Short answers, pulled from the story.
The development of agriculture began about 12,000 years ago, when humans switched from nomadic hunter-gatherer lifestyles to permanent settlements and farming. Wild grains had been collected and eaten from at least 104,000 years ago, but domestication came much later, with the earliest evidence of small-scale cultivation around 21,000 BC.
Agriculture began independently in at least eleven separate regions of the Old and New World, each an independent center of origin. The eight Neolithic founder crops, including emmer wheat, einkorn wheat, and hulled barley, were cultivated in the Levant by around 9500 BC.
The eight Neolithic founder crops were emmer wheat, einkorn wheat, hulled barley, peas, lentils, bitter vetch, chickpeas, and flax. They appear more or less simultaneously on Pre-Pottery Neolithic B sites in the Levant after 9500 BC, with wheat possibly the first grown on a significant scale.
Localised climate change is the favoured explanation for the origins of agriculture in the Levant. After the last ice age around 11,000 BC, long dry seasons favoured annual plants that leave dormant seeds or tubers, and an abundance of storable wild grains and pulses let hunter-gatherers form the first settled villages.
The Americas lacked large-seeded grains like wheat and barley and lacked large domestic animals for agricultural labor. Prehistoric American agriculture usually cultivated many crops close together using only hand labor, and after thousands of years of selective breeding maize produced two and one-half times more calories per acre than wheat and barley.
The British Agricultural Revolution between the 17th and mid-19th century raised wheat productivity from 19 bushels per acre in 1720 to around 30 bushels by 1840. The Haber-Bosch method for synthesizing ammonium nitrate, commercialized by Carl Bosch in 1910, and the Green Revolution led by Norman Borlaug, credited with saving over a billion people from starvation, transformed crop yields.