Can humans really extinguish all life on Earth? It's complicated

You’ve heard it before: we’re well into the sixth mass extinction of life on earth. Only this time, unlike the other five big ones, humans are overwhelmingly the killers responsible. As we continue to set the world on fire, regardless of a certain administration's attempts to pretend it's not happening, scientists warn we are in unprecedented territory that could result in a whole bunch of death, especially for impoverished people in the global south and the ongoing "biological holocaust" happening to nature.

But are we truly the masters of our domain, this little blue planet? And could we really end all life on Earth? What if we dropped every nuke at once? Would that get every bug, including roaches and bacteria?

“No way, no chance, no prayer — there's not the slightest possibility that we could wipe out all of life,” said Dr. David Jablonski, professor of geophysical sciences at the University of Chicago. "I mean, there are microbes living under glaciers and a kilometre down in the crust [of the Earth.]"

When I feel despair, and a kind of profound bottomless sadness, at the unchecked destruction of millions and millions of years of irreplaceable biodiversity going on right now, I take a little comfort imagining that from the smoking ruins of whatever remains once we've finally, foolishly removed the last biological underpinnings that keep us alive, complex life will likely re-emerge. It's done so after at least five other mass extinctions, all long before humans arrived on the scene. And it will do so after the Anthropocene extinction, which is being caused by human activities. Our downward spiral notably includes the spread of invasive species; overexploitation of species; habitat modification, fragmentation and destruction; pollution; and, of course, climate change. Though Jablonski is more bullish on the survival prospects of unicellular life, there is some comfort for the multicellular among us too.

Jablonski studies patterns of evolution, including the ways in which life rebounds after mass extinctions. It's been observed that following these mega-death events, not only is there not nothing, but in fact in the years — actually, the hundreds of thousands or millions of years, because we're talking geological time here — following a mass extinction, there is often an explosion of biodiversity, with surviving species evolving new branches on their evolutionary trees.

It's not always an absolutely gobsmacking profusion of uncontrollably bizarre life, as occurred with what's aptly called the Cambrian Explosion, a period about 540 million years ago when evolution got extremely creative. Nature began evolving entirely new body plans, with innovations like hard shells and backbones that have even survived up to today, and the ancestors of pretty much all the major groups (phyla) of animals. Of course, plenty of branches on the evolutionary tree fizzled out too, which is why creatures like Hallucigenia, a worm bug with spikes on its back and tentacles and legs on its front, are no longer with us.

We don't really know why the Cambrian was so exceptionally diverse. The ancestors of groups that diversified so much during the Cambrian existed before. But it wasn't till the Precambrian extinction 544 million years ago that they burst through their previous limits, generating new species that found new ways to live.

Sometimes, as with mammals after the non-avian dinosaurs died out, the diversity that occurs post-extinction isn't so much a question of numbers of species but of a group diversifying functionally (expanding the range of what new species in the group can do) or morphologically (expanding their possibilities of size or physical form) into new ecological niches. This can result in many things growing startlingly big, for example. It's almost as if the mass extinction process were clearing the way for this wild flourishing of new life you've planted. Almost.

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That's because, explained Jablonski, the patterns of diversity in the life we see after extinction do not relate to the patterns of diversity that existed before that extinction. After the Cretaceous–Paleogene extinction that killed off most of the dinosaurs, for example, the top carnivores for a while were not the mammals that ultimately replaced them but giant, flightless birds known as "terror birds." Also known as Phorusrhacids, these nightmare creatures grew up to ten feet tall and weighed 200 lb, and they could hunt you down by chasing you across the South American or Antarctic plains with their great hooked beaks at a pace of 30 mph.

The terror birds, which out-competed the mammals in occupying this particular ecological niche, may even have driven mammalian predators to take to the forests instead. The point here is that if we kill off all the animals in hopes of sparking some glorious diversification as life rebounds in a few millions years' time, there's no guarantee of what we're going to get.

"There's no way our world looks the way it did in the Mesozoic, even without the fact that we no longer have T-Rex and Triceratops. It's also that in the oceans, the lineage, the modes of life that were most diverse no longer have the most species in them," Jablonski told Salon.

"It's so different from the picture that you sometimes get" of post-extinction rebound, where it was seen as a simple recovery of populations, "essentially just recreating the vanished world," Jablonski went on. By contrast, the modes of life (the different ways organisms adapt to their environment) that exist seem to rebound pretty well, but entirely different types of organisms may occupy them (a terror bird, for example, when you were hoping for a cheetah.) And that's important if we were thinking that, given that life's so robust and resilient, we might as well go for broke, destroy everything we can, and enjoy all the new life in a few million years' time.

"The bottom line is that the selectivity of extinction and modes of life has nothing whatsoever to do with the convenience or well-being of humans. The things that survive and probably diversify will of course be the rats and the ragweed and the cockroaches, and so that probably is going to be the shape of the world in the future, unless some really concerted work is done to ameliorate some of the most extreme forces," Jablonski said, noting that it isn't even an issue of having the right numbers of species, but the right modes of life. If you were playing God, for example, you'd want to stash as many pollinators in your ark as you can, and as many plant species that are really good at storing carbon as you can.

"There's no reason whatsoever that the survivors or the rebounders will do that for us unless we actually engineer that," Jablonski said, referring not to bioengineering but to the urgent task of carefully designing nature reserves and migration corridors to protect exploited species at risk from human pressures by conserving large enough areas that the species you prioritize can survive in, taking into account their ability to get there and the expected climate.

"I was talking to someone just last week who was like, 'Oh yeah, cockroaches have survived every extinction event, and they'll be here after everything else dies'. And I'm like, 'Well no, not really'," Dr. Dominic Evangelista, an evolutionary biologist and principal investigator at The Roach Brain Lab at the University of Illinois Urbana-Champaign, told Salon in a video interview.

Evangelista has found that cockroaches, of which there are some 7,000 species, only ten of which are city pests, are younger than previously believed. In the past, fossils of insects that were likely ancestors of both praying mantises and cockroaches (and termites, which are a subgroup of cockroach) called roachoids may have been incorrectly identified as roaches. Now it is believed that the earliest real cockroach fossil is just 125 million years old, meaning it lived well after the Permian-Triassic extinction, when some 90% of all species on Earth were wiped out, and after the Triassic-Jurassic extinction that cleared the way for the dinosaurs. The Cretaceous period was the time of the cockroaches, and they thus survived just one of the five mass extinctions that occurred before the one we're living in right now. Nor are they particularly resistant to radiation compared to most other insects. Not such tough guys after all.

So are humans capable of driving Earth's cockroaches to absolute extinction?

"One hundred percent," Evangelista told Salon. "And we have already seen cockroaches go extinct."

The biggest danger to these supposedly hardy creatures is habitat loss in the tropics, where species diversity is extreme and where the thousands of different species of cockroach tend to specialize, so that different species will be endemic to different areas, meaning that they are found in that region and nowhere else. High levels of endemism make it extremely easy to extinguish a species from existence: burn down one lush, lavishly biodiverse forest and you may have wiped out thousands of species so specialized they live nowhere else and may be unknown to science. Burn enough forest and you risk driving not one, but all 7,000 species to extinction.

In one prescient example, a particular cave in Guinea, West Africa that once sheltered the Simandoa cave roach (Simandoa conserfariam) was destroyed a bit more than a decade ago in a bauxite mining operation. With it went the cave roach, which is now extinct in the wild. That insect, which exists now as a relic in captivity and can be purchased on the internet by hobbyists, is a beautiful creature with rust-coloured legs and a black body, the prothorax outlined in white, concealing a striking black-and-white striped abdomen.

Entire species may vanish before humans have even had the chance to christen them with a name, let alone to understand their behavior, their role in the ecosystem, or (to center our petty human lives again) their potential to, say, fight antibiotic resistant bacteria, or teach robots how to walk. Given that scientists keep finding new species of cockroach, even in highly-sampled areas, that seems more than likely: Evangelista's lab is at work describing species unknown to science from Guyana, an area of pristine and biodiverse forests that are relatively understudied, as well as from neighboring French Guiana, already known as a cockroach diversity hot spot in South America.

So, we can kill cockroaches after all. And in fact, we're doing it all the time. Evangelista said it's hard to prove that something has gone extinct if you didn't know it existed to begin with.

"Personally, I am certain that not only have humans caused some cockroaches to go extinct, but we've probably caused hundreds or maybe thousands of cockroaches to go extinct, and we don't even know about it," Evangelista said. Getting rid of roaches is one thing — but are humans really so powerful as to destroy all life on the third rock from the Sun?

Perhaps it would help to consider what, other than humans or divine intervention, might kill everything on Earth.

The tiny organisms Jablonski mentioned living within the Earth's crust or under glaciers are extremophiles, tiny organisms that live and thrive in the most extreme environments we can imagine (a rather human-centric definition, mind.) There are extremophiles that can thrive under the extraordinarily high pressure at the bottom of the Mariana Trench; in environments more alkaline than pH 11 and more acidic than pH 0.06 (which are both high ends of the spectrum); at up to 252º F in the Earth's crust or in scorching hydrothermal vents; in super-dry, super-salty, or super-cold environments; or under the effects of ionizing radiation.

Boiling off all the world’s oceans seems to be THE initial outcome to worry about on the way to total extinction, at least according to a study from Oxford University titled "The resilience of life to astrophysical events." Researchers David Sloan, Rafael Alves Batista, and Avi Loeb considered the various things that could cause such a calamity in reference to the impact that could be expected upon the tardigrade, a particularly hardy, bizarrely cute micro-animal also known as a water bear.

SEM Micrograph of a water bear, Tardigrade (Getty Images/Cultura RM Exclusive/Gregory S. Paulson)Technically, tardigrades are not extremophiles, because while they can survive conditions that would kill anything else, that doesn't mean they thrive in them. But they're pretty darn tough. The easiest way to kill off the tardigrades, the researchers argue, would be to sterilize the entire planet, adding 5.6 × 1026 J of energy to make the oceans boil off. It would require even more energy to remove the Earth's atmosphere, and you'd also need more radiation to kill a tardigrade hiding deep enough under the sea. Only a large asteroid impact, supernova explosion, or deadly gamma-ray bursts (GRBs) offer much chance of evaporating the oceans. The researchers estimate the likelihood of the various events serious enough to do this at a probability of less than 10⁻⁷ per billion years.

It’s unlikely that there’s anything we puny humans can do that would have the impact of these extremely extreme events, which, as Sloan, Batista and Loeb concluded, were unlikely to finish off the water bear anytime soon. By soon, they mean any time before the sun engulfs us all, which will happen sometime in the next five billion years.

Our star dying is an event, the authors say, that even tardigrades are not going to pull through. Luckily for them, by the time that happens the tardigrades may well have hitched a ride into outer space and traveled well beyond the sun’s overheated grasp. We, on the other hand, will be long gone by then, shriveled by some far more minor cataclysm like the fragile little primates we are.

So if tardigrades (never mind hardier extremophiles) are likely to survive gamma-rays, asteroid impacts and exploding supernovae, it seems highly likely that we're not going to be the ones who strike the fatal blow to all life on Earth.

On the other hand, we're trying really hard.