Ideas on the climate during the end of the last glacial age (the era between the Pleistocene the modern Holecene era the last 11,000 years) and have been central to different theories on the origins of agriculture.
In the last glacial age the earth was dry. During the Last Glacial Maximum the Sahara desert was much larger than today extending into areas today covered by tropical forests. The waxing and waning of the Sahara provides the basis of the ‘Sahara Pump‘ theory of how during harsh times it drove waves of emigration ‘Out of Africa’ including of course Man.
Vast stocks of the earth’s water was trapped in ice during glacial ages. Furthermore permafrost locked up much of the earths CO2 in terms of frozen vegetation and gases from the previous interglacial. These two factors meant that the earth was not as lush a place as it is today. Lack of rain and CO2, and sharp changes in climate over short time periods. restricted the pattern of growth of vegetation, especially at the termperate fringes most conducive to easy habitation today. Steppe tundra dominated latitudes away from the tropics, covering much larger areas than today. Intense sunlight during summers and loess soils encouraged mosses, lichens, grasses, and low shrubs that fed mammoths, horses, bison, giant deer, aurochs and reindeer. Such plants were less suited to the stomachs of humans who evolved to eat berries and meat on the savannah and forest edge. So man was forced to hunt magafauna and gather what berries and edible plants they could at temperate latitudes.
As the ice retreated the earth became an increasingly lush, and wet planet with greater humidity and CO2. The earth quite literally became a greenhouse leading to increased potential for plant growth – we call these phases pluvial periods. During the Pleistocene, CO2 levels were less than 200 parts per million, this rose to above 250 ppm as the earth pulled out of the glacial age. This may have increased plant productivity by up to fifty percent
This scenario, derived from modern data from ice and ocean cores, is the precise opposite to the ‘Oasis Theory’ (V.G.Childe) of the origins of agriculture, of the post glacial period being dry with humans and animals forced into contact around retreating sources of water. It also challenges the ‘Tuber Theory’ that these conditions favoured annual plants which die off in the long dry season, leaving a dormant seed or tuber. These plants put more energy into producing seeds than into woody growth.
These ideas are more applicable to dry periods, starting around 3,900 BC, when the ‘Green Sahara’ subsided and deserts advanced. Climate change at this time may help explain the origins of civilisation, but not the origins of agriculture which best estimates show occurred during a pluvial period at the beginning of the Holcene.
We tend to discuss a ‘neolithic revolution’ – a transition from hunting and gathering to agriculture and settlement during the Neolithic (New Stone Age) period. Whilst not disputing the revolutionary changes that occurred in this period the term ‘neolithic revolution’ can confuse the issues. It is important to look at the relationship and different pressures and paces of the different but related transitions, from hunting and gathering to cultivation and domestication of animals, from nomadism to settlement. The problem with the catch all phrase the ‘neolithic revolution’ is that it can easily lead to a caricature of before the revolution hunting and gathering after agriculture and settlement.
To such a sudden shift of all social relations and ways of subsistence one would have to ask why would people choose to undertake such a change, some have argued that it is by no means obvious that such a shift had obvious benefits – indeed Jared Diamond has called it ‘The Worst Mistake in the History of the Human Race’. We are wrong if we assume that the change from hunter gathering to farming bought an improvement in the quality of the human life or in the humans themselves. Skeletal evidence reveals that hunter gatherers were in fact, taller, better nourished, suffered less disease and lived longer than farmers. The gathering of wild grains produces more calories of food for each calorie of energy invested than any form of agriculture. Settled agriculturists can survive at higher population densities estimated to be 10-100 times greater than hunter-gatherers. So the question is what population pressures and pressures for resources led to higher population densities requiring agriculture.
The ‘worst mistake’ conceptualisation I believe is wrong but understanding why you need to unpick the separate strands, why for example there were a transition to pastoral nomadism, rather than settled agrarianism, in some areas, and what are the connections between the origins of agriculture and the origins of settlement?
The term ‘neolithic revolution’ (again a phrase from Vere Gordon Childe in the 1920s) also confuses the key changes we believe began during the beginning of the Holecene era (which began at the end of the last glacial age around 11,700 years ago) from those that fully evolved during the Meolithic (Middle Stone Age 10,000 years ago, to around 6,000 years ago), and into the Neolithic that followed. Rather than a single revolution there were a series of changes which helped trigger each other and came to fruition in the Neolithic. By the Neolithic only one of the several sub-species of humans has survived.
Humans in many different areas of the earth took up farming in what is, set against the 500,000 year age span of modern humans, a very short time. There may even have been independent discovery in some regions. This is the most compelling evidence that global climate change, and the resultant adaptations by vegetation, were the key trigger to the beginning of agriculture.
Peter Richerson, Robert Boyd, and Robert Bettinger. have advanced the thesis that agriculture was impossible during the pleistocene but mandatory during the Holocene “Was Agriculture Impossible during the Pleistocene but Mandatory during the Holocene?“. ( Richerson, Peter J.; et al. (2001) American Antiquity 66 (3): 387–411)) This hypothesis has been widely popularised in Ronald Wrigtht’s Book ‘A Short History of Progress‘.
I believe this thesis to be essentially correct but incomplete. It needs to be underpinned it needs to be underpinned by a hard nosed understanding of pressure on resources and demographic and technological change, and in particular a parallel analysis of the origins of social control and political power. In other words reconstructing political economy from the origins of human societies.