Life on Earth has so far passed through five distinct crises that threatened to wipe it out, typically referred to as mass extinctions. The direst one was some 252 million years ago when 96 percent of marine species and 70 percent of life on land became extinct following a yet unconfirmed series of cataclysmic events known as the end-Permian extinction. Mass extinction is an alarming, scary term, but while it’s natural to shiver at the idea, it’s worth considering that we’re actually living through a mass extinction, the sixth so far. This one is different though. While past mass extinctions were triggered by cataclysmic events, the sixth great extinction will be the only one caused single-handedly by a singular species: us. If this wasn’t sad enough, the sixth extinction might be remembered (if there’s anyone left to recollect) as the ‘invisible’ extinction.

The sixth grand extinction

The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the Pleistocene to the Holocene epoch. Among the main causes hypothesized by paleontologists are natural climate change and overkill by humans. Image: Wikimedia Commons

Humans can be beautifully creative, yet brutally inconsiderate at the same time of the world they live in and share with other creatures. It’s precisely our uncanny intellect and creativity – which we’re so proud of and which are often trumpeted as key differential qualities that separate us from beasts – that is dooming this planet. Because our technology and means of manipulating resources have evolved so rapidly, few species can keep up. A growing number of ecosystems are faced with situations where the environment is no longer predictable, either because of direct human interference (deforestation, loss of habitat, pollution) or indirect, through global warming (shorter seasons, droughts, ocean acidification, and so on).

Biologists and paleoecologists estimate that humans have driven roughly 1,000 species extinct in our 200,000 years on the planet. Since 1500 we have killed off at least 322 types of animals, including the passenger pigeon or the Tasmanian tiger. Only yesterday ZME Science reported how the Northern White Rhino is down to only five specimens as a result of human poaching. Another 20,000 or more species are now threatened with extinction according to the International Union for the Conservation of Nature, which keeps a list of all the known endangered plants and animals on the planet. The population of any given animal among the five million or so species on the planet is, on average, 28 percent smaller, thanks to humans.

A thought experiment

As if this wasn’t bad news enough, there’s another chilling prospect that might make the whole event even more frustrating: no one might ever know that most of these species existed. Say a doomsday event was to shatter human knowledge and civilization as we know it, forcing the survivors to start from square one. Within a couple of generations, common knowledge about the world we take for granted today (like the existence of lions, rhinos or turtles) might vanish. No problem, post-meltdown paleontologist will be able to piece together the puzzle of past existence on Earth by studying fossil remains. According to Roy Plotnick, a palaeontologist at the University of Illinois at Chicago, this might be useless for most animals on Earth.

One study found that although human population has doubled in the past 35 years, the number of invertebrate animals – such as beetles, butterflies, spiders and worms – has decreased by 45% during that same period. “We were shocked to find similar losses in invertebrates as with larger animals, as we previously thought invertebrates to be more resilient.” said Ben Collen of the U.K.’s University College London, one of the study authors. Image: Science

Of the 715 mammal species listed on the International Union for the Conservation of Nature (IUCN) Red List only 90 of them are represented in the fossil record or 13%. The rest of them will be gone without a trace.

“We assume a lot about the persistence of human recordings. Who can read an 8-inch floppy disk these days? The rock record is a much more persistent way of recording information. I’m a reader of science fiction, and I think about post-apocalyptic worlds. What would we make of what had happened in the long-ago past?” said Plotnick in an interview for Nature.

According to Plotnick, rodents and bats are the most vulnerable at becoming completely forgotten. Body size is an important factor as far as fossils go, but geographical distribution is by far the most significant. You have much less chance of getting caught in the fossil record if you’re only found in one place. Birds and reptiles are also particularly vulnerable, especially island species.

“As a palaeontologist I think about the appearance and extinction of species — that shows how we do geological time. One big signal is the mass extinction of mammals in North America, Europe and Australia, when hunters arrived. That would be the stratigraphic marker.” Plotnick says.

Plotnick ends his interview by striking a humorous chord – we’ve bred so many pigs all over the world and, consequently, scattered their bones all over the place that a post-current era civilization might conclude this planet was once dominated by hogs. They wouldn’t be that far away from the truth.