Why did human civilization arise now?
(Where by now, I mean ten thousand years ago).
New climatologic research (well, new in the past couple decades) suggests that civilization arose in a unique climate:
The new view of climate was reinforced by one of the last great achievements of the Soviet Union, an ice core drilled with French collaboration at Vostok in Antarctica. The record reached back through nearly four complete glacial-interglacial cycles — and drastic temperature changes peppered almost every stretch of data. This Antarctic record was too fuzzy to say whether any of these changes had come and gone on the decade-size timescale of the Younger Dryas. But warm interglacial periods had certainly been subject to big swings of temperature lasting for centuries. Especially striking to the researchers, by contrast, was our own era, the ten thousand years since the last glaciation. It was, "by far, the longest stable warm period recorded in Antarctica during the past 420 [thousand years]." When Bryson, Schneider, and others had warned that the century or so of stability in recent memory did not reflect "normal" long-term variations, they had touched on an instability grander than they guessed. (See above) The entire rise of human civilization since the end of the Younger Dryas had taken place during a period of warm, stable climate that was unique in the long record. The climate known to history seemed to be a lucky anomaly.
It turns out that the ice-age period of the last evolutionary leap of humanity wasn't a smooth and continuous era that gradually came to an end. Instead, for the better part of the past sixty thousand years, the climate of this planet has whipsawed back and forth between two extremes - the current warm,wet period and ice-age-like periods that were cold, dry, and windy - most often on 750 or 1500-year cycles.
What's more, it seems the transition times into and out of these violent extremes of climate were neither long nor easy. Instead, the planet gradually warmed up until a certain threshold was reached - like the slow lifting of a light switch until it clicks - and when that threshold was reached, the planet was suddenly plunged into an ice age. Once the switch was thrown, the typical transition time from warmth to ice was between three and twelve years.
I think humans naturally tend to a more anthrocentric viewpoint, where civilization arose because we had evolved enough to create it. It is fascinating and somewhat disturbing to think that we may have had the capacity for tens of thousands of years, but the weather just wasn't cooperating.
By the way, this same climate theory suggests that the greatest danger from global warming may be global cooling. That is, anthropogenic climate increases could hasten the onset of the switch in climate modes, changing ocean circulation and plunging us into an ice age. How credible such a theory is depends on your time scale. It is certainly tempting to look at the history of human civilization and dismiss a climate pattern that has never been seen, but if you zoom out an order of magnitude, you'll find that such weather is the historical norm, not the exception.
It will be quite ironic if we go through the switch, and CO2 emission becomes a positive externality, subsidized for its useful warming characteristics.