I don't care to explain like to a five year old, but the authors of the paper propose that only the modern human lineage have a total of four premolars in the lower jaw with a single root each. They claim these were never present in other hominid species.

I was under the impression that it was because humans learned to control fire, meaning we could eat softer food which altered the human teeth. However, 7.2m years massively predates our current understanding of when fire was first utilised..