Modern cells are highly intricate systems. They contain internal scaffolding, tightly controlled chemical processes, and genetic instructions that guide nearly everything they do. This complexity allows them to survive in diverse environments and compete based on their fitness. In contrast, the earliest cell-like structures were extremely simple. These primitive compartments were essentially tiny bubbles, where lipid membranes enclosed basic organic molecules. Understanding how such simple protocells eventually gave rise to the complex cells we see today remains a central question in origin-of-life research.
A recent study led by researchers at the Earth-Life Science Institute (ELSI) at Institute of Science Tokyo takes a closer look at how these early structures might have behaved on ancient Earth. Instead of proposing a single explanation for how life began, the researchers focused on experiments that simulate realistic environmental conditions. Specifically, they examined how variations in membrane composition affect protocell growth, fusion, and the ability to retain important molecules during freeze/thaw cycles.
Building Model Protocells With Different Lipids
To investigate this, the team created small spherical compartments known as large unilamellar vesicles (LUVs). These were built using three types of phospholipids: POPC (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine; 16:0-18:1 PC), PLPC (1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine; 16:0-18:2 PC), and DOPC (1,2-di-oleoyl-sn-glycero-3-phosphocholine; 18:1 (D9-cis) PC).
"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," said Tatsuya Shinoda, a doctoral student at ELSI and lead author.
Although these molecules are similar, their structures differ in subtle but important ways. POPC contains one unsaturated acyl chain with a single double bond. PLPC also has one unsaturated acyl chain, but with two double bonds. DOPC includes two unsaturated acyl chains, each with one double bond. These differences influence how tightly the molecules pack together. POPC tends to form more rigid membranes, while PLPC and DOPC produce membranes that are more fluid.
Freeze-Thaw Cycles Drive Growth and Fusion
The researchers then exposed these vesicles to repeated freeze/thaw cycles (F/T), mimicking temperature changes that could have occurred on early Earth. After three cycles, clear differences emerged. Vesicles rich in POPC clustered together without fully merging. In contrast, those containing PLPC or DOPC fused into larger compartments. The more PLPC present, the more likely the vesicles were to merge and grow.
This behavior highlights the role of membrane chemistry. Lipids with more unsaturated bonds make membranes less tightly packed, which appears to encourage fusion. "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." remarked Natsumi Noda, researcher at ELSI.
Mixing Molecules and Retaining DNA
Fusion is important because it allows the contents of separate compartments to mix. On early Earth, where organic molecules were scattered in the environment, this kind of mixing could have brought key ingredients together. That interaction might have supported chemical reactions leading toward more complex, cell-like systems.
The team also tested how well these vesicles could capture and retain DNA. They compared vesicles made entirely of POPC with those made entirely of PLPC. The results showed that PLPC vesicles were better at trapping DNA even before freeze/thaw cycles. After repeated cycles, they continued to hold onto more DNA than POPC vesicles.
Icy Environments as a Possible Cradle for Life
Traditionally, scientists have focused on environments such as drying pools on land or hydrothermal vents in the deep ocean as likely settings for the origin of life. This study adds another possibility. It suggests that icy environments may also have played a meaningful role.
On early Earth, freeze/thaw cycles could have occurred repeatedly over long periods. As water froze, growing ice crystals would push dissolved molecules into the remaining liquid, concentrating them in small spaces. This process could have increased the likelihood of interactions between molecules and vesicles. At the same time, membranes made of more unsaturated phospholipids would have been more prone to fusion, promoting mixing. However, there is a trade-off. While fluid membranes support fusion, they can also become unstable during freeze-thaw-induced stress, leading to leakage.
Balancing Stability and Evolution in Early Cells
For early protocells, maintaining a balance between stability and permeability would have been crucial. Membranes need to hold onto their contents, but also allow interactions that drive chemical change. The most successful membrane compositions likely depended on environmental conditions.
"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," concludes Tomoaki Matsuura, Professor at ELSI and principal investigator behind this study.
Together, these findings suggest that simple physical processes like freezing and thawing may have helped guide the transition from basic molecular compartments to the first evolving cells.