Kolbert explains to the reader that until the late 1700s it was believed that prehistoric mass extinctions had never occurred. It was believed then that just as specification occurred gradually over immense periods of time, so too, extinctions must occur only gradually, so gradually that given the tiny time period of homo sapiens, it would be unlikely that even a single extinction could be witnessed. However, as geology has developed, it has become clear in the fossil record that there have been five catastrophic periods of mass extinctions. In the course of the book, she describes each of these: one caused by the earth being hit by a an asteroid, others by glaciation and or global warming.
Very, very occasionally in the distant past, the planet has undergone change so wrenching that the diversity of life has plummeted. Five of these ancient events were catastrophic enough that they’re put in their own category: the so-called Big Five. In what seems like a fantastic coincidence, but is probably no coincidence at all, the history of these events is recovered just as people come to realize that they are causing another one. When it is still too early to say whether it will reach the proportions of the Big Five, it becomes known as the Sixth Extinction.
She continues: "Conditions changes so drastically or so suddenly (or so drastically and so suddenly) that evolutionary history counts for little."[…] mass extinctions are defined as events that eliminate a ‘significant proportion of the world’s biota in a geographically insignificant amount of time’
While there is always a background rate of extinction, it is nothing like the rate of mass extinctions. For example:
Today amphibians enjoy the dubious distinction of being the world’s most endangered class of animals; it’s been calculated that the group’s extinction rate could be as much as forty-five thousand times higher than the background rate ... extinction rates among many other groups are approaching amphibian levels. It is estimated that one-third of all reef-building corals, a third of all fresh-water mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles and a sixth of all birds are headed toward oblivion.What makes the sixth extinction stand out is that it is being caused by a single species of animal, human beings. Summing up in her final chapter, Kolbert says:
Right now in the amazing moment that to us counts as the present, we are deciding, without quite meaning to, which evolutionary pathways will remain open and which will forever be closed. No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy. The Sixth Extinction will continue to determine the course of life long after everything people have written and painted and built has been ground into dust and giant rats have—or have not—inherited the earth.From what I have said so far, it may seem that this is a horribly depressing book to read, but in fact, Kolbert’s writing is so clear and her travels while writing and documenting her claims so incredible that I came away from the book feeling like I have a much firmer grasp of evolution than before. Not since Stephen Jay Gould’s Ever Since Darwin have I learned so much about the continually unfolding story of the evolution of life on earth.
Each chapter presents the latest beliefs in geography, biology, astrophysics, but in a language even the lay person can grasp. While the book is clearly a warning, it is not a warning of what will come, but a description of what has been happening for at least the last two hundred years. Kolbert quotes with admiration the Stanford ecologist, Paul Ehrlich: “In pushing other species to extinction, humanity is busy sawing off the limb on which it perches.”
As she meticulously lays out her case for the Sixth Extinction, she points out many interesting little facts about earth’s history, for instance, in explaining why there is so much diversity in the many incarnations of the Amazon rainforest, some version or other of which has existed for millions of years, lots of time for diversity to accumulate,
By contrast, as recently as twenty thousand years ago, nearly all of Canada was covered in ice
a mile thick. So was much of New England, meaning that every species of tree now found in Nova Scotia or Ontario or Vermont of New Hampshire is a migrant that’s arrived (or returned) just in the last several thousand years.
Kolbert has packed into three hundred pages an amazing array of statistics and descriptions of scientific projects that left this reader on the edge of his chair turning pages as if reading a mystery thriller. It is a great book, and one that we can all read, and should all read.