life, and scientists are working to reconstruct the history of two essential requirements whmake tlanet hatablee first is free oxygen in the atmosphere. the other is a moderate and stable climate. paleontologist andy knoll has spent much of his career

life, and scientists are working to reconstruct the history of two essential requirements whmake 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 it returned to warmer conditions and has maintained a stable and moderate climate. both of these scientists look to the deep past for clues to the present enriching our understanding of the one place we know in the universe where life flourishes -- earth,he habitab

life, and scientists are working to reconstruct the history of two essential requirements whmake 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 it returned to warmer conditions and has maintained a stable and moderate climate. both of these scientists look to the deep past for clues to the present, enriching our understanding of the one place we know in the universe where life flourishes -- eah's inedler