Mirror-image life

Mirror-image life (also called mirror life) is a hypothetical form of life using mirror-reflected molecular building blocks.^[1] This alternative life form has never been discovered in nature, although certain mirror-image components of molecular machinery have been synthesized in the laboratory and efforts to chemically synthesize a mirror-image ribosome have been ongoing since 2016.^[2][3] In principle, entire mirror organisms could be created,^[4][5] although "the creation of a mirror-image organism lies well beyond the reach of present-day science".^[6]

Concept

Homochirality

Many of the essential molecules for life on Earth can exist in two mirror-image forms, often called "left-handed" and "right-handed", where handedness refers to the direction in which polarized light skews when beamed through a pure solution of the molecule, but living organisms do not use both.^[7] RNA and DNA contain only right-handed sugars; proteins made by the ribosome^[a] are exclusively composed of left-handed amino acids. This phenomenon is known as homochirality.^[8] It is not known whether homochirality emerged before or after life, whether the building blocks of life must have this particular chirality, or indeed whether life needs to be homochiral.^[9] Protein chains built from amino acids of mixed chirality tend not to fold or function well, but mirror-image proteins have been constructed that have identical function but on substrates of opposite handedness.^[8]

Possibility of mirror-image life

The possibility of mirror-image life has been discussed since Louis Pasteur's 1860 work on molecular asymmetry.^[10][11]

Advances in organic chemistry and synthetic biology may, in the distant future, lead to the possibility of fully synthesizing a living cell from small molecules, which could enable synthesizing mirror-image cells from mirrored versions (enantiomers) of life's building-block molecules. Some important proteins in the central dogma of molecular biology have been synthesized in mirror-image versions, including polymerase in 2016.^[3]

Reconstructing regular lifeforms in mirror-image form, using the mirror-image (chiral) reflection of their cellular components, could be achieved by substituting left-handed amino acids with right-handed ones, in order to create mirror reflections of proteins, and likewise substituting right-handed with left-handed nucleic acids.^[12] Because the phospholipids of cell membranes are also chiral, American geneticist George Church proposed using an achiral fatty acid instead of mirror-image phospholipids for the membrane.^[12][b]

Electromagnetic force (chemistry) is unchanged under such molecular reflection transformation (P-symmetry). There is a small alteration of weak interactions under reflection, which can produce very small corrections that theoretically favor the natural enantiomers of amino acids and sugars,^[15] but it is unknown if this effect is large enough to affect the functionality of mirror-image biomolecules or explain homochirality in nature.^[16]

Potential risks and debates

In December 2024, 38 scientists, including several synthetic biology researchers and two Nobel laureates, warned that the creation of mirror-image life could cause "unprecedented and irreversible harm" to human health and ecosystems worldwide.^[17][18] The potential for mirror-image bacteria to escape immune defenses and invade natural ecosystems might lead to "pervasive lethal infections in a substantial fraction of plant and animal species, including humans." Given these risks, the scientists concluded that mirror-image organisms should not be created without compelling evidence of safety.^[17]

In a Science News story,^[19] Andrew Ellington of the University of Texas at Austin criticizes the above article: "I think it's irresponsible for [the authors] to make this policy call. It's like banning the transistor because you're worried about cybercrime 30 years later." He also argues that it remains uncertain whether mirror-image organisms would ever pose a significant threat. Biosecurity expert Gigi Gronvall of Johns Hopkins University describes the concerns raised in the paper as "very theoretical". While supportive of open discussions about potential risks, she contends that research and funding bans are premature: "That really puts the cart before the horse."

In a Nature News story,^[20] Sven Klussmann of Aptarion Biotech, a company that develops mirror-image nucleic acid drugs, says: "we should not panic yet, and we should not restrict research too early." David Van Valen of the California Institute of Technology and founder of Aizen Therapeutics, a company that develops mirror-image peptide therapies, says: "I think most of the concerns that people are raising are overblown."

In a Nature Comment piece titled "Mirror of the unknown",^[6] Ting Zhu of Westlake University, one of the leading scientists in mirror-image molecular biology, notes that "all biological structures, functions and even organisms could be recreated in their mirror image, the possibilities — good and bad — in a looking-glass world are endless". He seeks to bridge divergent views amid growing debates: "In the face of vast unknowns, the noble path of pre-emptively protecting humanity from potential risks in the distant future can be slippery. And we should tread cautiously." Zhu emphasises: "It is crucial to distinguish mirror-image molecular biology from the creation of mirror-image organisms", and proposes: "Holistic guidelines could be developed for research on synthetic or semi-synthetic molecules, biological entities and modified organisms — irrespective of their chirality." He adds: "Scientific exploration is not a glorious march towards increasingly precise understandings of a universal truth. It has a long and difficult history of trials and errors, uncertainties and risks, controversies and doubts. Yet through rational dialogue and objective analysis, a responsible, open and rich human adventure can be charted, for the world of the unknown is infinite."^[6]

Direct applications

Direct application of mirror-image organisms could be mass production of enantiomers (mirror-images) of molecules produced by normal life.

• Enantiopure drugs: Some pharmaceuticals have shown different activity depending on enantiomeric form.^[21]
• Aptamers (L-ribonucleic acid aptamers): "That makes mirror-image biochemistry a potentially lucrative business. One company that hopes so is Noxxon Pharma in Berlin. It uses laborious chemical synthesis to make mirror-image forms of short strands of DNA or RNA called aptamers, which bind to therapeutic targets such as proteins in the body to block their activity. The firm has several mirror-aptamer candidates in human trials for diseases including cancer; the idea is that their efficacy might be improved because they aren't degraded by the body's enzymes. A process to replicate mirror-image DNA could offer a much easier route to making the aptamers, says Sven Klussmann, Noxxon Pharma's chief scientific officer."^[22]
• L-glucose, enantiomer of standard glucose: Tests showed that it tastes likes standard sugar, but is not metabolized the same way. However, it was never marketed due to excessive manufacturing costs.^[23] More recent research allows cheap production with high yields; however the authors state that it is not usable as a sweetener due to laxative effects.^[24]

In fiction

The creation of a mirror-image human is the basis of the 1950 short story "Technical Error" by Arthur C. Clarke.^[25] In this story, a physical accident transforms a person into his mirror image, speculatively explained by travel through a fourth physical dimension. H. G. Wells' The Plattner Story (1896) is based on a similar idea.

In the 1970 Star Trek novel Spock Must Die! by James Blish, the science officer of the USS Enterprise is replicated in mirror-image form by a transporter mishap. He locks himself in the sick bay where he is able to synthesize mirror-image forms of basic nutrients needed for his survival.^[26]

An alien machine that reverses chirality, and a blood-symbiont that functions properly only when in one chirality, were central to Roger Zelazny's 1976 novel Doorways in the Sand.^[27]

On the titular planet of Sheri S. Tepper's 1989 novel Grass, some lifeforms have evolved to use the right-handed isomer of alanine.^[28]

In the Mass Effect series, chirality of amino acids in foodstuffs is discussed often in both dialogue and encyclopedia files.

In the 2014 science fiction novel Cibola Burn by James S. A. Corey, the planet Ilus has indigenous life with partially-mirrored chirality. This renders human colonists unable to digest native flora and fauna, and greatly complicates conventional farming. Consequently, the colonists have to rely upon hydroponic farming and food importation.^[29]

In the 2017 Daniel Suarez novel Change Agent, an antagonist, Otto, nicknamed the "Mirror Man", is revealed to be a genetically engineered mirror-image human. Serving as an assassin due to his complete immunity to neurotoxins, which he coats himself with in the form of a cologne-like aerosol, he views other humans with disdain and causes them to feel an inexplicable repulsion by his very presence.^[30]

The concept is used during Ryan North's 2023 run on Fantastic Four as an existential threat towards the human population.^[31]

See also

• Xenobiology
• Mirror matter – A hypothetical form of matter that interacts only weakly with normal matter, which could form mirror planets, potentially inhabited by mirror matter life.
• Shadow biosphere
• Peptidomimetic § D-peptides

Notes

1. Many bacteria and fungi are able to synthesise non-ribosomal peptides containing right-handed amino acids, as the example of peptidoglycan synthesis shows.
2. An achiral version of phospholipids is not strictly required, as both chiralities of phospholipids are already used in the cell membrane of existing life forms: eukaryotes and bacteria use one chirality (G3P) while archaea use the other (G1P). The two have even been mixed using genetic engineering, producing viable modified E. coli.^[13] Genetic evidence for a natural mixed-membrane system have also been found, pending definitive proof by chemical analysis.^[14]