The origin of life on Earth remains one of science’s most compelling questions, with multiple theories attempting to explain how its fundamental building blocks first emerged. Traditionally, researchers have assumed that the chemical interactions necessary for early biological development required warm environments. However, a new study challenges this long-standing view, proposing that frozen conditions may have played a decisive role.
Through laboratory experiments, scientists demonstrated that lipid membranes with a higher degree of unsaturation promote vesicle fusion and preserve genetic material during repeated freeze–thaw cycles. These findings highlight icy environments as potential drivers in the evolution of protocells.
Modern cells are extraordinarily complex, containing a cytoskeleton, tightly regulated chemical processes, and genetic material that governs nearly every aspect of their function. In contrast, early cell-like structures were likely simple lipid bubbles encapsulating essential organic molecules. Understanding how such primitive assemblies evolved into fully developed cells remains a central challenge in origin-of-life research.
Rather than advocating a single origin theory, the research team examined how variations in membrane chemistry influence protocell growth, fusion, and biomolecule retention under dynamic, non-equilibrium conditions resembling those of early Earth.
The scientists constructed large unilamellar vesicles using three types of phospholipids—POPC, PLPC, and DOPC—molecules similar to those found in modern cellular membranes. According to Tatsuya Shinoda, a doctoral researcher at the Earth Life Science Institute of the Tokyo Institute of Science, phosphatidylcholine was selected due to its chemical continuity with modern cells and its ability to retain essential molecular contents.
Although structurally related, these lipids differ in the number and arrangement of double bonds in their fatty acid chains, which affects membrane fluidity. POPC forms relatively rigid membranes, whereas PLPC and DOPC produce more fluid structures.
When subjected to repeated freeze–thaw cycles simulating early Earth temperature fluctuations, vesicles rich in PLPC and DOPC fused into larger compartments, while POPC vesicles formed dense aggregates. The greater the degree of unsaturation, the stronger the tendency for fusion and growth.
Researcher Natsumi Noda explains that ice crystal formation exerts mechanical stress on membranes, forcing structural reorganization during thawing—a process that may facilitate vesicle fusion.
These findings are significant because vesicle fusion allows trapped molecules to mix and potentially react. Experiments further revealed that PLPC vesicles retained more DNA than POPC vesicles after freeze–thaw treatment.
While most origin-of-life scenarios have focused on environments such as hydrothermal vents or wet–dry cycling surfaces, this study suggests that frozen regions of early Earth—where repeated freezing and thawing likely occurred over long periods—may also have been crucial.
Tomoki Matsuura, professor at ELSI and lead investigator of the study, notes that successive selection of vesicles through freeze–thaw cycles, combined with division mechanisms, could eventually have led to protocells capable of Darwinian evolution.