Charles Petzold

On one level, this book is a joyous celebration of science. Elizabeth Kolbert is the type of science writer who doesn’t hesitate to travel to exotic and dangerous places where she gets her hands dirty and her feet wet. Readers of this book tag along as the author goes to Panama to look for frogs, hikes through the treacherous terrain in the Southern Uplands of Scotland, swims through the cold waters of the Tyrrhenian Sea, explores a bat cave in the Adirondacks, trudges through the mountain ranges of Peru, snorkels off the coast of One Tree Island at the southern tip of the Great Barrier Reef, and even walks the “pie crust” of the Reef itself.

The goal of these expeditions is largely to talk with scientists in their laboratories of the great outdoors, and to let us hear the scientists themselves tell us of their research.

These scientists have become experts in comparatively narrow ranges of study, but continue to maintain an awareness how their research fits in the bigger picture — and this book is nothing if not about very big pictures. They’ve chosen to work in these extreme environments for reasons I can only describe as love — love for a particular species or ecological environment, love of knowledge, and love of understanding how it all fits together.

The enthusiasm of these scientists for their work is infectious. I can easily imagine young people reading Elizabeth Kolbert’s book and getting turned on to pursue a career in scientific field research (or maybe even journalism).

Yet, there is an undercurrent to this book that is not so joyous.

Many of the species the scientists in this book have chosen to study are dying out. The Sixth Extinction: An Unnatural History (Henry Holt, 2014) is about a process sometimes known as “biotic attrition” (pg. 172) but more generally understood as a loss of biodiversity. Extreme losses of biodiversity result when many species becoming extinct, and when this happens in a relatively short period of time (in geological time scales) it becomes an extinction event.

The concept of extinction as regards species is fairly recent. Just 250 years ago it was widely believed that every type of living thing that had ever existed was still alive today. In a history that weaves through the first several chapters, Elizabeth Kolbert shows how fossil discoveries upset these assumptions. She identifies Georges Cuvier (1760 – 1832) as the scientist who began interpreting fossils as remnants of espèces perdues, lost species that no longer existed. It took Darwin to synthesize the idea that (in Kolbert’s words) “Extinction and evolution were to each other the warp and weft of life’s fabric, or, if you prefer, two sides of the same coin.” (pg.54)

Today we recognize five major extinction events — the Big Five. Generally they mark dividing lines in geological periods and eras. The most recent occurred roughly 66 million years ago and caused the transition from the Mesozoic to the Cenozoic era. About 75% of all plant and animal species became extinct around this time. As a result of pioneering work by Walter Alvarez and his father Luis beginning in the 1970s, it is now generally accepted that at least a major cause of this event was a massive asteroid impact.

Extinction may have been unheard of 250 years ago, but today’s children know all about extinction, for that’s what brought about the demise of the dinosaurs. Despite a loss of 75% of all species, the Earth eventually rebounded in a very dramatic way. The elimination of the dinosaurs allowed mammals to flourish and become larger. More mammalian species came about, including primates. In a very real sense, Homo sapiens owes its existence to that wayward comet.

The idea of a sixth extinction occurring right now is not new with Elizabeth Kolbert. It has been the subject of previous speculations and books. Yet, The Sixth Extinction brings the concept to a very human level. Usually extinctions are slow and occur on geological timescales — not human timescales. So if we can see extinctions occurring almost before our eyes (looking through the pages of this book), something is seriously wrong.

This sixth extinction seems to be largely the result of various influences by “one weedy species” (pg. 6) as two scientists have termed us. This impact of humans on the Earth has become part of the scientific understanding of changes in the Earth and its inhabitants.

For example, we term the current geological epoch the Holocene, which is recognized as an interglacial period of the past 11,700 years. Yet, in the past few decades the concept of the Anthropocene has taken shape to acknowledge the influence of this one species in changing the planet, including transforming over a third of the land surface, damming or diverting most of the world’s major rivers, and using more than half the world’s fresh water runoff. (Paraphrased from pg. 108)

Humans have caused other species to become extinct in a variety of ways. Sometimes species have been hunted (for either food or sport) to extinction. This seems to be the case for the Megatherium (a ground sloth) and the American mastodon. The last great auk was seen and captured in 1844 and killed by its captors. For some endangered species, poaching continues to the present day.

Some ancient acts by humans might even today be classified as “genocide.” It is now widely believed that early Homo sapiens procreated with Homo neanderthalensis, but there also seems to be evidence that humans exterminated the Neanderthals as well.

One of the many unintended consequences of the Anthropocene has been the pruning of our own family tree. Having cut down our sister species — the Neanderthals and the Denisovans — many generations ago, we’re now working on our first and second cousins. By the time we’re done, it’s quite possible that there will be among the great aps not a single representative left, except, that is, for us. (pg. 255)

Another problem results from increased mobility, including faster and more extensive transportation. Humans have brought themselves — as well as animals, plants, and pathogens—all over the world. Thomas Friedman has used the phrase “the earth is flat” as a metaphor for globalized culture and commerce. In The Sixth Extinction a similar concept is the New Pangaea. The original Pangaea was a single continent that originally existed on the Earth. The New Pangaea results from the ease by which we can leap across oceans and land masses.

As species indigenous to a particular region are spread to other regions, local biodiversity increases, but global biodiversity tends to decrease as invasive species and invasive pathogens go to work. (pg. 212) Pathogens that have been spread through human mobility are apparently the cause of devastating extinctions going on in frog populations and bat populations.

The problem for North American bats — discussed in Chapter 10 of The Sixth Extinction — is a fungus that was somehow accidently brought to the U.S. from Europe. Interestingly, it was first noted among bats in Howe Caverns — which New Yorkers will immediately recognize as a popular tourist attraction with over 200,000 visitors a year. From there, the fungus has spread out in concentric circles and killed many more bats.

Climate change is associated with many transitions between geological eras and epochs. Our own era the Holocene is defined by climate change as well, and characterized as an interglacial period of warmth. Climate change is also very closely associated with extinction events of the past. Yet, it is a characteristic of the Anthropocene that humans can influence even the climate. The concentration of CO₂ in the atmosphere has increased by 40% over the past two centuries, and methane has doubled. Both these gases — and methane much more than CO₂ — contribute to the greenhouse effect that warms the planet and causes changes in weather patterns.

Consequently, the theme of climate change runs throughout this book, as both an historical driving force of change and extinction, and also as one of several trends in which this “one weedy species” is influencing this history. Some of the increased atmospheric CO₂ dissolves in sea water and increases ocean acidification. Historically, ocean acidification was an influence in two, maybe three, of the previous Big Five extinctions (pg. 120), and looks to be playing a role in the sixth one as well.

Despite the ostensibly grim subject matter, Elizabeth Kolbert manages to maintain a fairly light touch, even introducing some scientist-based humor along the way. “If Dicerorhinus sumatrensis has a future,” she writes at one point, “it’s owing to [Dr. Terri] Roth and the handful of others like her who know how to perform an ultrasound with one arm up a rhino’s rectum.” (p. 221-2)

A little gallows humor isn’t out of place, either. As Elizabeth Kolbert is leaving one of the caves where bats are studied, one of the scientists tells her “Don’t step on any dead bats.” (p. 210) It sounds like good advice, but it is hopeless advice, because the floor of the cave is covered with their corpses, and the author has already slipped and fallen onto a layer of mushy dead bats.

Rarely has such a lively book been written that is ultimately so much about death.