In hunter gathering the amount of hunting that can be maintained is set by the surplus from gatering and the yield from hunting. As populations intensify big game is hunted to the level which cannot maintain hunting by big game specialists (Winterhalder and Lu).
This required a change in diet. In part an increased consumption of small animals (Stiner et al) and an intensification of plant gathering which precedes agriculture proper
There was a dramatic increase in range of small topped and groundstone tools after 15,000 BP, reflecting the need to diversify hunting and gathering. People only began to use the technologioes that underpinned agriculture only after 15,000 Bp (Bettinger 2000). Many of the tools needed were similar, whether digging for tubers became the space and hoe, or cutting of crop became the synth.
Hunter gathering managed risk of yield variation, for example risk of a failed hunt, or of a storm disrupting gathering, through sharing. Human group living, the tribe or clan, indeed the family is a form of insurance against bad times. The support of the group helped minimise risk of starvation for everyone. Man did not emerge as a methodological individual but as a social animal.
The shift to agriculture involved an expanded strategy for coping with yield variation. Multiple crops, a variety of fields, and development of alternative food sources, such as through contined but specialised hunting (such as fishing) and domestication of animals.
Although agriculture developed in the Near East in the early holocene about 9-11,000 BP. and then rapidly diffused to Western Asia, there were around 8-10 independent sources of agriculture, with China appearing to be an independent source about the same time.
The lead modern thesis is that climate change at the end of the last Glacial Age played a critical part.
The reduction in climate variability, increase in CO2 content of the atmosphere, and increases in rainfall rather abruptly changed the earth from a regime where agriculture was impossible everywhere to one where it was possible in many places. (Richerson, Boyd, & Bettinger 2001)
Population pressure theories (Cohen 1977) posit slowly accumulating global scale population pressure as responsible for origin of agriculture.
But as Richerson et al ask why not earlier, humans essentially modern 30, 000 years earlier. Under their models Asia is filled up to its population carrying capacity from early agriculture in only 1,400 years.
The ending of the last ice age seems to provide an explanation, it could not have happened sooner. What is more previous interglacials appear to have been short lived and highly variable, without several thousand years of steeled stable, warm, wet conditions encouraging increased plant gathering.
This sets out a potential, but what triggered the innovation?
With a human population living at low hunter gatherer densities and its food sources only able to sustain low population & pressure on population from declining hunting requiring increased plant gathering any improved method in plant intensification, either in terms of methods of proto-agriculture/intensified gathering or agriculture itself
The Buserup thesis states – any group that that can use land more effiiciently will, all things being equal, be able to evict a less efficient group.
‘An agricultural frontier will tend to expand at the expense of hunger gatherers as rising population densities on the farmer side of the frontier motivate pioneers to invest in acquiring land from less efficient users’
‘become richer through farming, or a dismal choice of flight, submission or military defence at long odds against a more numerous foe’. (Richerson, Boyd, & Bettinger 2001)
The ‘Africa pump’ allowed new wave of immigration out of africa by homo sapiens adapted to intensified huinting and gathering tactics in warmer latitudes. A warming earth allowed their own frontier to extend northwards as reduction of steppe habitats drove megafauna south. The result extinction. The extinction may have been most rapid in North America because man came from the north not south and so would have required a greater proportion of meat in their diets and proportionately less on gathering/proto agriculture. The tools and techniques were more specialised for hunting. But the success of the invasion of the Clovis people sealed their own fate as they destroyed their source of food so rapidly.
The rapid extinctions drove methane into the atmosphere and this increased warming, rapid feedback may have increased the rate of global warming and helped ensure that the non-glacial period was extended. Man had helped create a climate window ensuring his rise to dominence.
Plant intensive communities as they became dominant could drive out hunter gatherers along an expanding margin of cultivation, as they could support much higher populations. Individual hunter gatherers may have been better fed, but they were few. Groups that had innovated because poor local conditions forced innovation or death at times of variabilty of food supply could then advance into better territory.
Robert Braidwoods Research, and modern geentic testing of wheats, suggest that the slopes of the Zagros Mountains in South East Turkey were a key source of innovation spreading down to the dertile crescent (much wetter and fertile than it is today).
Around 11,000 years ago hunter gatherers were collecting wild seeds, probably the ancestors of Wheat and Barley, and were hunting the ancestors of sheep and goats, by 9000 BP they were settled in villages and were cultivating early varieties of Wheat and Barley. There is evidence from several part of the world of settlement prior to agriculture, but only where intensification of plant gathering made nomadic hunting and gathering unnecessary.
How then were seeds and animals brought into the domestic sphere. The key is understanding the co-evolution of those plants and animals and humans.