From Harvesting the Biosphere:
During the late Paleolithic, the climate was too cold and C02 levels too low for agriculture to develop.
The shift from foraging to pastoralism meant a 100x increase in the number of people that could be supported for the same amount of land.
Pastoralism also unlocked previously inaccessible energy stored in grassland. By grazing animals, then eating them, people could survive in places too arid for cropping.
Shifting cultivation produced between 6-30x the energy expended doing the farming.
Traditional agriculture hit a ceiling around 1850. It could not produce enough food for expanding populations, rapid industrialization and urbanization.
Before the discovery of the Haber process, soil replentishment was the major bottleneck. Nitrogen and phosphorus depletion in the soil limited crop yield and how often you could grow. Slash and burn agriculture and crop rotation provided limited workarounds.
Modern agriculture (1900s-1950) has ratcheted up the people-per-land ratio even more. The Green Revolution, synthetic NPK fertilizer, industrial automation and new crop varieties are the driving forces. There are now 7.4bn of us.
Related: fundamental needs.