The Sixth Extinction: An Unnatural History (2014)

“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. But 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 freshwater 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.” 

Once considered the greatest survivor on the planet, amphibians have not been able to adapt to a rapid change in environment and climate, or survive the influx of invasive species and diseases that have been carried around the world by human beings. 

“On the basis of a few scattered bones, Cuvier had conceived of a whole new way of looking at life. Species died out. This was not an isolated but a widespread phenomenon.” 

Cuvier’s theory, formed via his career as a “fossilist,” was that species went extinct during certain epochs because of cataclysmic events that occurred in those moments, such as the giant meteor that wiped out the dinosaur species. This theory of catastrophism was widely mocked by many scientists at the time, but in fact, it has since been accepted as a cause of extinction.

“An animal that was born with, say, teeth or sense organs that were somehow different from its parents’ would not be able to survive, let alone give rise to a whole new kind of creature.” 

The development of species and changes in species took place over long periods of time and did not happen immediately, as some scientists had initially theorized. 

“The changes that had caused extinctions must therefore have been of a much greater magnitude—so great that animals had been unable to cope with them. That such extreme events had never been observed by him or any other naturalist was another indication of nature’s mutability: in the past, it had operated differently—more intensely and more savagely—than it did at present.” 

Of great significance was the finding that a primary reason for extinction was the introduction of non-native or invasive species or the transfer of new fungi and diseases to organisms that were not biologically equipped to resist these new intrusions. 

“In fact, the American mastodon vanished around thirteen thousand years ago. Its demise was part of a wave of disappearances that has come to be known as the megafauna extinction. This wave coincided with the spread of modern humans and, increasingly, is understood to have been a result of it. In this sense, the crisis Cuvier discerned just beyond the edge of recorded history was us.” 

The most destructive force encouraging extinction on the planet turns out to be human beings. The demise of the American mastodon shortly after the spread of human beings around the world is not coincidental, but the result of one species hunting another to extinction.

“Darwin saw that the key to understanding coral reefs was the interplay between biology and geology. If a reef formed around an island or along a continental margin that was slowly sinking, the corals, by growing slowly upward, could maintain their position relative to the water. Gradually, as the land subsided, the corals would form a barrier reef. If, eventually, the land sank away entirely, the reef would form an atoll.”

The vital importance of the coral reefs was realized by Darwin when he noticed that the reefs, living organisms themselves, were crucial to the development of, sustenance of, and future of a large variety of other marine life. The reefs would grow as a reaction to environmental changes, continuing to live on their own while providing food and security to other organisms. 

“Newton argued for a ban on hunting during breeding season, and his lobbying resulted in one of the first laws aimed at what today would be called wildlife protection: the Act for the Preservation of Sea Birds.” 

Scientists began to recognize that many species, such as the great auk, went extinct because humans hunted them mercilessly, and that these species were especially vulnerable during their mating seasons. One way to prevent the extinction of an entire species was to enact a law prohibiting hunting during breeding seasons.

“But how, then, to make sense of cases like the great auk or the Charles Island tortoise or, to continue the list, the dodo or the Steller’s sea cow? These animals had obviously not been done in by a rival species gradually evolving some competitive advantage. They had all been killed off by the same species, and all quite suddenly—in the case of the great auk and the Charles Island tortoise over the course of Darwin’s own lifetime.” 

This quote provides specific proof and examples of species that were killed off due to human behavior and human behavior alone. The extinctions happened suddenly, and all due to the same invasive species.

“On an otherwise ordinary day sixty-five million years ago, an asteroid six miles wide collided with the earth. Exploding on contact, it released energy on the order of a hundred million megatons of TNT, or more than a million of the most powerful H-bombs ever tested. Debris, including iridium from the pulverized asteroid, spread around the globe. Day turned to night, and temperatures plunged. A mass extinction ensued.” 

The Alvarezes advanced Cuvier’s theory of catastrophism with further specifics: a giant asteroid had abruptly ended the Cretaceous era and eliminated many species, such as the dinosaurs.

“ ‘Those eleven years seemed long at the time, but looking back they seem very brief,” Walter told me. “Just think about it for a moment. Here you have a challenge to a uniformitarian viewpoint that basically every geologist and paleontologist had been trained in, as had their professors and their professors’ professors, all the way back to Lyell. And what you saw was people looking at the evidence. And they gradually did come to change their minds.’ ” 

This quote concerns the concept of the “paradigm shift,” wherein people are reluctant to change their minds and accept new information or theories until the very last moment. This process is one that is frequently found in the sciences, and particularly in arguments forwarded about extinction.

“Whatever the explanation, the contrasting fate of the two groups raises a key point. Everything (and everyone) alive today is descended from an organism that somehow survived the impact. But it does not follow from this that they (or we) are any better adapted. In times of extreme stress, the whole concept of fitness, at least in a Darwinian sense, loses its meaning: how could a creature be adapted, either well or ill, for conditions it has never before encountered in its entire evolutionary history?”

Catastrophism in many ways counteracts Darwin’s theory of natural selection. How can an organism possibly adapt to circumstances it has never encountered previously? Some did, or life would not exist on earth today, but the majority of species do not and cannot.

“Data that did not fit the commonly accepted assumptions of a discipline would either be discounted or explained away for as long as possible. The more contradictions accumulated, the more convoluted the rationalizations became …. But then, finally, someone came along who was willing to call a red spade a red spade. Crisis led to insight, and the old framework gave way to a new one. This is how great scientific discoveries or, to use the term Kuhn made so popular, ‘paradigm shifts’ took place.”

Paradigm shifts mark significant progress and changes in scientific theory and thought. We now have a much better idea of the geological and biological history of the planet because of paradigm shifts away from earlier accepted theories.

“If twenty-five years ago it seemed that all mass extinctions would ultimately be traced to the same cause, now the reverse seems true. As in Tolstoy, every extinction event appears to be unhappy—and fatally so—in its own way. It may, in fact, be the very freakishness of the events that renders them so deadly; all of a sudden, organisms find themselves facing conditions for which they are, evolutionarily, completely unprepared.” 

There is no one blanket reason why extinctions occur. Science has now proven that there are myriad reasons, such as both small and large cataclysmic events, as well as the impact that the human species has had on the earth.

“Ocean acidification is sometimes referred to as global warming’s ‘equally evil twin.’ The irony is intentional and fair enough as far as it goes, which may not be far enough. No single mechanism explains all the mass extinctions in the record, and yet changes in ocean chemistry seem to be a pretty good predictor. Ocean acidification played a role in at least two of the Big Five extinctions (the end-Permian and the end-Triassic) and quite possibly it was a major factor in a third (the end-Cretaceous).” 

This quote reflects the effects that changes in marine environments have had on previous massive extinction points in the Earth’s history. The fact that ocean acidification and an increase in water temperatures are happening right now is indicative of a Sixth Extinction.

“By burning through coal and oil deposits, humans are putting carbon back into the air that has been sequestered for tens—in most cases hundreds—of millions of years. In the process, we are running geologic history not only in reverse but at warp speed.”

Because of damaging human activities to the environment, the rate of change in the Earth’s atmosphere is substantially increased, so quickly that other species do not have time to adapt to those changes before reaching extinction.

“The way corals change the world—with huge construction projects spanning multiple generations—might be likened to the way that humans do, with this crucial difference. Instead of displacing other creatures, corals support them. Thousands—perhaps millions—of species have evolved to rely on coral reefs, either directly for protection or food, or indirectly, to prey on those species that come seeking protection or food. This coevolutionary venture has been under way for many geologic epochs. Researchers now believe it won’t last out the Anthropocene.” 

The danger to the coral reefs is very real, and current scientific prediction is that many, including the Great Barrier Reef, will not last another thirty years. Because of the coral reefs’ importance to the life cycle of other marine species, their extinction will cause many more extinctions in reaction.

“But global warming is going to have just as great an impact—indeed, according to Silman, an even greater impact—in the tropics. The reasons for this are somewhat more complicated, but they start with the fact that the tropics are where most species actually live.” 

Too often global warming is associated with the impact on northern areas with icebergs and ice caps. However, the greatest impact global warming will have is in the tropics, as that is the area of the world where the most species thrive.

“Any species (or group of species) that can’t cope with some variation in temperatures is not a species (or group) whose fate we need be concerned about right now, because it no longer exists.” 

Many scientists have come to terms with the fact that some species extinctions are already happening and that nothing can be done to prevent them. Instead, some scientists say that more time and effort should be spent on those species for whom there is still time to ward off extinction.

“If we assume, very conservatively, that there are two million species in the tropical rainforests, this means that something like five thousand species are being lost each year. This comes to roughly fourteen species a day, or one every hundred minutes.” 

This quote relays the numerical realities of the rate of extinction in a tropical area like a rainforest. Climate change and deforestation are responsible for the loss of fourteen species a day.

“The process of remixing the world’s flora and fauna, which began slowly, along the routes of early human migration, has, in recent decades, accelerated to the point where in some parts of the world, non-native plants now outnumber native ones. During any given twenty-four-hour period, it is estimated that ten thousand different species are being moved around the world just in ballast water. Thus a single supertanker (or, for that matter, a jet passenger) can undo millions of years of geographic separation.” 

It’s the fault of human beings, our spread around the globe, and our methods of transportation that some species have died off or are in competition with non-native species. 

“With introduced pathogens, the situation is much the same. Long-term relationships between pathogens and their hosts are often characterized in military terms; the two are locked in an ‘evolutionary arms race,’ in which, to survive, each must prevent the other from getting too far ahead. When an entirely new pathogen shows up, it’s like bringing a gun to a knife fight. Never having encountered the fungus (or virus or bacterium) before, the new host has no defenses against it. Such ‘novel interactions,’ as they’re called, can be spectacularly deadly.” 

Species that are exposed to new pathogens that are not native to their environments are more likely to suffer and possibly die off. These species have no natural defenses to fight back against a pathogen that they’ve never encountered before. 

“If climate change drove the megafauna extinct, then this presents yet another reason to worry about what we are doing to global temperatures. If, on the other hand, people were to blame—and it seems increasingly likely that they were—then the import is almost more disturbing. It would mean that the current extinction event began all the way back in the middle of the last ice age. It would mean that man was a killer—to use the term of art an ‘overkiller’—pretty much right from the start.” 

If humanity is mainly at fault for increasing global temperatures, then that has been its role from the beginning of its existence. Should natural global temperature change be the main culprit, then humanity seems more than willing to advance an extinction event with its activities that drive global temperatures up further and faster. 

When the project is completed, it should be possible to lay the human genome and the Neanderthal genome side by side and identify, base pair by base pair, exactly where they diverge. Neanderthals were extremely similar to modern humans; probably they were our very closest relatives. And yet clearly they were not humans. Somewhere in our DNA must lie the key mutation (or, more probably, mutations) that set us apart—the mutations that make us the sort of creature that could wipe out its nearest relative, then dig up its bones and reassemble its genome.” 

As many modern humans have traces of Neanderthal in their DNA, it would stand to reason that the Homo sapiens and Neanderthal species intermixed, but humans, probably through some genetic change, were able to cause the Neanderthal to go extinct. We are fascinated with our past relatives, even though they died because of us.