We got serious about agriculture about 10,000-12,000 years ago. Jared Diamond wrote a well-known popular article for Discovery Magazine in 1987 about that transition called “The Worst Mistake in the History of the Human Race.” It’s still quite valid, except that Diamond didn’t understand that agriculture wasn’t a choice, it was an adaptation to the decline and/or extinction of the large herbivores our ancestors lived on for the first two million years. He documents the evidence that humans got smaller and less healthy, as well as more numerous. Once there was land and resources to hoard and protect, social inequity and intergroup conflicts, wars, began.
Large populations affect the regions they inhabit, typically with habitat degradation. That’s been documented many times. However, paleoecologist William Ruddiman carries this concept much further. He presents evidence that when we started agriculture, we began to measurably add to the greenhouse gas content of the Earth’s atmosphere. In 2005, Scientific American published his article “How Did Humans First Alter Global Climate?“