HUMAN actions look insignificant in terms of geological time: against spans of millions of years, we have been on the earth momentarily.
The Sixth Extinction: An Unnatural History
Nevertheless, such is humankind’s impact that some scientists now suggest this brief era, the Holocene, may be characterised primarily as the time of the planet’s sixth great extinction. Indeed some suggest our era should be known as the Anthropocene, since, as the New Yorker’s Elizabeth Kolbert writes: “No creature has ever altered life on the planet in this way before.”
The first great extinction of species was in the late Ordovician period, 450 million years ago; the fifth and most recent was at the close of the Cretaceous 50 million years ago and killed the dinosaurs. None of the individual species whose fate Kolbert examines are as spectacular as T Rex. But her evidence is worrying.
She starts with the seemingly obscure golden frogs of Panama. Despite the fact that amphibians are some of nature’s most adaptable survivors, they have been disappearing worldwide at thousands of times their “natural” extinction rate, even in apparently pristine wildernesses.
The Panamanian frogs turn out to be the victims of a chytrid fungus spread by humans – one of thousands of instances of alien species unwittingly spread across the globe to new environments. Birds and bats are under severe threat in places too; a quarter of mammal species and a fifth of reptiles are also at risk.
Kolbert traces the history of extinctions, and of biologists’ growing awareness since the 18th century of what happened to mastodons, cave bears, giant sloths and giant marsupials. These animals, we now know, were wiped out by early humans – a key point, since we tend to think of pre-industrial peoples living in harmony with nature. Yet after they arrived in what is now New Zealand in the 13th century AD, the Maori wiped out the moas, giant flightless birds, within a century. That process has greatly accelerated. But the extinction of the dodo in the 17th century and the Great Auk in the 19th were less dramatic than the effects on habitat of 20th-century industrialisation.
Kolbert swims off coral reefs and down to natural carbon dioxide vents off Italy to see the effect of the acidification of the oceans, caused largely by the sea absorbing around a third of human-generated CO2. And then there are the related effects of climate change. Yet while plants and animals can evolve to cope with a hotter world, that will take far too long for humans.
That is ultimately what makes this engaging study scary: Kolbert believes the processes that she traces are too big and too far advanced to reverse. The Earth will continue, even if it ends up populated by giant rats rather than humans. But the sixth great extinction will alter it for millions of years.