Courtesy of Irene Chen and Jack Szostak

In this computer model, cells with self-replicating nucleic acids (red and blue) also take in more water. They compensate for increased pressure by capturing fatty acids from the membranes of their non-replicating neighbors (white).

Jack Szostak and his colleagues at Harvard Medical School are seeking to understand the origin of life through a series of audacious experiments intended to build a basic living cell from scratch (see “What Came Before DNA?” Discover, June 2004). Using a simple experiment, they now demonstrate that one of the key steps—creating a simple growing cell by tucking self-reproducing molecules into a membrane—may be startlingly simple.


The new research rebuts the widespread belief that cells have to evolve elaborate molecular machinery to enable them to grow, one of the basic characteristics of living things. Szostak and his colleagues started with chemicals thought to have been common on early Earth: nucleic acids (the building blocks of DNA) and fatty acids. One interesting property of fatty acids is that they spontaneously form bubbles, or vesicles, that allow water molecules to pass back and forth but trap larger molecules. In the Harvard experiment, vesicles that contained relatively high concentrations of nucleic acids expanded like balloons, while nucleic acid-poor vesicles shrank. The growing vesicles cannibalized fatty acids from the shrinking ones, so they were able to keep growing without popping.

Previously, researchers have shown that some simple RNAs, the smallest about twice as long as those of the Szostak group’s simple cells, can replicate without help from other molecules. The group’s new observation is that packing a membrane with more nucleic acids makes it expand; this mechanism could provide the cells with a simple method for evolutionary competition. If some of these model cells contained nucleic acids that could replicate themselves, even inefficiently, they would have grown at the expense of competitor cells. The more effectively the nucleic-acid molecules can replicate, the more rapidly their surrounding membranes will grow. “What we showed was that you can get a Darwinian competition to emerge just from the basic physical properties of the system,” says Irene Chen, a graduate student in Szostak’s lab. “It doesn’t require biological machinery.”