What's the Most Important Invention to Bring Back in Time?

We've never been able to selectively breed a new breed of corn from the original Tia site. This suggests it might be really hard if you're trapped in the past to bring back the corn you enjoy. It took us a long time as a species to figure out what the organs do because most cultures have a taboo against cutting open bodies. I'm told it feels really weird. But it's not medicine that are the most important invention, in my opinion.