An Experiment for the Ages
Inside a lab at Michigan State University, the world’s longest-running evolution experiment is taking place. The Long-Term Evolution Experiment, or LTEE, began on February 24, 1988. On that day, biologist Richard Lenski took a single Escherichia coli bacterium and used its descendants to seed 12 identical populations in separate flasks. For more than three decades, these populations have been evolving independently in a controlled environment.
The daily routine is meticulously simple. Each morning, a researcher transfers 1% of the bacterial culture from each flask into a new flask containing a fresh supply of a glucose-limited growth medium. The bacteria thrive for a few hours until the glucose is consumed, then enter a state of starvation until the next day's transfer. This daily cycle produces about 6.64 generations of bacterial growth. The experiment has now surpassed 80,000 generations. To preserve this evolutionary history, samples from each population are frozen every 500 generations, creating a "frozen fossil record." Scientists can thaw bacteria from any point in the past and compare them directly to their living descendants.
A Taste for Something New
The growth medium in the flasks contains a second potential food source: citrate. Wild E. coli cannot metabolize citrate when oxygen is present, which is an important diagnostic feature of the species. For over 31,000 generations, none of the 12 bacterial lines could touch it. Then, something remarkable happened in the flask designated Ara-3.
Around generation 31,500, the liquid in that flask became significantly more cloudy, a sign of a much larger bacterial population. Analysis showed that these bacteria had evolved the ability to eat citrate, tapping into the abundant food source their ancestors could not use. This new skill, called Cit+, allowed the population to grow much larger than the other 11 lines.
By studying the frozen fossil record, researchers discovered the evolution of this trait was a two-step process. An initial "potentiating" mutation appeared sometime after generation 20,000. This first mutation did not enable citrate consumption, but it made a subsequent mutation—a gene duplication event that activated the function around generation 31,500—possible. This discovery showed how a complex new trait can arise through a combination of historical contingency and selection. Even after tens of thousands of additional generations, none of the other 11 populations have repeated this specific evolutionary feat.