At Nautilus, science reporter Zach Zorich examines the following question: If the world began again, would life as we know it exist? In science and evolution, this is a discussion of convergence vs. contingency. Scientists like Richard Lenski, an evolutionary biologist at Michigan State University, are conducting experiments in the lab to test out their theories. In 1988, Lenski separated a single population of Escherichia coli bacteria into 12 separate flasks and has been studying them for 26 years:

In 11 of Lenski’s flasks, the E. coli cells grew physically larger, but bacteria in one flask divided itself into separate lineages—one with large cells and the other with small cells. “We call them the smalls and the larges,” says Lenski. “They have coexisted now for 50,000 generations.” No other population in the experiment did the same; a historically contingent event seemed to have taken place. Even 26 years later, none of the other E. coli lineages evolved it. In this case, contingency seems to have won out over convergence.

In 2003, another contingent event took place. The number of E. coli in one of the flasks increased to the point where the normally translucent nutrient solution turned cloudy. At first Lenski thought that the flask had been contaminated, but it turned out that the E. coli, which normally just feed on glucose in the solution, had developed a way to consume a different chemical in the flasks, called citrate. After 15 years, or 31,500 generations, just one of the populations was able to consume the substance.2 Its population size quickly expanded by a factor of five.

This “historical contingency” gave Lenski and his graduate student Zachary Blount a chance to examine the likelihood that it would happen again if they rewound the tape. Blount went to the archive of frozen E. coli, and selected 72 samples collected at different periods in the experiment from the population that later evolved citrate metabolism. He thawed them out, and let them grow. Eventually, four out of the 72 samples acquired the ability. What’s more, the mutations only occurred in populations that had been frozen after 30,500 generations. Genetic analysis showed that several genes had undergone mutations that “potentiated” the evolution of citrate metabolism before that point. In other words, the ability to consume citrate was contingent upon other mutations that had come before it. Those formed a fork in the road, altering the path that generations after would be able to travel.

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