Review: A Brief History of Earth by Andrew Knoll

 Andrew Knoll tells the story of Earth, its formation, its changes over time, and especially, the story of the interaction of life and geology over that time.  Few if any are more steeped in that story or have contributed more to unravelling it.  The early Earth was not much like Earth today and we could not have lived on it.  Once the crust cooled enough to solidify and for water to condense and form oceans, there were oceans and probably relatively few and small bits of land.  There was no oxygen, so nothing for us to breathe.  A steady stream of potent UV radiation would have made it impossible for land plants and animals of today to live even if there had been oxygen.

Nonetheless, the first signs of life appear in the oldest rocks only about a billion years after the planet formed.   How these were found and identified is one of the great detective stories of natural science, and it is told well here.  The subsequent evolution of cyanobacteria is one of the great watersheds in the life history of the Earth, because they were the first to use a form of photosynthesis that produced oxygen as a byproduct.  It would take much of a billion years before significant oxygen would appear in our atmosphere, but even then, not much – perhaps only one percent or so of present values, and not enough for even a beetle to live, but, apparently enough for the first eukaryotes, the first cells like those we are made of, with complex internal structures including the mitochondria that power our cells.

Geology, especially the movement of tectonic plates, was already making thing complicated.  Several times vast ice sheets covered the planet producing snowall Earths with even much of the oceans frozen.  After the last one, oxygen increased dramatically, and with the Cambrian Explosion, larger animals with hard parts appeared in vast profusion.  Later, plants and then animals would invade the land, and a world much more like our own took shape.  Amphibians developed, and later dinosaurs and mammals.

Knoll is a graceful and immensely erudite author, and he tells this story well.   His focus is always on the interplay of geology and life, and how their story has been deciphered.  His last chapter focuses on the latest dominant species and the vast and perilous changes it is (we are) making to the Earth.

From his acknowledgments: THIS BOOK DISTILLS the fruits of a lifetime spent trying to understand our planet and the life it sustains. Through research on five continents and teaching, first at Oberlin College and then for nearly four decades at Harvard, I have learned a tremendous amount about Earth’s past, present, and probable future. 

Knoll, Andrew H.. A Brief History of Earth (p. 231). Custom House. Kindle Edition.

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