A Chicago scientist has shed some light on one of evolution’s most stubborn mysteries by resurrecting 800-million-year-old genes in the lab.

The basics of evolution are pretty well understood: random genetic mutations over time lead to ever-more complex living things. But at tiny scales, there’s a puzzle: Scientists have had a hard time explaining how tiny, complicated structures in our cells referred to as “molecular machines” evolved.

Joe Thornton, a professor of human genetics at the University of Chicago, used a technique he calls “molecular time travel” to recreate ancient genes and actually built the primitive cellular machine. Then he observed how common processes led to highly intricate structures.

“Increases in complexity don’t require these very low probability, hard to imagine mutations,” said Thornton, who is also on faculty at the University of Oregon.. “Complexity can increase through very routine mechanisms which happen all the time.”

Explaining that complexity would address a point raised by skeptics of evolution. Some proponents of “intelligent design” have asserted that molecular machines are “irreducibly complex,” and therefore could not have evolved from a simpler form. 

Thornton’s results are published in the journal, Nature.