This preview shows pages 1–2. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Chapter 18 Life in the Universe 529 the big picture Putting Chapter 18 into Context Throughout our study of astronomy, we have taken the big picture view of trying to understand how we fit into the universe. Here, at last, we have returned to Earth and examined the role of our own generation in the big picture of human history. Tens of thousands of past human generations have walked this Earth. Ours is the first generation with the technology to study the far reaches of our universe, to search for life elsewhere, and to travel beyond our home planet. It is up to us to decide whether we will use this technology to advance our species or to destroy it. Imagine for a moment the grand view, a gaze across the centuries and millennia from this moment forward. Picture our descendants living among the stars, having created or joined a great galactic civilization. They will have the privilege of experiencing ideas, worlds, and discover- ies far beyond our wildest imagination. Perhaps, in their history lessons, they will learn of our generationthe generation that history placed at the turning point and that managed to steer its way past the dangers of self-destruction and onto the path to the stars. summary of key concepts 18.1 Life on Earth When did life arise on Earth? Fossil evidence puts the origin of life at least 3.5 billion years ago, and carbon isotope evidence pushes this date to more than 3.85 billion years ago. Life therefore arose within a few hundred million years after the last major impact of the heavy bombardment, and possi- bly in a much shorter time. How did life arise on Earth? Genetic evidence suggests that all life on Earth evolved from a common ancestor, which was probably similar to microbes that live today in hot water near undersea volcanic vents or hot springs. We do not know how this first organ- ism arose, but laboratory experiments suggest that it may have been the result of natural chemical processes on the early Earth. Once life arose, it rapidly diversified and evolved through natural selection ....
View Full Document
- Spring '08