Cities can't rise unless food and water exist in abundance to feed large groups of people day after day.

So, agriculture makes a city possible. The first cities, thousands of years ago, did not develop until people in the area had mastered the art of sowing seeds and harvesting crops.

Between 10,000 and 12,000 years ago, people started cultivating crops in the area we call the Fertile Crescent--Iran, Turkey, Syria, the Jordan River Valley, the Zagros Mountains. It's not called "fertile" because it's lush and green--just the opposite. But it is where men and women first deliberately started planting seeds, making sure the plants got water, building fences to keep animals from eating the plants, and picking the grain when it was ripe.

Maybe they figured out how to make pots for storage before they started planting fields. In fact, maybe that's why they started planting--because they had a place to keep food. Or maybe pottery came later. No one knows.

It's a massive change. Think about it: Everywhere in the world for tens of thousands of years, people followed the food. In small tribes, in family groups, they followed herds of animals, or made the rounds of different areas where food was plentiful at different times of the year. In spring, they looked for the wild carrots and edible greens. In late summer, they went where the fruit trees and the berry bushes grew. Theyknew where to fish at different times, and where to hunt.

Life revolved around moving.

RIght after evidence of deliberate agriculture and plowed fields, comes archaeological remains of bigger villages, with pottery and houses. Why did people decide to build houses and stay put when their lives and traditions were built on mobility?

No one knows. Farming is more labor intensive than hunting and gathering, in most areas. Our ancestors who traveled around and foraged for food probably enjoyed more leisure time than we do.

Simply by picking the grain they wanted--the fatter stalks, where the grain stuck to the stalk, people changed the grain. It was evolution in action. Because early farmers picked, then planted, the fatter stalks of grain, over and over, domesticated grain very quickly became different than wild grain.

Within 200 or 300 years, the wild varieties of plants disappeared from an area, indicating that all plants were being grown from stored seeds.

When people continued to live in the same area, they also began keeping domesticated animals, because they needed the meat and dairy products. Sheep hearding started in the Zagros Mountains around 9,000 BC.

Scientists think that all sorts of social controls and institutions arose after this, mainly to control and protect the food supplies. The famous walls of Jericho, for instance, went up around 8,000 BC.

Besides the fact that everyone had to work harder when they settled in one area and planted crops, there are some other problems. Growing only one crop, or a few select crops on the same ground year after year depletes the ground of nutrients. The planting and cultivation drives away other plants and animals.

A growing population becomes dependent upon the food grown, because nothing else is available. If there's a drought or famine or blight or pest, the entire village can starve.

So . . . why did this agricultural revolution happen?

Our ancestors could not look ahead and see the benefits of 10,000 years of civilization. So why did they all change their lives?

It may be something we can't even guess at: climate change, enemies, flood . . . who knows?