Researchers at the Earth-Life Science Institute in Tokyo have shown through experiments that repeated freezing and thawing could have driven the growth and fusion of primitive cell-like structures on early Earth. Vesicles made with certain lipids fused into larger compartments and retained DNA more effectively during these cycles. The findings suggest icy environments played a role in life's origins.
A team led by Tatsuya Shinoda at the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo created large unilamellar vesicles (LUVs) using three types of phospholipids: POPC, PLPC, and DOPC. These model protocells mimicked simple compartments enclosing organic molecules on ancient Earth. The researchers exposed them to repeated freeze-thaw cycles to simulate early environmental conditions. Shinoda explained, 'We used phosphatidylcholine (PC) as membrane components, owing to their chemical structural continuity with modern cells, potential availability under prebiotic conditions, and retaining ability of essential contents.' After three cycles, POPC-rich vesicles clustered but did not fully merge, while those with PLPC or DOPC fused into larger structures. The more PLPC present, the greater the fusion. Natsumi Noda noted, 'Under the stresses of ice crystal formation, membranes can become destabilized or fragmented, requiring structural reorganization upon thawing. The loosely packed lateral organization due to the higher degree of unsaturation may expose more hydrophobic regions during membrane reconstruction, facilitating interactions with adjacent vesicles and making fusion energetically favorable.' PLPC vesicles also captured and held DNA better than POPC ones, even before cycles and especially afterward. Fusion allowed mixing of contents, potentially enabling complex chemistry. The study proposes icy settings with freeze-thaw as a cradle for life, alongside traditional sites like hydrothermal vents. Tomoaki Matsuura, the principal investigator, concluded, 'A recursive selection of F/T-induced grown vesicles across successive generations may be realized by integrating fission mechanisms such as osmotic pressure or mechanical shear. With increasing molecular complexity, the intravesicular system, i.e., gene-encoded function, ultimately may take over the protocellular fitness, consequently leading to the emergence of a primordial cell capable of Darwinian evolution.' The research appears in Chemical Science.
