Mind the Gap!

In 1835, the French naturalist Felix Dujardin started crushing ciliates under the microscope and observed that the tiny cells exuded a jellylike, water-insoluble substance, which he described as a ‘‘gelée vivante’’ and which was eventually christened ‘‘protoplasm’’ by the physician Johann E. Purkinje and the botanist Hugo von Mohl. Fifty years after Dujardin’s observations, the possibility that living organisms were the evolutionary outcome of the gradual transformation of lifeless gel-like matter into protoplasm was so widespread that it found its way into musical comedies. In 1885, the selfimportant Pooh-Bah, Lord Chief Justice and Chancellor of the Exchequer, declared in Gilbert and Sullivan’s The Mikado that ‘‘I am in point of fact, a particularly haughty and exclusive person, of preAdamite ancestral descent. You will understand this when I tell you that I can trace my ancestry back to a protoplasmal primordial atomic globule.’’ As a number of contributors argue in the book Protocells: Bridging the nonliving and living matter (edited by Steen Rasmussen, Mark A. Bedau, Liaohai Chen, David Deamer, Norman H. Packard, and Peter F. Stadler), life on Earth may have descended from primordial globules formed in the primitive oceans. This thick volume collects papers by 31 authors that were presented at two different international workshops, in which theoretical and experimental models of protocells were analyzed in considerable detail. As summarized by molecular biologist Martin M. Hanczyc in the opening chapter, mesmerizing reports of lifelike behaviors by microscopic droplets of different compositions led Jerome Alexander, Stéphane Leduc, and Alfonso L. Herrera to the conclusion that physicochemical models of protoplasm might provide insights into the origin of life. Other scientists, like the Dutch biochemist Bungenberg de Jong, argued that the colloid properties of droplets, which he termed coacervates, formed by the spontaneous aggregation of biological macromolecules, could explain the properties of protoplasm. And in a book published in Russian in 1936, Aleksandr I. Oparin went further and proposed these droplets as precursors of the first cells. Oparin considered enzymatic-based assimilation, growth, and reproduction traits of life, but not nucleic acids, whose role as the material basis of inheritance was not even suspected. He assumed that biological inheritance was the outcome of growth and division of the coacervate drops that he viewed as models of precellular systems. In hindsight, this may seem naı̈ve, but Oparin’s commitment to coacervates resulted in part from his refusal to assume that life can be reduced to a single compound such as the randomly formed ‘‘living gene’’ proposed by the American geneticist Herman Muller. Thirty years ago, laboratory simulations and the discovery of a large array of amino acids and nucleobases in meteorites had reinforced the idea that life arose from a primitive soup, but few dared to discuss how the huge gap separating organic molecules of prebiotic origin from the first living systems could be closed. Oparin’s coacervates were seen by many as historical curiosities with little, if any, significance for the study of the appearance of life, which had become focused on the origin of genetic replication. Whether the earliest genetic polymers were enclosed within membranes is not yet clear, but as summarized by biochemist David W. Deamer in this volume, he and others, like the late Spanish chemist Juan Oró, were convinced that this was the case. Their assumptions were buttressed over 30 years ago when they independently achieved the abiotic synthesis of lipids under mild conditions; they were reinforced a few years later when Deamer and Pashley reported the presence of membrane-forming lipidic compounds in a sample of the Murchison meteorite. Lipids slowly but steadily began to glide toward origin-of-life scenarios and, at the same time, started to receive attention from industry, as when it was shown that micelles and liposomes behave as microreactors and could be used to deliver interferon and other medicaments into cells. There are more things in lipids that are dreamt of in our laboratories. The spontaneous self-assembly of amphiphilic molecules into bilayers, micelles, and vesicles are well-known, but as the ETH-based chemist Pier Luigi Luisi and his associates write in this book, 20 years ago he and his colleagues demonstrated the feasibility of autocatalytic formation and self-replication of micelles. Micelles and liposomes do not reproduce and do not store genetic information, but they replicate by a