Neil deGrasse Tyson, author and host of the NOVA series, "Origins, Fourteen Billion Years of Cosmic Evolution", gives a galvanizing tour of the cosmos revealing what the universe has been up to while turning part of itself into us. He writes a monthly column called "Universe" for Natural History magazine and is the author of several books, including "One Universe: At Home in the Cosmos" (2000) and "The Sky is Not the Limit: Adventures in an Urban Environment" (2000).
His most recent work is the book (published by W.W. Norton & Co.) and NOVA PBS four-part series, "Origins". Chapter fifteen, entitled "The Origin of Life on Earth" is excerpted here with publisher permission.
The search for life in the universe begins with a deep question: what is life? Astrobiologists will tell you honestly that this question has no simple or generally accepted answer.
Not much use to say that we'll know it when we see it. No matter what characteristic we specify to separate living from nonliving matter on Earth, we can always find an example that blurs or erases this distinction. Some or all living creatures grow, move, or decay, but so too do objects that we would never call alive.
Does life reproduce itself? So does fire. Does life evolve to produce new forms? So do certain crystals that grow in watery solutions.
We can certainly say that you can tell some forms of life when you see them - who could fail to see life in a salmon or an eagle? - but anyone familiar with life in its diverse forms on Earth will admit many creatures will remain entirely undetected until the luck of time and the skill of an expert reveal their living nature.
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