How to Identify a Common Ancestry of Extraterrestrial Life?
Today, I was asked by an early-career biophysicist who follows my work on the search for extraterrestrial life:
“If a verified non-terrestrial biological sample were ever found to use the same nucleobase set as terrestrial life (A/T/C/G), would you read that primarily as evidence for shared ancestry, convergent chemistry, or directed panspermia — and what single measurement do you think would most cleanly distinguish those possibilities?”
My reply was straightforward:
“It could either be a sign of shared ancestry or a universal path for life. But note that DNA/RNA chirality has a 50% chance of being right-handed as it is on Earth. This causes the double helix to twist exclusively to the right. Similarly, amino-acids are left-handed in all terrestrial lifeforms.
Hence, chirality will be a test of a common ancestry. If we study many extraterrestrial life samples and half of their double helix building blocks are left-handed, then we would know that they do not have a common ancestry, even if the nucleobase set is the same.”
Albert Einstein famously argued: “God does not play dice!”, which in the context of astrobiology can be paraphrased to say: “A unique chirality for multiple extraterrestrial lifeforms may indicate an interstellar gardener.” However, if the symmetry breaking originated on Earth from some physical process that operates on exoplanets, we might get the same chirality under similar circumstances.
The OSIRIS-REx mission landed on the asteroid Bennu and on September 24, 2023 delivered back to Earth 121.6 grams of Bennu’s rocky material (as discussed here and here). Analysis of the pristine Bennu sample revealed that amino acids are racemic — meaning they feature an equal mix of left-handed and right-handed molecular structures with no measurable biological contamination (as reported here).
This discovery of evenly split chirality upends previous theories that amino acids formed in space naturally favor left handedness. For decades, scientists suspected that the handedness bias of life on Earth (homochirality) might have been inherited directly from amino acids delivered by meteorites. Because Bennu’s amino acids are split half and half between the left and right handedness, the origin of molecular homochirality in life on Earth remains a mystery. Despite the lack of chiral preference, Bennu’s sample includes 14 of the 20 amino acids used by life on Earth, along with all 5 DNA and RNA nucleobases. This confirms that complex prebiotic chemical building blocks can naturally form in space and exist in pristine environments.
And now, to my advice:
“If you ever encounter aliens, never shake their hands or hug them.”
Mirror-image bacteria could evade natural immune systems, resist antibiotics, act as unstoppable invasive species, and cause pervasive, lethal infections across global ecosystems. Human immune systems rely on chiral recognition. Mirror bacteria would likely go completely undetected by our innate immune receptors, leaving complex life defenseless. Since mirror organisms operate on reverse chemistry, standard medical countermeasures and antibiotics would be ineffective. If accidentally released, these mirror organisms could consume resources in the soil and water, destroying ecosystems and potentially leading to the death of most complex life on Earth.
Owing to these existential dangers, policy researchers argued in a Science Forum article published here that mirror bacteria cannot be developed safely in the laboratory, and advocated for a strong, international scientific consensus to halt research on mirror life.
In blind dates of interstellar proportions, we should practice caution and not get into physical contact with alien lifeforms, unless we realize that we have a common ancestry. In case we are all related, we can hug our extraterrestrial visitors as if they were lost siblings of our family.
In case we have the same chirality and biological building blocks, the urgent follow-up questions will be: `Who created us all?’ and `Were we created in its image?’
Genesis 1:27 states: “So God created mankind in his own image, in the image of God he created them.” Secular astrobiologists might wonder whether direct panspermia was involved in us sharing the same genetics as our extraterrestrial counterparts, and if so — who is our shared parent or interstellar gardener?
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, former director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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