One question I have always thought interesting is why agriculture cropped up independently in Mesopotamia, China, North America, and central Africa, all between 10,000 and 5,000 years ago. Anatomically modern humans developed in East Africa something like 100,000 to 200,000 years ago. They then left Africa something like 80,000 years ago. For something like the first 100,000 years of our existence, we didn't develop agriculture, but within the past 10,000 years we developed it in at least 4 areas independently. What happened 10,000 years ago that created this shift?
There is no consensus on this question. Here are some relevant facts: 1) While technically the Earth's been in an ice age for the past 2.6 million years, we're currently in the midst of a relatively warmer time called an "interglacial" period. This warmer period began 10,000 years ago. Prior to that, from 110,000 years ago to 10,000 years ago, was a "glacial" period, in which glaciers covered much of Europe and North America. 2) There was a great number of extinctions ("Late Pleistocene Extinction Event") of large animals between 50,000 and 10,000 years ago. These extinctions tend to coincide with the arrival of humans into each animals ecosystem. For the most part these large animals were naive to human hunters and did not have time to evolve to be afraid of those spindly humans. For the most part this did not occur in Africa, where large mammals were exposed to human hunters from the very beginning and were thus able to evolve as the humans' hunting skills improved.
Everything below this line is pure speculation.
It would seem that the almost simultaneous (geologically speaking) development of agriculture at multiple locations around the globe occurring soon after the last glacial period ended is not a coincidence. The changing weather and ecosystems that came with the warmer climate must have somehow led to agriculture. And with hunting becoming increasingly difficult as easy to kill naive large mammals were depleted, humans everywhere likely became increasingly dependent on plants for calories.
Another factor that may have come into play: once humans had spread around the globe, it is thought that human population reached equilibrium at somewhere around one million people. Equilibrium of course means that humans were competing with each other for limited resources, thus human tribes were likely in close proximity to each other, fighting over territory, some tribes starving to death or being decimated by their neighbors, etc. In such an environment, agriculture would be ripe for development as, once it was developed, it would allow for much higher population densities. It seems that something about the end of the ice age combined with hunter/gatherer humans existing at maximum capacity must have led to the development of agriculture.
Another interesting point that I read about last year in The Economist was a computer model that demonstrated the importance of population density in maintaining knowledge. This model showed that in a non-literate culture, where technology is learned directly from other humans, knowledge will tend to be forgotten over the millennia if there is an insufficient population density to keep that knowledge alive. Thus perhaps the beginnings of agriculture may have developed in many different places at many different times, but couldn't survive to maturity unless it was in a region where lots of humans were living. Thus, it could be that something about the end of the ice age created better conditions for hunter/gatherers in Mesopotamia, China, and the Americas that allowed for higher population densities, thus allowing agricultural technology to mature.