I first reported on Lenski’s experiment 12 years ago, and since then I’ve revisited it every few years. The bacteria have been evolving in all sorts of interesting ways, and Lenski has been able to reconstruct the history of that evolution in great detail, thanks to a frozen fossil record. Every 500 generations Lenski and his students sock away some bacteria from each flask in a freezer. They can thaw out these ancestors whenever they wish and compare them to their youngest descendants. Biotechnology has improved drastically since 1988, giving Lenski an increasingly powerful evolutionary microscope. When he started out, it could take months to identify just one of the many mutations that arose in each lineage. These days, he and his colleagues can sequence an entire E. coli genome for a few hundred dollars and find every single new mutation in its DNA.

Four years ago, I wrote here about one particularly fascinating episode in the evolution going on in Lenski’s lab. It started in 2003, when the scientists there noticed something odd in one of the 12 flasks. It had become much more cloudy than the others. In a microbiology lab, that’s a sure-fire sign that the bacteria in a flask have experienced a population explosion.

At first the team suspected that some other species of bacteria had slipped into the flask and was breeding quickly. But they found that the flask was packed with E. coli—descendants of the original ancestor that Lenski had used to start the entire experiment. Somehow the bacteria in this one flask had evolved a way to grow much faster than the other bacteria.

The scientists determined that the bacteria had made a drastic switch: from feeding on the glucose to another compound, called citrate. Citrate is an ingredient in the broth where Lenski’s E. coli grows. It’s not food; instead, it helps keep the minerals in the broth in the right balance for E. coli to grow.