When NASA dispatched a spacecraft to gather samples from an asteroid, scientists were keenly looking forward to the chance to study pieces of the celestial body in a laboratory setting. Their aspiration? To reveal insights into some of humanity’s deepest inquiries. For Danny Glavin, a senior sample scientist, this mission held personal significance. He aimed to resolve a conundrum central to his profession: Why are all recognized living organisms constructed solely from the left-handed variants of amino acids, the building blocks of proteins?
Almost ten years later, Glavin and his colleagues finally received their opportunity. They investigated material from Bennu, a carbon-rich asteroid made up of loosely connected boulders. However, the results were surprising. Rather than backing the dominant theory—that the early solar system favored left-handed amino acids, distributing them to a young Earth—the samples did not demonstrate any such preference.
“I must confess, I felt somewhat disheartened or let down,” Glavin admitted. “It seemed to undermine 20 years of research in our lab and my career.”
### The Puzzle of Molecular “Handedness”
Amino acids, whether utilized in biological processes or not, exist in two mirror-image forms: left-handed and right-handed. This characteristic, called chirality, resembles the difference between left and right hands—they are alike but cannot be superimposed. On our planet, life exclusively utilizes left-handed amino acids and right-handed sugars, with the latter contributing to DNA’s iconic right-handed spiral.
This consistency has long baffled researchers, as both left- and right-handed forms are abundantly available in nonliving chemical mixtures. It is theoretically possible for life to have evolved using right-handed amino acids. So, why didn’t it? Is this uniformity a crucial aspect for life, or purely a cosmic coincidence? Did the preference for left-handed amino acids originate in space, or did it develop subsequently on Earth?
“A key question for everyone is whether life had to be the way it is,” remarked Iris Chen, a chemical and biomolecular engineering professor at UCLA who did not participate in the study. “Is the universe predisposed to support our kind of life, or is our biology merely a product of chance and random events?”
### Bennu: A Time Capsule of the Solar System
NASA’s $800 million OSIRIS-REx mission (an acronym for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) was crafted to gather and analyze materials from Bennu. Scientists were convinced that the asteroid’s mineral fragments, possibly older than the 4.6-billion-year-old solar system, could provide insights about the chemical foundations of life. These particles of stardust may have come from dying stars or supernovas that ultimately contributed to the formation of the sun and planets.
To analyze the samples from Bennu, researchers prepared a “Bennu tea,” boiling small portions of the asteroid’s rocks and dust in water and acids to extract organic components. By employing mass spectrometry, they identified organic molecules, including 14 of the 20 amino acids crucial for life. Their results, published in *Nature Astronomy*, unveiled an equal distribution of left- and right-handed amino acids—contradicting previous beliefs.
### A Challenge to Long-Known Theories
For years, scientists have scrutinized meteorites—space rocks that land on Earth—and found that these often have a higher concentration of left-handed amino acids, sometimes exceeding 60% more. This led to the hypothesis that ancient asteroids delivered these compounds to Earth, where they engaged in chemical processes near deep-sea vents to create the first cells. The rest, as the story goes, is evolution.
Given the prevalence of left-handed amino acids in meteorites, researchers anticipated that Bennu would exhibit a similar trend. One theory proposed that polarized light in space might have favored left-handed molecules, enhancing the difference over time. However, Bennu’s samples displayed no such preference, prompting scientists to reconsider whether previous meteorite studies had been influenced by Earth-based proteins.
Jason Dworkin, the project scientist for the OSIRIS-REx mission, provided an alternative explanation: “Bennu represents a type of future meteorite that is too delicate to endure landing on Earth, hence it’s missing from our collections.”
### A Coin Flip for Life?
The discoveries from Bennu indicate that life’s molecular arrangement may have been dictated by chance. Once a successful configuration—like left-handed amino acids—was established, it likely endured through evolutionary processes. Proteins and enzymes, the molecular machinery within cells, interlock like pieces of a jigsaw puzzle. Transitioning to right-handed amino acids mid-evolution would have disrupted this fragile equilibrium.
Uniformity also brings practical benefits. If humans were based on right-handed amino acids, they would struggle to digest plants or animal products composed of left-handed amino acids.
Curiously, researchers have synthesized mirror versions of biological proteins using right-handed amino acids in laboratories. These molecules operate similarly but are significantly more challenging to break down, as enzymes tailored for left-handed molecules are ineffective against their right-handed variants.