Better access to chemistry from space keeps changing how we understand life.

How did life start? There may not be a bigger question. To learn the secret of our origins means going back beyond the earliest forms of biological life, past simple bacteria, and down to the chemistry of the building blocks that came earlier.

Most people have heard DNA’s double helix described as the blueprint for life, but its single-stranded relative RNA is also critical for transmitting genetic information. Both are present in the cells of all living organisms, and many scientists suspect that RNA was the original genetic material, coming on the scene before DNA, more than four billion years ago during a period scientists call “RNA world.”

But to build the RNA world, RNA and other biomolecules had to come together in the first place. Their constituent parts have a distinctive chemical property called chirality that’s related to how their atoms are arranged. And a debate has broken out about how life’s chirality got started: is it the product of the chemical environment of the early Earth, or did life inherit its chirality from space?

For some scientists, homing in on how a chain of genetic material was able to come together to start terrestrial life now involves looking away from Earth. One idea being explored in astrobiology is whether some prebiotic organic molecules could have been delivered to Earth by meteorites or dust grains. Recent discoveries in interstellar space may be providing some support for this.

In 2011, NASA published a study of meteorites suggesting that they contain nucleobases, chemicals that are components of both DNA and RNA. Thus, a critical starting material for life may have been seeded to early Earth from space. A year later, a team at the University of Copenhagen reported finding a sugar molecule in interstellar space that can be chemically transformed into ribose—the “R” in RNA. Last year, the same team uncovered a more complex molecule (methyl isocyanate) in a star-forming region more than 400 light years away from Earth.

And in 2016, two postdoctoral researchers, Brett McGuire (National Radio Astronomy Observatory, Virginia) and Brandon Carroll (California Institute of Technology), working with astronomers at the Parkes Observatory in Australia, reported the detection of a molecule in interstellar space, near the center of the Milky Way, that could have distinct consequences for the narrative of terrestrial life.

Where no chiral molecule has gone before

McGuire and Carroll discovered a molecule called propylene oxide (molecular formula: C3H6O) 25,000 light years away from Earth, in a star-forming region of our galaxy called Sagittarius B. But it wasn’t the chemical itself that was surprising; this propylene oxide bears a property that has been associated exclusively with life on Earth.

Propylene oxide is what is known as a “chiral” molecule (pronounced KY-ral, from the Greek word cheir for hand), which means that it comes in two forms: right- and left-handed. Chiral molecules have the same chemical formula, and their structures are nearly identical except for certain atoms that are attached on different sides of the three-dimensional molecule. In the case of propylene oxide, it’s the methyl group (CH3) that can attach to one of two carbons, as shown below.

The two forms of a chiral molecule cannot be superimposed on each other on a level plane, much like when you place one hand on top of the other and a thumb sticks out at either end—the hands are mirror images of each other. The French microbiologist Louis Pasteur discovered this quirk of nature more than 150 years ago.
What he didn’t realize was that he happened upon a fundamental feature of organic matter: as molecules get more complex, chirality is all but guaranteed. While it doesn’t change the number or types of atoms in that molecule, the differences in how those atoms attach can impact a molecule’s function. One example is limonene, the a key component of the scent of citrus fruit. The right-handed version tastes like lemon, while the left-handed one like orange. Ditto for the molecule carvone: in caraway seeds, the left-handed version binds to a receptor in neurons that line the base of your nose that send a signal to your brain telling it that it has smelled rye bread; the right-sided carvone signals your brain that it has smelled spearmint.

Beyond smell and taste, chirality determines the shape of our large-scale biological structures. The famous double helix of a DNA strand twists right, along with the sugars that comprise its backbone; the amino acids in proteins twist left. Despite the fact that these molecules naturally occur in both orientations, all the living organisms on Earth appear to have DNA that is built on the blueprint of it twisting right—perhaps descended from a single right-handed twist in the ancient RNA world.

The enzymes that help our body use amino acids and DNA bases work because they recognize the specific shapes of these molecules. An amino acid with a different chirality would have a different shape, keeping those enzymes from interacting properly with it. If you were served a burger of protein that had right-handed amino acids, your body would not be able to break it down.

This deep bias that permeates all life must have had a beginning. And McGuire and Carroll suggest that their discovery of chiral propylene oxide—as well as the earlier discoveries of methyl isocyanate and glycoaldehyde—shows that space may have had a “hand” in life’s origins.

“This is the first chiral molecule detected in outer space,” said McGuire, who is the Jansky Postdoctoral Fellow with the National Radio Astronomy Observatory. Its detection suggests that a bias toward one form of chirality is not limited to life on Earth, as has been previously thought, and lends evidence to the idea that material from elsewhere in the Solar System—possibly including some much older than Earth or even our Solar System—may have seeded the earliest chemicals necessary to form life on our planet.

Of course, chirality isn’t the only problem you have to solve—the chiral molecules we’ve seen in space are much less complex than most biomolecules.

The molecular puzzle

Ever since the Watson and Crick discovery of the structure of DNA, scientists have wanted to understand how simple atoms combined to form the double helix. Myriad experiments from the 1950s on (with the Miller-Urey being the most famous among them) showed that heating gases that were likely present on early Earth, such as methane, ammonia, and hydrogen, creates a “primordial soup” that includes amino acids, the building blocks of proteins. Followup and related experiments showed that nucleotides (which form the base pairs of DNA and RNA) could also form in similar conditions.

Combined with the later discovery that RNA could catalyze chemical reactions, this paved the way for a chemical theory about the origin of life: the RNA world. Basic chemistry could allow RNA precursors to form and possibly combine to make RNA. Once RNA formed, the biomolecule could catalyze a self-copying reaction to make more of itself. Over time, even more sophisticated chemistry could arise from the pool of self-copying RNA molecules.

It sounded logical, but it hit some roadblocks when scientists began to consider how the pieces fit together. It turned out you couldn’t solve the chemistry without solving chirality, too.

Gerald Joyce, who is currently professor at Salk Institute for Biological Sciences, was a young biochemist in the 1980s when he began to investigate for his doctoral research how nucleobases can come together to form complex biomolecules. In a Nature paper from 1984, he described how he tried to get simple chemical molecules to coalesce into larger biological complexes. Replicating the 1950s experiment, he found that the right catalysts allowed his primordial soup (carbon-, nitrogen-, oxygen-, and hydrogen-containing molecules) to generate the RNA bases adenine, guanine, cytosine, and uracil.

These are all chiral molecules, and both the right- and left-handed versions were present in equal proportion in the “soup.” When they came together to form an RNA molecule, there was no mechanism to establish any consistency in how they attached—right could follow left, left right, and so on. This random assortment meant that nucleotides of different chirality were present on the molecule’s sugar backbone; Joyce found that the addition of further nucleotides was very inefficient. Other scientists who built mathematical models showed that adding a nucleotide of the wrong-handedness stops the chain from extending, confirmed Joyce’s research.

So starting with both right- and left-handed versions (known as enantiomers) of nucleotides didn’t allow complex RNA molecules to form. “Without homochirality we would not have complex biological structures,“ said Donna Blackmond, a biochemist at Scripps Institute. (Blackmond was not involved in the CalTech discovery or in Joyce’s research.)

Blackmond thinks that both versions of chiral molecules were present on ancient Earth, but at some point a bias toward one form had to creep in, creating what’s referred to as enantiomeric excess. “Symmetry had to be broken, and we had to have some significant enantiomeric excess before prebiotic reactions could start,” she told Ars. She suspects that excess of one enantiomer over another could, over time, weed out the unneeded enantiomer and establish “homochirality” (only one kind of enantiomer) for certain key molecules.

Even if you have a tiny majority of one enantiomer, lab experiments have shown that it can lead to a much higher excess. The first experiment to show this dates to the late 1950s. Kenso Soia in Japan found that enantiomers can favor their own production. If both the right- and left-hand versions are copied independently, then the amount of both kinds would grow. Soia showed that the enantiomers tend to pair up in both same type and different type units, and the two aren’t copied at equal rates. When a right-handed form pairs up with a left-handed one, they become inactive and stop replicating.

Through this process, a small excess of one enantiomer can grow into a much larger surplus. More recent mathematical models have supported Soia's idea. Over time, the amplification process can result in the dominance of one chirality and eventually perhaps lead to the minority one disappearing. When biomolecules form from the resulting mix, they would condense with an overwhelming majority of their parts having a single chirality. Self replication could then establish a population with just a single chirality, which makes the building of an RNA strand more efficient.

Laurence Barron, a chemist at the University of Glasgow, (he occupies the chair once held by Lord Kelvin of absolute zero fame) says that if you start out with uneven amounts of different chiral forms, it is necessary to have some sort of amplification process to allow a small majority of one enantiomer to grow into a much wider margin. This has been shown to work in laboratory through autocatalysis.

Detailed dissent

But there’s disagreement about the details of how an excess of one enantiomer grew and where it might have happened.

McGuire and Carroll say most scientists have assumed that “homochirality doesn’t occur in space, and is a hallmark of life, and [is] therefore something that must have its origins on Earth.” But they speculate about a possible alternative: “chirality in space could have kickstarted a homochiral process on Earth.”

They suggest that space could have provided the initial impetus through light. As light travels, its oscillating electric and magnetic fields can trace a corkscrew, called circular polarization. Polarized light interacts with different enantiomers differently. Barron explained that circularly polarized ultraviolet light (known as UV–CPL) will decompose enantiomers that rotate the same way the light does, at least in the lab. This results in an abundance of molecules with the opposite chirality. So if a cloud of gas in space that contained both right- and left-handed molecules met a ray of polarized light, the mixture could have ended up having a majority of one enantiomer.

The timing on that is flexible. Barron notes that, while some might think that enantiomeric enrichment would occur as the young Earth cooled, the process could also have been happening for billions of years before the planet was formed, with chiral chemicals later delivered to the early Earth by comets.



Some evidence for this comes from a meteorite that was found in Murchison, Australia in 1969. We don’t know the precise origins of meteorites, which are thought to have formed very early in the Solar System’s history. Having them come back to Earth is like a time capsule that can give us a peek into the chemistry that existed then. If meteors have enantiomeric excesses, it would support the idea that homochirality arrived on Earth via molecules from space.

Samples from the Murchison meteorite show that amino acids had a 10 percent surplus of the left-handed enantiomers. While not a strong excess, it’s clear that space can have an enrichment of one type of chirality.

“This turns out to be a small effect, but over time it can add up,” McGuire said.

And University of Glasgow’s Barron added that “UV-CPL is not common in the cosmos, but it has been detected, inter alia, in star-formation regions.”

Stefanie Milam, an astrochemist at NASA, said the discovery of chiral propylene oxide in space was exciting because it supports the possibility that the “bias happened independent of biological processes.” Space could have given Earth a head start. “If a meteorite seeds a planet with water and chiral molecules, then you’re starting with sophisticated chemistry,” she said.

An expert on the star-forming region where propylene oxide was found, called Sagittarius B2, Milam says it’s 25,000 light years away from Earth and located in the middle of our galaxy. It’s a region where stars are still forming, very much what our Solar System’s neighborhood may have looked like 4.6 billion years ago. Looking at Sagittarius B2 is “like looking back in time, at what early Earth would have looked like, too,” Carroll added.

“Looking” might be a bit optimistic, though. Scientists cannot actually “see” molecules like propylene oxide or other molecules found there, like glycoaldehyde or methyl isocyanate (reported by the Copenhagen team). ALMA (Atacama Large Millimeter/submillimeter Array) in Chile (used by the Copenhagen team) and the Green Bank Telescope in West Virginia, where McGuire is a postdoctoral fellow, are highly sensitive radio telescopes. They do not obtain colorful images of distant galaxies like the Hubble; these telescopes pick up photons at frequencies emitted or absorbed by molecules, telling us what’s there. But there are limits to what we can learn; the larger the molecule, the fainter the signal, and their detection via radio astronomy becomes more difficult.

“Propylene oxide is one of the simplest chiral molecules we can find,” McGuire explained. “It is possible that there are many more chiral molecules in space, but it’ll have to await our ability to resolve their presence.” The Green Bank data also wasn’t able to tell McGuire and Carroll whether the propylene oxide was a right- or left-handed enantiomer.

That level of detail may have to wait until actual molecules are retrieved from space and analyzed. The most promising possibilities come from planned NASA space probes that will attach themselves to bodies such as comets. Comets are chunks of planetary bodies that formed at the birth of the Solar System and, consequently, contain the chemistry as it existed then—Milam calls them “pristine relics of when the Sun actually formed" that "have volatile material on them.” It’s also possible that comets from exosolar systems got pulled into our Sun’s gravitational field. They can carry information about chemistry that is happening in another solar system.

An examination of molecules in outer space was part of the recent Rosetta mission. Rosetta followed comet 67P (GG), which is thought to be 4.6 billion years old—the same age as our Solar System. Rosetta sent back mass spectrometry data showing that glycine, the simplest amino acid, was present on the comet. Although glycine is not chiral, it’s possible that other probes will find more complex molecules. Osiris Rex is due to retrieve sample material from asteroid Bennu in 2018, the first time that we are due to receive actual material from an asteroid without having it land on earth first and risk contamination.

Jason Dworkin of NASA, who studies meteors (which are meteorites that have not yet reached Earth’s atmosphere and therefore are not contaminated with earthly matter), said that over the past decade technology has improved so much, we can have confidence that any chiral molecules we find there are truly from space.

“Having access to sample material from an asteroid would allow us to see the kind of complex chemistry that was available on early Earth,” Dworkin said.

And it would also help scientists pinpoint other places that life may still start. Our galaxy, the Milky Way, is filled with many exosolar systems, each with planets orbiting a star. Some might enjoy the same Goldilocks conditions that made life on Earth possible, like liquid water and moderate radiation. Other regions in our galaxy are actively forming new stars and can provide models for the early Solar System. These include IRAS 16293-2422, a star 400 light years away from Earth where the glycoaldehyde and methyl isocyanate were detected, and Sagittarius B2, a star-forming region 25,000 light years away that we mentioned above. In addition to being where the propylene oxide molecule was detected, it contains billions of liters of alcohol.

While our imaging capabilities aren’t yet at the point where we can tell for certain, it’s possible that some newly forming systems recreate the environment of early Earth—a place where life is able to originate. (Some have termed these “originable zones.”) Studying the molecules present in these zones can help us understand what happened on Earth.

McGuire and Carroll are hopeful that the next few years will give us access to more chemistry from outer space, such as amino acids that can give more certain information about the provenance of life. Hopefully, this will only shed more light on how homochirality developed—or even change our notion of its terrestrial origins.