The article summarizes the study (which is at journals.plos.org):

The study shows that from about 7000 to 5000 years ago, the Persian Gulf extended farther inland, and tides pushed freshwater twice daily far into the lower reaches of the Tigris and Euphrates. The scholars propose that the early communities must have harnessed this dependable hydrology using short canals to irrigate crops and date groves, enabling high-yield agriculture without the need for large-scale infrastructure.

rivers built deltas at the head of the Gulf, tidal access to the interior was cut off. The resulting loss of tides likely triggered an ecological and economic crisis"one that required an ambitious societal response. The extensive works for irrigation and flood protection that followed ultimately came to define the golden age of Sumer.

"We often picture ancient landscapes as static," says [one researcher]. "But the Mesopotamian delta was anything but. Its restless, shifting land demanded ingenuity and cooperation, sparking some of history's first intensive farming and pioneering bold social experiments."