The Lost Worlds of Planet Earth 1 NEIL DEGRASSE TYSON: Yes, this is home. This is Earth. Having trouble finding a familiar continent? The past is another planet. Actually, many. I'm standing on the great expanse of time that has elapsed since the Big Bang. In order to think about it, we've compressed it all into a single year. It's the early morning of December 23 on this Cosmic Calendar of ours, or about 350 million years ago, when our world was a mere four billion years old. Earth looks so different. You might not even know the place. The stars wouldn't help you. Even the constellations would have been different back then. The dinosaurs were still more than 100 million years in the future. There were no birds, no flowers. And the air was different, too. The atmosphere had more oxygen than at any other time in Earth's history, before or since. This allowed insects to grow much larger than they do today. How? Insects don't have lungs. Life-giving oxygen is taken in through openings in the outside of their bodies and transported through a network of tubes. If an insect were too large, the outer reaches of these tubes would absorb all the oxygen before it could ever get to its internal organs. But during the Carboniferous Period, the atmosphere had almost twice the oxygen as today. Insects could then grow much bigger and still get enough oxygen in their bodies. That's why the dragonflies here are as big as eagles and the millipedes the size of alligators. So why was there so much oxygen back then? It was produced by a new kind of life. DEGRASSE TYSON: What kind of life could've changed the Earth's atmosphere so dramatically? Plants that could reach for the sky-- trees. In their competition for sunlight, trees evolved a way to defy gravity. Before trees, the tallest vegetation was only about waist-high. And then something wonderful happened. A plant molecule evolved that was both strong and flexible, a material that could support a lot of weight, yet bend in the wind without breaking. Lignin made trees possible. Now life could build upward. And this opened a whole new territory, a three-dimensional matrix for communities far above the ground. Earth became the Planet of the Trees. But lignin had a downside: it was hard to swallow.