Will Humans Ever Go Extinct?
The species Homo sapiens evolved some 300,000 years ago and has come to dominate Earth unlike any species that came before. But how long can humans last?
Eventually humans will go extinct. At the most wildly optimistic estimate, our species will last perhaps another billion years but end when the expanding envelope of the sun swells outward and heats the planet to a Venus-like state.
But a billion years is a long time. One billion years ago life on Earth consisted of microbes. Multicellular life didn’t make its debut until about 600 million years ago, when sponges proliferated. What life will look like in another billion years is anyone’s guess, though one modeling study published in 2021 in Nature Geoscience suggests that Earth’s atmosphere will contain very little oxygen by then, making it likely that anaerobic microbes, rather than humans, will be the last living Earthlings.
If surviving to see the sun fry Earth is a long shot, when is humanity likely to meet its doom? Paleontologically, mammalian species usually persist for about a million years, says Henry Gee, a paleontologist and senior editor at the journal Nature, who is working on a book on the extinction of humans. That would put the human species in its youth. But Gee doesn’t think these rules necessarily apply for H. sapiens.
“Humans are rather an exceptional species,” he says. “We could last for millions of years, or we could all drop down next week.”
Opportunities for doomsday abound. Humans could be wiped out by a catastrophic asteroid strike, commit self-destruction with worldwide nuclear war or succumb to the ravages caused by the climate emergency. But humans are a hardy bunch, so the most likely scenario involves a combination of catastrophes that could wipe us out completely.
Pick Your Poison
Some species killers are out of our control. In a 2021 paper in the journal Icarus, for example, researchers describe how asteroids comparable to the one spanning 10 to 15 kilometers in diameter that killed off the nonavian dinosaurs hit Earth approximately every 250 million to 500 million years. In a preprint paper posted on the server arXiv.org, physicists Philip Lubin and Alexander Cohen calculate that humanity would have the ability to save itself from a dino-killer-sized asteroid, given six months’ warning and an arsenal of nuclear penetrators to blow the space rock into a cloud of harmless pebbles. With less warning or a larger asteroid, Lubin and Cohen suggest that humanity should give up and “party” or “move to Mars or the Moon to party.” Currently, the biggest asteroid that scientists know of with the potential of striking Earth is called (29075) 1950 DA. It is a mere 1,300 meters across and has a one-in-50,000 chance of hitting our world in March 2880, according to a 2022 risk analysis by the European Space Agency.
Incoming space rocks aside, many threats to humanity are of our own making: nuclear war, the climate emergency, ecological collapse. Our own tech might do us in in the form of sentient artificial intelligence that decides to snuff out its creators, as some AI critics have suggested.
An all-out nuclear war could easily destroy humanity, says François Diaz-Maurin, associate editor for nuclear affairs at the Bulletin of the Atomic Scientists. The last time humans dropped nuclear bombs on one another, only one country, the U.S., had nuclear warheads, so there was no risk of nuclear retaliation. That’s not the case today—and the bombs are a lot bigger. Those bombs, which struck the Japanese cities of Hiroshima and Nagasaki in 1945, packed the equivalent of 15 and 21 kilotons of TNT, respectively. Together they killed an estimated 110,000 to 210,000 people. A single modern-day, 300-kiloton nuclear weapon dropped on New York City, for example, would kill a million people in 24 hours, Diaz-Maurin says. A regional nuclear war, such as one between India and Pakistan, could kill 27 million people in the short term, whereas a full-scale nuclear war between the U.S. and Russia could cause an estimated 360 million direct deaths, he adds.
The threat to humanity’s very existence would come after the war, when soot from massive fires ignited by the bombings would rapidly alter the climate in a scenario known as nuclear winter. Fears of nuclear winter may have receded since the end of the cold war, Diaz-Maurin says, but research shows that the environmental consequences would be severe. Even a regional nuclear war would damage the ozone layer, block out sunlight and reduce precipitation globally. The result would be a global famine that might kill more than five billion people in just two years, depending on the size and number of detonations.
“That possibility of destroying humanity is still here and real,” Diaz-Maurin says.
Death by ecological contamination or the climate emergency would be slower but still within the realm of possibility. Already humans are facing health stressors from chronic pollution that have been exacerbated by the additional heat brought by climate change, says Maureen Lichtveld, dean of the School of Public Health at the University of Pittsburgh. Hotter temperatures force people to breathe more rapidly to dispel warmth, which draws more pollution into their lungs. The climate emergency also deepens existing problems around food security—for instance, persistent drought can devastate cropland—and infectious disease. “The interconnectedness of climate change and health inequities and inequities in general is what is impacting our global population,” Lichtveld says.
The Perfect Storm
Will these inequities eventually lead to a species-wide downfall? It’s not easy to calculate the likelihood that, say, the climate emergency will kill us all, says Luke Kemp, a research associate at the Center for the Study of Existential Risk at the University of Cambridge. But it’s probably not realistic to consider risks individually anyway, Kemp says.
“When we look at the history of things like mass extinctions and societal collapses, it’s never just one thing that happens,” Kemp says. “If you’re trying to rely on a single silver bullet to kill everyone in a single event, you have to write sci-fi.”
The end of humanity is far more likely to be brought about by multiple factors, Kemp says—a pileup of disasters. Though apocalyptic movies often turn to viruses, bacteria and fungi to wipe out huge swathes of population, a pandemic alone is unlikely to drive humanity to extinction simply because the immune system is a broad and effective defense, says Amesh Adalja, an infectious disease physician at the Johns Hopkins Center for Health Security. A pandemic could be devastating and lead to severe upheaval—the Black Death killed 30 to 50 percent of the population of Europe—but it’s unlikely that a pathogen would kill all of humanity, Adalja says. “Yes, an infectious disease could kill a lot of people,” he says, “but then you’re going to have a group [of people] that are resilient to it and survive.”
Humans also have tools to fight back against a pathogen, from medical treatments to vaccines to the social-distancing measures that became familiar worldwide during the COVID pandemic, Adalja says. There is one example of a mammalian species that may have been entirely wiped out by an infectious disease, he says: the Christmas Island rat (Rattus macleari), also called Maclear’s rat, an endemic island species that may have gone extinct because of the introduction of a parasite.
“We are not helpless like the Christmas Island rat who couldn’t get away from that island,” Adalja says. “We have the ability to change our fate.”
If infectious disease contributes to the downfall of humanity, it’s likely to be as just one piece of a larger puzzle. Imagine a world pushed to upheaval by sea-level rise and disruption to agriculture from climate change. The humans of this climate-ravaged world attempt a geoengineering solution that goes wrong. The situation worsens. Resources are scarce, and a bunch of countries have nuclear weapons. Oh, by the way, the mosquitos that carry yellow fever now range as far north as Canada in this scenario. It’s not hard to see how the human population could decline and disappear in the face of an arsenal of challenges, according to Kemp.
Worst-case scenarios are understudied, Kemp says. In climate science, for example, there is a lot of research into what the world might look like at two or three degrees Celsius warmer than the preindustrial average but very little looking at what an increase of five or six degrees C might look like. This is partly because scientists have a hard time predicting the effects of that much warming and partially because climate scientists feel pressure from politicians not to appear alarmist, Kemp says. Models of future worst-case scenarios also tend to do an inadequate job of predicting the cascading effects of a disaster. “The general field of existential risk is relatively new, nascent and just understudied,” he says.
There are questions as to how much humans should worry about something as big-picture as extinction. While some see the question as pressing—controversial tech billionaires such as Elon Musk and Peter Thiel have funded organizations dedicated to studying the risks of transformative technologies—others argue that today’s problems are urgent enough. Already humans are heating the globe, overexploiting and destroying nature, using land and water unsustainably and creating chemicals that are harmful to all life, often in service to the globally well-off, says Sarah Cornell, who studies global sustainability at the Stockholm Resilience Center at Stockholm University.
“Today’s reality is that some human beings are undermining or even destroying living conditions of many, many other people,” Cornell says. “From a human-scale perspective, this is an existential crisis already, not a risk somewhere up ahead.”