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5.0 out of 5 stars From the Oldest Fossils to the Rise of Animals - Glimpses into Our Dark Past, April 17, 2006
This review is from: Life on a Young Planet: The First Three Billion Years of Evolution on Earth (Princeton Science Library) (Paperback)
This is the story of life in the Proterozoic Eon, which is characterized by an extreme scarcity of fossils. Sites from the early Proterozoic are rare and much changed from their original condition, so it's hard to know anything about what life and geology were like. The picture slowly clears as the story moves forward. For example, when eukaryotes arrived, they were larger and more varied than earlier organisms and so their fossils are easier to distinguish from mineral deposits. By the end of the story, animals are on the verge of the Cambrian "explosion". There are still more questions than answers, particularly about what the ancestors of the Cambrian animals were like, and why the "explosion" happened when it did. It is the questions, and the discussion of possible answers, that give this book its appeal.

One major question was how much oxygen the oceans held. Photosynthesis - the main source of free oxygen on Earth - began long before the evolution of multicellular life. What caused the delay? Was some process scrubbing oxygen from the ocean? The evidence needed to discuss these questions includes the presence of certain minerals and the ratios of certain isotopes. Knoll discusses the evidence and indicates what we can know so far. There are other questions than the amount of oxygen, and Knoll treats them similarly.

The lead-in to the "explosion" gets special attention. Important considerations include the nature of late pre-Cambrian animals, oxygen levels, mass extinction, and the proliferation of developmental genes. There is much debate these issues and Knoll describes the debate, giving his evaluation of various positions. This, for me, was the most exciting part of the book. If you too find it exciting, I recommend the book Endless Forms Most Beautiful by Sean Carroll for insight into how the evolution of developmental genes led to increasingly rapid diversification of animals.

As you can tell from the above, Young Planet is somewhat technical. I had to pause frequently to think about what I was reading and I had to go back from time to time to review some point. That's what I usually want in a science book; it's more about thinking than about description. And it's not hard to read; it's just about right for a non-scientist who wants to learn about the subject. (It may help you to have an idea of my background. Click on my name, above, and you might want to red my other reviews.)
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