Asteroid Bennu’s Building Blocks of Life Challenge Our Understanding of Chirality

By SWA S. W. Alexander January 30, 2025

NASA’s Discovery: The Chemistry of Life in an Ancient Asteroid

In a groundbreaking discovery, NASA scientists have confirmed that fragments from asteroid Bennu contain the essential building blocks of life, including all five nucleobases that form DNA and RNA and 14 of the 20 amino acids found in proteins. This finding, revealed after researchers carefully opened a sealed canister containing Bennu’s material under inert gas conditions to prevent contamination, strengthens the idea that life's raw ingredients may have arrived on Earth from space.

However, Bennu’s amino acids hold a surprising twist: unlike life on Earth, which predominantly uses left-handed (L) amino acids, the asteroid’s amino acids exist in nearly equal amounts of left- and right-handed (D) forms. This unexpected balance challenges long-standing theories that asteroids like Bennu may have played a direct role in shaping Earth’s biochemistry.

The Mystery of Life’s Handedness

Chirality: The Left-Handed Bias of Life on Earth

Chirality is a fundamental property of many biological molecules, meaning they exist in two mirror-image forms—just like our left and right hands. However, life on Earth has a peculiar rule:

Amino acids, the building blocks of proteins, are almost exclusively in the L-form (left-handed).
Sugars, which form the backbone of DNA and RNA, are almost always in the D-form (right-handed).

This one-sided preference is a mystery because non-biological processes typically produce equal amounts of both forms. Scientists have long theorized that meteorites and asteroids, rich in organic compounds, might have delivered a bias toward left-handed amino acids, helping life get started with its distinct molecular "handedness."

But Bennu’s equal mix of L- and D-amino acids now complicates this theory. If asteroids provided Earth's first amino acids, why didn’t they come with the same bias that life later adopted?

Implications for the Origins of Life

Bennu’s unexpected chirality balance raises several possibilities about how life’s left-handed rule emerged:

1. Did Earth Create Its Own Chiral Bias?

If asteroids like Bennu delivered amino acids in equal amounts, then some process on early Earth must have selectively favored left-handed molecules. Possible explanations include:

Catalytic minerals – Certain naturally occurring crystals may have absorbed or reacted more efficiently with one form over the other.
Polarized light from the Sun – Circularly polarized UV light could have selectively broken down one type of amino acid more than the other.
Self-replicating molecules – Early precursors to life, such as RNA, may have worked more efficiently with left-handed amino acids, leading to a gradual preference.

2. Is Chirality an Earth-Specific Quirk?

If the preference for left-handed amino acids is not universal, then life elsewhere in the universe may follow completely different biochemical rules. Imagine a world where proteins are built from right-handed amino acids—such alien life would be fundamentally incompatible with our biology.

3. Could Another Process Have ‘Tweaked’ the Balance?

If asteroids delivered amino acids in equal amounts, but life on Earth later evolved a preference, then some unknown factor may have pushed biology toward left-handed molecules. Some scientists speculate that intense radiation, deep-sea hydrothermal vents, or even meteorite impact conditions could have created the necessary imbalance.

Chirality in Medicine: Why This Matters Beyond Space

Understanding chirality isn’t just about solving the mystery of life’s origins—it also has major implications for modern science, especially in medicine. Many drugs exist in both left- and right-handed forms, but the body often responds differently to each:

Thalidomide – One form worked as a sedative, while the other caused severe birth defects.
Ibuprofen – Only the left-handed version effectively relieves pain.
Methamphetamine – One form is a dangerous stimulant (illegal meth), while the other is used in nasal decongestants.

By studying chirality in space and on Earth, scientists can develop better drugs and even understand how life itself evolved.

What’s Next? The Future of Astrobiology

Bennu’s organic-rich fragments offer a rare glimpse into the chemistry of the early Solar System, and ongoing studies will help scientists determine whether this asteroid’s molecular makeup is common among other space rocks. If similar findings emerge from future missions—such as the upcoming sample return mission from Mars' moon Phobos or the study of Enceladus' ice plumes—our understanding of life's origins may need a radical update.

For now, Bennu’s unexpected mix of left- and right-handed amino acids leaves us with more questions than answers. Did life’s chirality evolve on Earth? Or is there an unknown cosmic process that tipped the balance? One thing is certain—asteroid Bennu has just made the search for life’s origins even more exciting. 🚀

Credit: NASA / Robert Markowitz – Researchers opening Bennu’s sample canister in a super-clean lab under inert gas to prevent contamination.

Reference: Asteroid fragments upend theory of how life on Earth bloomed

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