Yesterday, we discussed the chirality, or handedness, of molecules, especially amino acids and proteins. The homochirality of organic molecules refers to their requirement for the same handedness, a mystery that has puzzled scientists since 1848, when Louis Pasteur first observed that some life-essential molecules had mirror-image forms. Amino acids used by living cells and the proteins they build have left-handed chirality, while DNA, RNA, and the sugars forming their building blocks are all right-handed. These facts raise a question for those who believe life arose spontaneously.
Left-handed proteins and right-handed DNA are found in all living organisms. The spiral structure of DNA needs to twist in a specific direction to function properly, but what caused the right-handed sugar molecules to come together? When half of all amino acid molecules in nature exhibit right chirality and the other half left, what force could have gathered only the left-handed ones to form the first proteins?
Scientists have speculated that cosmic rays or polarized light might have triggered this process. However, even if such forces created an initial bias for the same-handedness, they could not sustain and amplify it enough to produce a significant number of homochiral molecules to form the first living cells. In 1999, researchers proposed that electron spin created magnetism, causing left-handed peptides (short chains of amino acids) to bind to magnetic surfaces like magnetite. But this still did not explain how the homochirality of organic molecules could be amplified sufficiently to generate living cells.
Some scientists consider RNA to be the key to the origin of life. In 2009, a group of researchers studying RNA molecules suggested that a crystal called RAO could react to produce two of RNA’s four nucleotides. In 2023, other researchers used magnetism to produce RAO crystals with homochirality. However, this process required a magnetic field 6,500 times stronger than Earth’s magnetic field. Additionally, RAO has only been shown to produce two of RNA’s four nucleotides, still falling short of generating the complete homochirality of molecules needed for life.
As 2024 ended, NASA-funded research identified a problem with the so-called “RNA world” hypothesis. Simulating early Earth conditions, they found that RNA did not show a chemical bias toward selecting homochiral amino acids. The current thinking is that the homochirality of organic molecules “could have emerged through later evolutionary pressures.” In other words, we have the “evolution-of-the-gaps” theory because, of course, evolution can do anything you can imagine.
Currently, NASA scientists are analyzing samples brought back from asteroid Bennu, hoping that molecular evidence from meteorites and asteroids will demonstrate that the building blocks for life came from beyond Earth. I suspect that the force responsible for bringing life to Earth did originate outside our planet, and even beyond the universe.
— Roland Earnst © 2025
References: science.org and nasa.gov
