追溯。（总算离开大脑了。。） Hints： 第一句前加 ... （连接上篇） America's Yellowstone National Park

...is actually the result of immeasurable activity inside our bodies.
We are performing literally thousands of different tasks every second of every day. Simply being is part of an intricately complicated balancing act, the mechanics of which are completely hidden from us. And just as our bodies hide from us the miracle of everyday life, so they hide from us an even bigger surprise.
It's the story of how we came to be the way we are. And it is an amazing story. It will change the way you think about your body, the way we live, the shape we are, even the way we think was decided for us thousands, millions even billions of years ago, when the human body didn't even exist, and planet earth was almost unrecognizable.
Here we are. 8,000 feet above sea level, in America's Yellowstone National Park. If you'd been on the surface of the planet earth three billon years ago, it would have looked something like this.

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