecedented’. An overused word – but here literally true.