The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

But the transition from an RNA strand to a self-replicating RNA molecule is no small evolutionary feat. Most likely, two such molecules were needed—one to act as the template (i.e., the information carrier) and the other to make a copy of the template (i.e., a duplicator). When these two RNA molecules—template and duplicator—met each other, it was, perhaps, the most important and explosive evolutionary love affair in the history of our living planet. But the lovers had to avoid separation; if the two strands of RNA were to float away from each other, there would be no duplication and, by extension, no cellular life. And so some sort of structure—a spherical membrane—was likely needed to confine these components. These three components (a membrane, an RNA information carrier, and a duplicator) might have defined the first cell. If a self-replicating RNA system were bound by a spherical membrane, it would make more RNA copies within the confines of the sphere and grow in size by enlarging the membrane. At some point, biologists believe, the membrane-bound spheroid would split into two, each carrying the RNA duplicating system. (In lab experiments, Jack Szostak and his colleagues have shown that simple spheroidal structures, bound by membranes formed by fat molecules, can absorb more fat molecules, grow, and eventually split into two.) And from that point on, the protocell would launch its long evolutionary march toward the progenitor of the modern cell. Evolution would selec...