funding for this program is provided by annenberg media. narrator: of all the planets in our solar system, none can support complex life except earth. but the conditions on earth were not always so suitable for life, and scientists are working to reconstruct the history of two essential requirements whke tlanet hatable. the first is free oxygen in the atmosphere. the other is a moderate and stable climate. paleontologist andy knoll has spent much of his career examining earth's rock record to piece together the history of early life and the rise of oxygen. there's this close waltz, if you will, between the history of earth's physical environments and the history of life. narrator: geologist paul hoffman studies the time in our planet's history when the climate was much more extreme, a period called the snowball earth. hoffman: during the maximum cold of the snowball earth, you have an earth that's more like mars than it is like earth. narrator: hoffman is seeking answers to how the earth entered this snowball and, more important for us, how

annenberg media ♪ annenberg media ♪ world war ii was over. gis were home raising the roof, hoping to raise a family. how could they realize the american dream, a home of their own? when foreign steelmakers were pricing america's largest steel mills out of the market, how could a minimill hope to compete? baseball. for millions, a pastime. for a few, a paycheck. why was this man's paycheck half a million dollars a year? a visit to a modern shopping center yields a bewildering display of goods and services, the products of a free market. who's to say this is what we want or what we need? markets-- do they meet our needs? economic analyst richard gill will help us examine that question on economics usa. i'm david schoumacher. the american economy-- the sum total of what goes on in shopping malls, car dealers, supermarkets, stock exchanges, 7-elevens, and gas stations all around the country. millions of buy-and-sell decisions everday, trillions of dollars worth every year. and those decisions affect every part of our lives. who decides between a

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california farm bureau federation] [captioned by the national captioning institute --www.ncicap.org--] annenberg media ♪ by: