In spite of various fringe historians claiming to have found remains of Ice Age civilizations on lost continents, Atlantis and what not, there is - to the best of my knowledge - no tangible evidence of any agricultural, city-state forming civilization before ca. 7000 BCE (Çatalhöyük and a few others). This is 40,000 years after the emergence of behavioral modernity with little change physiologically and psychologically to modern humans after this adaptation.
Then, however, agriculture and city-state civilizations were emerging independently many times in various locations within a few thousand years (dates of earliest evidence for agriculture and cities):
- Ancient Middle East (10000 - 7000 BCE, Çatalhöyük and others)
- Ancient China (9000 - 2000 BCE)
- Ancient Mexico (5000 - 2500 BCE)
- Ancient Peru (8000 - 3500 BCE)
- Prehistoric North Africa (6000 - 3500 BCE, perhaps also, possibly independently, Ethiopia and the Sahel) - (May have been influenced by ancient Middle East).
- Ancient India/Pakistan (7000 - 3300 BCE) - (May have been influenced by ancient Middle East).
- Prehistoric eastern woodlands in present day US (5000 BCE - 1000 CE) - (May have been influenced by ancient Mexico.)
- Ancient New Guinea (7000 BCE) - (Agricultural revolution only, no city states.)
Some of these cultures may perhaps have emerged under the influence of others but some were doubtlessly independent.
This pattern (40,000 years nothing, then eight or so events within 10,000 years) is statistically unlikely. It would seem that this is connected to the end of the last ice age (about 10000 BCE). But why would agriculture and city states not emerge during the Ice Age?
Is this question being discussed among historians and/or archaeologists? If so, what is/are the main theories?
Some possible explanations I could imagine:
Much fertile land was covered by ice sheets or had sub-polar climate, hence impractical for agriculture. Access to some land masses (the Americas) was further blocked by ice sheets (and oceans). On half as much land only half as many hunter-gatherer groups could exist, reducing the statistical probability of any of them making the transition to agricultural civilizations (but would it reduce the probability that much that it would explain the observed bias? Also with lower sea levels, the available land area would increase again).
I remember reading about the climate being significantly more arid during the Ice Age. This would lead to significantly lower population densities and would be a sufficient explanation (but then again there must have been wet, warm, tropical areas; otherwise tropical rain forest species would not have survived. And why would the climate have been more arid everywhere? It is not that there was no liquid water any more).
City states would have developed first in low lands and along costs - with lower sea levels this would all be under water today which may be why no direct evidence has been found (but why would there be no indirect evidence, e.g. no plant or animal species (except dogs) domesticated earlier than 10000 BCE?)
Some other explanation connected to absorption of energy from the sunlight? Earth's albedo would have been higher during the Ice Age, thus less energy should have been absorbed which would lead to a globally lower carrying capacity of the planet for plants, animals, humans, biomass in general (but again, this should probably not be enough to explain the observed bias).
Status update (March 4, 2017)
There were some very interesting discussions with many good arguments about technology, domestication, Yuval Harari's "Sapiens", the extinction of megafauna, and drowned lands.
There was one answer (by PhillS) that suggests the end of the Ice Age together with unsteady climate brought about by the Younger Dryas as the possible main reason for the development of agriculture (in time leading to city-states). It cites a work by Steven Mithen where this is argued. This is a very interesting aspect, summarized in a very good way. But it does not answer all the questions: 1. There were climatically turbulent periods (like the Younger Dryas) before, during the Ice Age. 2. The argument of the impact of the Younger Dryas seems to apply primarily to the development of agriculture in the ancient Middle East, but not to the other cited examples. 3. I agree that the end of the Ice Age likely played a part. But why?
An interesting comment (by mart) conversely states that the long period of climatic stability after the Ice Age (and compared to the Ice Age) may be a prerequisite of agriculture and complex societies. Wikipedia has a graph of the mean temperatures during the Last Glacial Maximum, it indeed shows periodic turbulence during the Last Glacial Maximum before it slowly stabilizes from ca 20000 BCE.
Another answer (by Carni) cites the extensive energy requirements for agriculture and specialization in city state societies, which were perhaps only available after the end of the Ice Age. As discussed in the comments to the question, this does, however, not address why these conditions would not be found anywhere during the Ice Age (if not in the subtropic region then perhaps in the tropics?).
Of some interest is a vegetation zone map during the Last Glacial Maximum (also from Wikipedia). It shows large areas of polar and tropical desert, but there are extensive savannahs around the Mediterranean and tropical grassland and rainforest in Sub-Saharan Africa, India, South East Asia, as well as Australia and New Guinea (and South America, but this would not have been settled at the time). It also shows woodlands etc. in China and Japan. Much of this actually sounds climatically rather pleasant.
Edit (Sep 2020, 3 years later)
One more thing I missed back then: Some evidence of earlier attempts at cultivation of "proto-weeds" during the ice age has been found. see Snir et al. (2015) on the 23000 year old Ohalo II site near the Sea of Gallilee. This would suggest that before climate stabilized after the end of the Ice Age, some hunter-gatherer communities were in a much better position to embark on an agricultural revolution and form city-states etc. than other groups ever before.