Nonreplicating Protocells

Prebiotic soup experiments have shown that the molecular building blocks of life can be built under prebiotically plausible conditions. From this starting point, researchers have launched continued studies of polymerization and explorations of the breadth of RNA function. Recently, effort has intensified to examine experimentally another stage of the origins of life: the assembly of the molecular parts into model protocells intended to represent the first primitive, cell-like systems to emerge on Earth. Although it may not be possible to recreate the precise sequence of events that led to cellular life, laboratory experiments have begun to show what was and was not possible. Prebiotically plausible lipid vesicles form easily and have many properties that are conducive to cellular function. In addition to protecting nascent replicating genetic systems from parasitic sequences, vesicles facilitate evolution. The data thus far suggest that prebiotically plausible vesicles could have grown, divided, and promoted competition between distinct chemical systems. Most protocellular studies to date have probed the role of self-replication, one feature of extant life in the emergence of the first cellular system. Undoubtedly replicating systems were crucial for protocellular evolution, but other features of life must have been important as well. For example, life does not exist in isolation. A living system must cope with and adapt to environmental fluctuations to survive. The protocell must have generated some of these fluctuations because cellular activity necessarily modifies its surroundings by selectively absorbing nutrients and releasing unwanted molecules. It seems likely that life would have faced this challenge early and either emerged in dynamic locales that continuously regenerated conditions conducive to life or exploited mechanisms to physically move to new areas not depleted in resources. Further studies that explore non-replication-based aspects of the origins of life could reveal a more complete picture of the transition from prebiotic chemistry to early life.