Which number is main idea? Please help me.
(1) If we compressed the entire history of life on the planet into a single year, the first modern human would not appear until December 31 at about 11:53 p.m., and the first civilizations would emerge only about a minute before the end of the year. (2) Yet humanity's achievements in its brief history on Earth have been remarkable. (3) Some 15,000 years ago, our ancestors practiced religious rituals and painted superb pictures on the walls of their caves. (4) Around 11,000 years ago, some human groups began to domesticate animals and plants, thereby freeing themselves from total dependence on hunting and gathering food. (5) About 6,000 years ago, people began to live in cities, to specialize in different forms of labor, to divide into social classes, and to create distinct political and economic institutions. (6) Within a few thousand years empires were created, linking isolated groups and bringing millions under centralized rule. (7) Advanced agricultural practices improved farming, resulting in growing populations and the emergence of large nation-states. (8) A mere 250 years ago the Industrial Revolution began, thrusting us into the modern world of factories and computers, jets and nuclear reactors, instantaneous global communications, and terrifying military technologies.
