The short story is that one of E.Coli's distinguishing traits is it's inability to metabolise Citrate, which has a very different molecular structure than E.Coli's primary food. After around 31,000 generations, all of a sudden this E.Coli started eating Citrate! With the frozen genetic samples, the researchers are able to go back in a frame-by-frame manner, and watch the mutations accumulate into this novel masterpiece.
A major evolutionary innovation has unfurled right in front of researchers' eyes. It's the first time evolution has been caught in the act of making such a rare and complex new trait.
And because the species in question is a bacterium, scientists have been able to replay history to show how this evolutionary novelty grew from the accumulation of unpredictable, chance events.
Twenty years ago, evolutionary biologist Richard Lenski of Michigan State University in East Lansing, US, took a single Escherichia coli bacterium and used its descendants to found 12 laboratory populations.
The 12 have been growing ever since, gradually accumulating mutations and evolving for more than 44,000 generations, while Lenski watches what happens.
So, there we have it. Genuinely new complexity from naturally selected random change! Woooo! Go Science!