In 1988, Richard Lenski, an evolutionary biologist now at Michigan State University, launched the longest running experiment on natural selection. It started with a single microbe–E. coli–which Lenski used to seed twelve genetically identical lines of bacteria. He placed each line in a separate flask, which he provisioned with a scant supply of glucose. The bacteria ate up the sugar in a few hours. The next day, he took a droplet of microbial broth from each flask and let it tumble into a new one, complete with a fresh supply of food. The bacteria boomed again, then starved again, and then were transferred again to a new home. Lenski and his colleagues have repeated this procedure every day for the past 24 years, rearing over 55,000 generations of bacteria.
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.