Why Life “Turned Left”: NASA Scientists Seek Answers

When NASA successfully launched a spacecraft to explore an asteroid, scientists eagerly anticipated the opportunity to analyze samples of this space rock in a laboratory setting, hoping it would shed light on some of humanity’s most profound questions about the origins of life.

For Danny Glavin, a senior sample scientist, the mission represented a chance to unravel a long-standing enigma in his career: Why are all known living organisms composed exclusively of left-handed forms of amino acids, the essential building blocks of proteins? This question has perplexed scientists for years, and Glavin was determined to find answers.

His long-awaited opportunity arrived nearly a decade later. Glavin and a dedicated team of researchers meticulously analyzed the gritty samples collected from Bennu, a carbon-rich asteroid characterized by its loosely bound boulders. However, their findings were unexpected and challenging. Instead of supporting the prevailing theory—that the early solar system favored left-handed amino acids and delivered them to primitive Earth—the results indicated a complete lack of preference for either form.

“I must admit, I felt somewhat disillusioned and disappointed,” Glavin reflected. “The results seemed to undermine two decades of research conducted in our lab and throughout my career.”

Amino acids, whether they play a role in biological processes or not, exist in two mirror-image configurations. Each molecule features a central carbon atom bonded to various atom groups, which can be oriented in one direction or its reverse. This unique property, known as chirality, resembles the distinction between left and right hands: while they are fundamentally similar, if stacked, their thumbs would point in opposite directions. Understanding this distinction is crucial for grasping the molecular basis of life.

In the realm of Earth-based life, amino acids are exclusively “left-handed,” while sugars—integral components of DNA—are consistently right-handed, contributing to the distinctive rightward twist of the double helix. The homogeneity observed among these molecules presents a perplexing puzzle for scientists, especially considering that both left and right-handed variants are readily available in nonliving chemical mixtures. This raises intriguing questions about the nature of molecular selection in the formation of life.

Practically speaking, if biological molecules had evolved in the reverse configuration, life might still have thrived. This begs the question: if life could have emerged with right-handed molecules, why didn’t it? Is the uniform “handedness” of biological molecules a vital ingredient in life’s recipe, and specifically, did life have to be left-handed? Did this preference originate in the cosmos, or was it shaped by processes occurring later on Earth?

“A fundamental question for all of us concerns whether life had to develop in the way we observe today,” remarked Iris Chen, a professor of chemical and biomolecular engineering at UCLA who was not involved in the asteroid study. “Is the universe inherently predisposed to support our type of life, or is our biological configuration the product of random accidents and chance?”

NASA chose carbon-rich asteroid Bennu to study the chemical origins of life.
Credit: NASA

Scientists had anticipated utilizing the materials collected by NASA’s $800 million OSIRIS-REx mission—an acronym for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer—to investigate the “handedness” of specific amino acids. The mineral fragments from Bennu could potentially predate the solar system, estimated at 4.6 billion years old. These grains of stardust may have originated from dying stars or supernovas, eventually contributing to the formation of the sun and planets.

To conduct their research, the scientists created a concoction they dubbed “Bennu tea,” boiling a small quantity of the asteroid’s rocks and dust in water and acids to extract organic compounds. Subsequently, they employed advanced mass spectrometry techniques to identify organic molecules, including 14 of the 20 amino acids integral to life’s protein synthesis, which carry out crucial genetic instructions. Some of the latest findings from this research were published in the journal Nature Astronomy this week.

“I have to admit, I was a little disillusioned or disappointed. I felt like this invalidated 20 years of research in our lab and my career.”

Over recent decades, researchers have discovered that meteorites—rocks that have traversed space and crash-landed on Earth—exhibit a higher prevalence of left-handed amino acids compared to right-handed ones, often by around 60 percent. This led to the hypothesis that space rocks delivered the essential compounds that underwent chemical reactions near Earth’s deep-sea vents, potentially leading to the formation of the first cells, with the rest unfolding through evolutionary processes.

These findings, combined with the knowledge that celestial bodies have bombarded Earth for eons, prompted scientists to speculate that ancient asteroids, regarded as the solar system’s time capsules, would likewise reveal a greater abundance of left-handed amino acids. If the solar system indeed contains more left-handed variants, it raises the possibility that polarized light in space could have influenced this disparity. A slight environmental bias might amplify over time, leading to a significant difference.

Scientists think meteorites and planetary body collisions may have delivered origins of life chemistry to early Earth, including left-handed amino acids.
Credit: NASA’s Goddard Space Flight Center Conceptual Image Lab illustration

However, the researchers studying Bennu discovered that left-handed and right-handed amino acids were found in equal proportions. This unexpected outcome has led Glavin to question the validity of previous studies on meteorites, suggesting they might have been contaminated with terrestrial proteins upon landing. Jason Dworkin, the project scientist for the OSIRIS-Rex mission, posits that there may be an alternative explanation for Bennu’s divergence from established trends.

“Bennu represents a type of future meteorite that is too fragile to endure landing on Earth, which is why it’s absent from our collections,” Dworkin explained, indicating that this fragility could account for the anomalies observed.

It raises the intriguing possibility that the design of life might have been influenced by sheer chance. Once a viable pattern took root, it persisted through the process of evolution. Proteins and enzymes, which serve as tiny catalysts within cells, must fit together perfectly like pieces of a jigsaw puzzle. If life originally emerged with left-handed amino acids, a shift to right-handed amino acids later could have disrupted the entire process. The advantages of uniformity are vast; for example, if life forms were based on right-handed amino acids, they would struggle to digest the left-handed amino acids found in plants and animals.

Researchers have successfully synthesized mirror versions of biological proteins using right-handed amino acids in laboratory settings. While these versions function similarly to their left-handed counterparts, they are significantly more resistant to degradation. Enzymes that typically break them down are rendered ineffective. This scenario is akin to using a hair dryer in a foreign country—if the plug and outlet don’t match, the device won’t work.

A diagram of a left-handed and right-handed version of an amino acid from a meteorite.
Credit: NASA illustration

Some scientists contemplating the ramifications of this dilemma have voiced concerns regarding the potential future creation of mirror cells in laboratory settings. If humans were to become infected with harmful mirror bacteria, their immune systems might be unprepared to respond effectively, potentially leading to severe consequences. A group of biologists recently published a comprehensive paper discussing these risks, as highlighted by The New York Times.

Despite Glavin’s disappointment over Bennu’s lack of chirality bias, the team remains committed to their research. Glavin and his collaborators plan to analyze additional samples from the asteroid to further investigate the handedness of other amino acids.

There may be a silver lining in these findings: some astrobiologists have proposed using the uneven distribution of molecular handedness as a potential biosignature. An equal ratio of both types in an extraterrestrial sample might indicate that the molecules were synthesized chemically without biological influence. Conversely, an excess of one type could hint at the presence of alien life. This perspective offers a new lens through which scientists can search for extraterrestrial existence.

“Frankly, this could simplify the quest for life in certain ways because we eliminate the risk of false positives,” Glavin noted. “If we observe a significant amplification of one type over the other, it may suggest biological activity at play.”