In essence, each cell is just a bubble of chemistry sealed within a membrane. In modern cells, that chemistry is extraordinarily complex, orchestrating about a billion reactions per second. Likewise, its container, the lipid membrane, is built and maintained by elaborate biochemical mechanisms. But even the first, much simpler cells would have also needed a bubble. Without a physical boundary corralling molecules into a small space, whatever became early life would have been too spread out to assemble the complex systems life requires, said Bruce Damer, an astrobiologist at the University of California, Santa Cruz. “You need a container before you can get the chemistry.” For some researchers, the cell membrane seems so essential to the functions of life that they propose it as the origin of life. In this framework, known as the “lipids first” hypothesis, the Earth went from a mess of primordial chemistry to something recognizably alive thanks to those containers. But many lipid advocates go even further. They argue that the bubbles did more than contain chemicals. The bubbles themselves began to evolve. Evolution requires a way for parents to pass on traits to their children. Modern life achieves this with complex biochemical substances such as DNA and RNA, which can encode and copy information in their sequences. In the 1990s, researchers imagined a different possibility: What if a chemical system, rather than a molecule, could carry heritable information and be subject to evolution? The idea begins with a puddle of water on early Earth. The puddle is filled with all kinds of small organic molecules, some homegrown and others released by dust and meteorites. Many of these molecules have a split personality. One end of them is attracted to water or “hydrophilic.” The other is repelled by water in the same way that oil is. These “hydrophobic” ends will come together to hide from the water, while the hydrophilic ends will face outward. These behaviors cause the molecules to form a small sphere: a bubble. The primordial pool also has other molecules, simpler raw ingredients that, under the right circumstances, can become something more of themselves. Here’s how it works: Imagine that a bubble contains two special molecules, molecule A and molecule B. Each of these molecules speeds up a chemical reaction that uses other ingredients in the puddle. Molecule A, in this case, speeds up the chemical reactions that produce molecule B. After a while, the bubble fills with B. These, in turn, can speed up the production of another type of molecule. Sometimes there can be a puddle where the B’s return the favor and get more A’s. The A’s do more B’s, then the B’s do more A’s. Each type keeps the other recharged in an endless loop. The bubble containing these A and B will grow, but the proportion of molecules will remain more or less stable. Over time, the bubble will split and its molecules will distribute into their little bubbles. Each of them can get some A’s and some B’s, and the symbiotic cycle will continue to work, building the parents’ compositional recipe more faithfully than chance alone would. Unlike DNA, which is copied exactly, letter by letter, a proportion of molecules are passed on to the next generation of bubbles. But evolution needs more than inheritance. You need to allow for change. In our chemical soup, that possibility comes from a missing C molecule, which, like a mutation, tilts a bubble toward a different molecular loop. Faster-growing bubbles divide more frequently and displace slower-growing bubbles, so fast-moving offspring will make up a larger and larger portion of the crowd. Slow recipes are eliminated. In 2000, Doron Lancet, a geneticist at the Weizmann Institute of Science, and Daniel Segrè, a computational biologist at Boston University, published the results of a computer model that explored whether these dynamics could lead to realistic behavior. They discovered that the bubbles can transmit their compositions to their daughters, who vary and compete, meeting the requirements of natural selection. At least in his computer model, the lipid world idea passed its first test. What molecules could form these bubbles? In the original simulation, molecules A, B and the rest would be different types of lipids. The team settled on lipids, the molecules that make up modern cell membranes, in part because they reflect the half-hydrophobic, half-hydrophilic nature required by the model. When lipids are immersed in water, they can form double-layered sheets. Under the right conditions, the sheets close on themselves, forming a sphere with a pocket of water trapped inside. But in the 25 years since Lancet, Segrè and their colleagues published the original lipid world hypothesis, scientists have wondered whether the first compartments had to be made of lipids. Couldn’t other types of molecules have formed a container first? “There really isn’t a distinct hypothesis about the lipid world anymore,” Damer said. Researchers have proposed that certain minerals, such as iron sulfides, could have formed the walls of a vessel, or that coacervates (liquid-like droplets where molecules gather and concentrate) could have acted as membrane-less vessels for early chemistry. Damer and David Deamer, a Santa Cruz biochemist who worked with Lancet and Segrè on the original hypothesis, have remained focused on lipids, but in a specific environment: hot spring pools. They now study whether repeated drying and filling of such pools could have compressed simple molecules between lipid layers to develop increasingly complex systems. Damer sees his most recent “hot spring hypothesis” as an extension of the world of lipids. Research into the origins of life is notoriously controversial, with scientists in the field debating the world of lipids versus other ideas about how life formed. “The idea of a primitive life form based solely on lipids has never made sense to me,” said Jack Szostak, a Nobel Prize-winning biochemist at the University of Chicago. He cited a lack of experimental evidence that a lipid-based system can evolve. Furthermore, he said, how even simple lipids would have formed on the early Earth is “the least developed aspect of all prebiotic chemistry.” Another criticism goes to the heart of the original computer simulation. A 2010 analysis suggested that replication of compositional information from one generation to the next would be too inexact for natural selection to work. However, none of these objections address whether the early years of life necessitated a bubble. Even Szostak, who favors an RNA-based explanation for the origin of life, agrees that what matters most to lipids is that compartments are necessary to hold related molecules together. Without them, he said, “Darwinian evolution could not work.”
