How did life begin? There may not be a greater demand. To learn the secret of our origin, we must go beyond the earliest forms of biological life, beyond simple bacteria, and go back to the chemistry of the building blocks that came before.
Most people have heard that the double helix of DNA is described as the blueprint for life, but the single-stranded relative RNA is also critical for the transmission of genetic information. Both are present in the cells of all living organisms, and many scientists suspect that RNA was the original genetic material that appeared on the scene before DNA more than four billion years ago during a period scientists call “RNA world.”
But to build the RNA world, first of all, RNA and other biomolecules had to come together. Their constituent parts have a distinctive chemical property called chirality, which is related to how their atoms are arranged. And a debate has erupted about how life’s chirality originated: 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 begin earthly life, now it involves looking away from the earth. One idea being explored in astrobiology is whether some prebiotic organic molecules may have been delivered to Earth from meteorites or dust grains. Recent discoveries in interstellar space may provide some support for this.
In 2011, NASA published a study of meteorites suggesting they contain nucleobases, chemicals that are components of both DNA and RNA. So it’s possible that a crucial resource for life on the early Earth may have been seeded from space. A year later, a team from the University of Copenhagen reported that it had found a sugar molecule in interstellar space that can be chemically converted into ribose — the “R” in RNA. Last year, the same team discovered 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 from the Parkes Observatory in Australia, reported the detection of a molecule in interstellar space, near the center of the Milky Way, which could have obvious implications for the story 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 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 has a property associated exclusively with life on Earth.
Propylene oxide is what is known as a “chiral” molecule (pronounced KY-ral, from the Greek word cheir apparent for hand), meaning it comes in two forms: right-handed and left-handed. Chiral molecules have the same chemical formula and their structures are almost identical except for certain atoms attached to different sides of the three-dimensional molecule. In the case of propylene oxide, it is the methyl group (CH3) that can attach to one of the two carbon atoms, as shown below.
The two shapes of a chiral molecule cannot be superimposed on a flat plane, much like when you put one hand on the other and stick out a thumb at each end – the hands are mirror images of each other. French microbiologist Louis Pasteur discovered this quirk of nature more than 150 years ago.
What he failed to realize was that he encountered a fundamental feature of organic matter: as molecules become more complex, chirality is virtually guaranteed. While it doesn’t change the number or types of atoms in that molecule, the differences in how those atoms attach can affect a molecule’s function. An example is limonene, an important component of citrus fruit fragrance. The right-handed version tastes like lemon, while the left-handed tastes like orange. Ditto for the carvone molecule: in caraway seed, the left-handed version binds to a receptor in neurons along the base of your nose that send a signal to your brain that the rye bread has smelled; the carvone on the right signals your brain that it smelled like spearmint.
In addition to smell and taste, chirality determines the shape of our large-scale biological structures. The famous double helix of a DNA strand twists to the right, along with the sugars that make up its backbone; the amino acids in proteins rotate to the left. Despite the fact that these molecules occur naturally in both directions, all living organisms on Earth seem to have DNA built on the blueprint of right-spinning — perhaps descending from a single right-handed twist in the ancient RNA world.
The enzymes that help our bodies 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, preventing those enzymes from interacting well with it. If you were given a burger of protein with right-handed amino acids, your body wouldn’t 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 — show that space may have had a “hand” in the origin of life.
“This is the first chiral molecule detected in space,” said McGuire, the Jansky Postdoctoral Fellow at the National Radio Astronomy Observatory. Its detection suggests that a preference for one form of chirality is not limited to life on Earth, as previously thought, and proves 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 needed to form life on our planet.
Of course, chirality isn’t the only problem to solve – the chiral molecules we’ve seen in space are much less complex than most biomolecules.