Must the Molecules of Life Always be Left-Handed or Right-Handed?
They are on Earth, but life on other planets could play by different rules
One of the strangest aspects of life on Earth—and possibly of life elsewhere in the cosmos—is a feature that puzzles chemists, biologists and theoretical physicists alike. Each of life’s molecular building blocks (amino acids and sugars) has a twin—not an identical one, but a mirror image. Just like your right hand mirrors your left but will never fit comfortably into a left-handed glove, amino acids and sugars come in both right and left versions. This phenomenon of biological shape selection is called “chirality”—from the Greek for handedness.
On Earth, the amino acids characteristic of life are all “left-handed” in shape, and cannot be exchanged for their right-handed doppelgänger. Meanwhile, all sugars characteristic of life on Earth are “right-handed.” The opposite hands for both amino acids and sugars exist in the universe, but they just aren’t utilized by any known biological life form. (Some bacteria can actually convert right-handed amino acids into the left-handed version, but they can’t use the right-handed ones as is.) In other words, both sugars and amino acids on Earth are homochiral: one-handed.
More than 4 billion years ago, when our home planet was in its fiery and temperamental youth, both the biological building blocks and their mirror reflections were present. In fact, both still coexist on Earth today—just not in life as we know it. Certainly, if you cook up a batch of amino acids, sugars or their precursor molecules in a laboratory, you’ll always get a 50-50 mixture of left and right. But somehow, as life emerged in the countless millennia that followed the Earth’s formation, only the left-handed amino acids and the right-handed sugars were selected.
Chiral molecules have even been found in interstellar space. In a landmark discovery announced by the National Radio Astronomy Observatory this June, scientists identified molecules at the center of the galaxy that could be used to construct either the right- and left-handed sugars. While they still have no clue whether there are more of one hand than the other, the finding sets the stage for further experiments that could illuminate more about the origins of handedness.
The big questions still remain: How and why did life choose only one of two mirror reflections to construct every single creature in her menagerie? Does life require homochirality to get its start, or could life forms exist that use both the earthly building blocks and their alter egos? Did the seeds of homochirality originate in the depths of interstellar space, or did they evolve here on Earth?
Jason Dworkin, who heads the Astrochemistry Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Maryland says that one challenge for scientists attempting to answer these questions is that “the early Earth is gone, and we have a string of very, very scant evidence of what it was like.” Four-or-so billion years of volcanic eruptions, earthquakes, meteor bombardments and, of course, the profound geological influence of life itself have so transformed the planet that it is nearly impossible to know how the Earth looked when life began. That is why Dworkin’s research group and many of his colleagues at NASA focus on meteorites—the remnants of space debris that find their way down to solid ground.
“These are time capsules from 4.5 billion years ago,” says Dworkin. “So what we collect in meteorites now is very similar to what was raining down on the Earth then.”
Dworkin is also the lead government scientist on the OSIRIS-REx mission to the near-earth asteroid, Bennu. The mission, which launches this September, will spend around a year taking measurements of the asteroid to better understand how it moves through our solar system. When the spacecraft’s time with Bennu is up, it will collect the ultimate prize: a sample from the surface of the asteroid, which it will bring it back to the Earth in the year 2023 so that scientists can study its chemical composition. “Everything we do supports getting that one sample,” says Dworkin.
The scientists chose Bennu in part because of its resemblance to a special type of meteorite that provides an intriguing (though by no means conclusive) clue to the origins of homochirality. Many meteorites contain carbon-based molecules including amino acids and sugars, which are just the right ingredients for life. Dworkin’s group analyzed the composition of these “organic” compounds in dozens of meteorites, and came to a surprising conclusion. Oftentimes both the left- and right-handed versions of, for example, an amino acid, were found in equal amounts—exactly what might be expected. But in many cases, one or more organic molecule was found with an excess of one hand, sometimes a very large excess. In each of those cases, and in every meteorite studied so far by other researchers in the field, the molecule in excess was the left-handed amino acid that is found exclusively in life on Earth.
Dworkin says that the sample from Bennu may provide even stronger evidence of this phenomenon. “Unlike meteorites, which, one, fall on the ground and then get contaminated, and, two, are separate from their parent body,” with Bennu, the scientists will know exactly where on the asteroid the sample came from. They are taking “extraordinary measures” confirm that nothing from Earth’s biology can contaminate the sample. “So when we get these (hopefully) excesses of amino acids on the Bennu sample in 2023, we can be confident that it’s not from contamination,” Dworkin says.
The evidence thus far from meteorites implies that perhaps there is a means of producing homochirality without life. However, Dworkin says, “We don’t know if the chemistry that lead to homochirality and life came from meteorites, from processes on the earth, or perhaps from both.” There is also still the question of how and why that excess developed in the meteorite or its asteroid parent or on the early Earth in the first place.
Hypotheses abound. For example, polarized light found on our side of the galaxy can destroy the right-handed version of many amino acids by a small, but noticeable amount. The slight excess of the left-handed amino acid, would then have to be drastically amplified to get to the levels found in living organisms on Earth.
It is this amplification process that intrigues Donna Blackmond of the Scripps Research Institute in La Jolla, California. Blackmond has been studying the potential chemicalorigins of homochirality for nearly her entire career. “I think it’s going to be some combination of chemical and physical processes,” she says. Blackmond’s group is currently trying to discover how chemical reactions that could have taken place on the early Earth may have been swayed to produce only life’s building blocks. In 2006, her team showed that they could amplify only the left-handed form of an amino acid starting from a small excess. In 2011, they showed that the amplified amino acid could then be used to produce a huge excess of a precursor to RNA, which is made right-handed by a sugar that is attached to it. (RNA is thought by many scientists to be the original biological molecule.) Blackmond and many other chemists have made strides in this type of chemistry, but they are still a long way from being able to model all of the chemistries and conditions that might exist on an asteroid or a juvenile planet.
Blackmond also notes that it’s far from clear that life needed total homochirality in order to get its start. “One real extreme would be to say that nothing could ever happen until we have a completely homochiral pool of building blocks, and I think that’s probably too extreme,” she says. “We could start making information type polymers”—like DNA and RNA—“possibly before we had homochirality.” For now, all scientists can do is keep asking questions about molecules here on Earth and on the celestial bodies that surround us. In the hopes of unlocking one more piece of this puzzle, researchers are now developing new technologies to determine if there are excesses of one hand in interstellar space.
In the meantime, life on Earth will continue, mysterious and asymmetric as ever.