People, like most animals, are naturally lazy. So the ascent of mankind is something of a

mystery. Humans who make their livings hunting and gathering in the traditional way do not have to put much effort into it. Farmers who rely on rain to water their crops work significantly harder, and lead unhealthier lives. But the real back-breaking is that carried out by farmers who use irrigation. Yet it was the invention of irrigation, at first sight so harmful to its practitioners that actually produced a sufficient surplus to feed the priests, scholars, artists and so on whose activities are collectively thought of as "civilization".

In the past 10,000 years, the world's climate has become temporarily colder and drier on several occasions. The first of these, known as the Younger Dryas, after a tundra-loving plant that thrived during it, occurred at the same time as the beginning of agriculture in northern Mesopotamia. It is widely believed that this was not a coincidence. The drying and cooling of the Younger Dryas adversely affected the food supply of hunter-gatherers. That would have created an incentive for agriculture to spread once some bright spark invented it.

Why farmers then moved on to irrigation is, however, far from clear. But Harvey Weiss, of Yale University, thinks he knows. Dr. Weiss observes that the development of irrigation coincides with a second cool, dry period, some 8,200 years ago. His analysis of rainfall patterns in the area suggests that rainfall in agriculture's upper-Mesopotamian heartland would, at this time, have fallen below the level needed to sustain farming reliably. Farmers would thus have been forced out of the area in search of other opportunities.

Once again, an innovative spark was required. But it clearly occurred to some of these displaced farmers that the slow-moving waters of the lower Tigris and Euphrates, near sea level, could be diverted using canals and used to water crops. And the rest, as the cliché has it, is history.

So climate change helped to intensify agriculture, and thus start civilization. But an equally intriguing idea is that the spread of agriculture caused climate change. In this case, the presumed criminal is forest clearance. Most of the land cultivated by early farmers in the Middle East would have been forested. When the trees that grew there were cleared, the carbon they contained ended up in the atmosphere as carbon dioxide. Moreover, one form. of farming—the cultivation of rice in waterlogged fields—generates methane, in large quantities. William Ruddiman, of the University of Virginia, explained that, in combination, these two phenomena had warmed the atmosphere prior to the start of the industrial era. As environmentalists are wont to observe, mankind is part of nature. These studies show just how intimate the relationship is.

The invention of irrigation is meaningful because it could help to

A．alleviate farmers' workload.

B．increase agricultural production.

C．make planting much easier.

D．get rid of human laziness.