No, it has nothing to do with Keith Urban. (Will the jokes be this bad all season?) It has to do with a seemingly irreversible trend in human history toward larger and larger social units. Data is spotty before 1950, but the evidence does seem to point toward some interesting trends. For millennia after human beings first left Africa and began to spread around the globe onto all of the continents save Antarctica, human population density may have fallen. The math is fairly easy to understand. As we spread out to inhabit more territory, our geographic reach outran our biological fecundity and our species may have lost ground in terms of population density; not that it mattered much at the time. For millennia after that, as our ancestors did the practical research that located good spots for finding
game and gathering food, we likely lived in small tribes as we spread out to exploit diverse niches around the planet. Population density likely stabilized for a long period. Then something unique happened in and around the alluvial flood plains of the Tigris and Euphrates rivers, the Indus river, the Yellow and Yangtze rivers and the Nile river; in the geographical blink of an eye, we learned to stop chasing after our food (hunting and gathering) and learned how to make the food grow where we wanted it (agriculture and animal domestication). Not all ground is fertile, so populations living on farmed products tended to increase around fertile fields and lush grazing lands. The first permanent settlements were established as we dug in (literally) to make the soil give us bread (and beer!) The first cities were born. Recorded history covers only this short and recent period of relatively settled life and it seems that urbanization and written language go hand in hand, so it is in some sense fair to note that history as a human practice is in fact an urban phenomenon.