For over a century, scientists have picked the microbe apart–sequencing its genes, cracking its genetic code, running experiments on its metabolism, earning Nobel Prizes off of it, and turning it into, arguably, the most-studied organism in history.

But as deep as scientists dive, they have yet to touch bottom. That’s in part because Escherichia coli is not fixed. It continues to evolve, and even in the most carefully controlled experiments, evolution leaves behind a complicated history.

Twenty-five years ago, Richard Lenski used a single microbe to seed twelve lines of bacteria. He fed each line a meager diet of glucose, and the bacteria have been adapting to this existence in his lab at Michigan State University ever since. (Here I’ve gathered together a few pieces I’ve written over the years about the 58,000-generations-and-counting Long-Term E. coli Evolution Experiment.)

In 2003, Lenski’s team realized that something utterly unexpectedhappened. One of the hallmarks of Escherichia coli as a species is that when there’s oxygen around, it can’t feed on a compound called citrate. But one day a flask turned cloudy with an explosion of E. coli that were doing just that. The change was so profound that it may mean these bacteria had evolved into a new species.