Agriculture began about 50 thousand years ago, because the ice sheet of the previous ice age was maximum about then. An ice age has been occurring like clockwork every 100 thousand years for 10 previous cycles.

The ice sheet would draw moisture out of the air, as dehumidifiers and air conditioners do. That's because cold air does not hold as much water, so it condenses and precipitates moisture in the air. The result is that while the ice sheet increases from the precipitated moisture, the rest of the air gets very dry; and dry air causes temperatures to increase due to less evaporative cooling and cloud cover.

So Planet Earth was very hot and dry everywhere else while the ice sheet existed, which of course would have been bad for agriculture, except in front of the ice sheet. The ice sheet would have formed a sharp cliff due to the dry air in front of it. As cold air dropped down from the ice sheet, it would have created a cold fog in the lowlands with hot dry air everywhere else—perfect conditions for creating agriculture from scratch.

The primitives could walk from cold wet areas to hot dry areas in front of the ice sheet. So they could pick conditions suitable for growing things; and could be metaphorically growing bananas alongside of wheat.

All types of vegetation would have migrated into most favorable areas, as conditions changed in front of the ice sheet. So humans could find any type of seeds and places to plant them within walking distances.