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During World War II, Londoners often sought shelter from German bombs in the city’s subway tunnels. There, they encountered another type of enemy: hordes of voracious mosquitoes. These weren’t your typical aboveground mosquitoes. They were natives of the metro, born in pools of standing water that pockmarked the underground passageways. And unlike their open-air cousins, London’s subterranean skeeters seemed to love biting humans.
Fifty years after the war ended, scientists at the University of London decided to investigate the subway population. They collected eggs and larvae from subway tunnels and garden ponds and reared both populations in the lab. The tunnel bugs, they confirmed, preferred feeding on mammals over birds. And when the scientists put males and females from different populations in close quarters designed to encourage mating, not a single pairing produced offspring. That sealed the deal: The underground mosquitoes were a whole new species, adapted to life in the subway tunnels people had built.
It’s stories like that one that got Joseph Bull thinking. As a conservation scientist at the University of Copenhagen, he hears a lot about how humans are driving other species extinct. If the current rate stays steady, the planet is on its way to its sixth mass extinction, a severe event on par with the meteorite impact that killed the dinosaurs. But he wondered if there might be a flip side. “I hadn’t really seen any kind of analysis of whether all these kinds of activities that humans get up to around the planet, whether and how they cause new species to emerge,” he says. The Anthropocene—while not quite yet an official geological epoch, still a supremely useful concept—is defined by the myriad ways in which humans affect the Earth. Civilization is destructive, but it’s generative too, sometimes in disturbing ways. A new world will emerge out of the Anthropocene, and it will be shaped by the species humans create and foster as well as the ones they kill off.
The most obvious way that people create new species is through domestication. By picking out the traits in a wild population that are most beneficial to humans and breeding for them, people can “force evolution in different species,” Bull says. Wolves become dogs, nubby grass becomes maize, wild boars become pigs.
But humans can drive speciation in other, less purposeful ways. “It’s important to think about the creation of new species as a process,” Bull says. One of the most dramatic ways people put that process in motion is by moving members of an existing species from one place to another. Sometimes those individuals die in the new environment. Sometimes they hang on and interbreed with native species. And sometimes, they take over, like kudzu in the American South or snakes on Guam. Over time, the new environment exerts different pressures on the invasive population, causing it to diverge from its ancestors. The invasive species might also change the game for native species, pushing them in new genetic directions (if, of course, it doesn’t just drive them extinct).
Although hunting is one good way to drive a species extinct (just ask the passenger pigeon), it can also spur evolution by removing certain types of individuals from a species gene pool—birds of an easy-to-see color, say, or fish large enough to be caught in a net. No new species is known to have been created through hunting alone, Bull says, but given enough time it’s far from impossible.
Finally, we have the process that created the underground mosquito: People’s propensity to create whole new ecosystems, including and especially cities. Populations of animals colonize these new environments and adapt to their demands, from mosquitoes developing a taste for mammals blood underground to city birds becoming better problem-solvers than their rural relatives.
Keeping these mechanisms in mind, Bull tallied up humans’ impact on species in a paper published today by the Proceedings of the Royal Society B. During the last 12,000 years, scientists have recorded 1,359 plant and animal extinctions. Meanwhile, humans have relocated 891 plant and animal species, and domesticated 743—for a total of 1,634 species. It seems that human-driven speciation could be as much a mark of the Anthropocene as extinction is.
Of course, extinction, like speciation, is hard to document as it’s happening. Many species likely disappear before scientists even know they are there. That’s why extinction rates are usually calculated with extrapolations and models, but even they give wildly different numbers. That’s all to say that many more than 1,359 lifeforms have likely gone extinct in the past 12,000 years. Though it’s possible humans create species without detecting them, too. Just think of the wild world of antibiotic-resistant microbes, which evolve so fast in response to drugs that it’s dangerously difficult to keep up.
Number of species, however, is just one way measure the effects humans are having on nature—and maybe not the best way. Drive keystone predators like wolves or sharks extinct and entire ecosystems collapse, no matter how many new species pop up to replace them. What’s more, older species can carry millions of years of evolutionary history in their genes; if they go extinct, that diversity is lost. “Anthropogenic species represent a nanosecond of the evolutionary time that many ‘natural’ species have passed through,” says Christopher Dick, an evolutionary biologist at the University of Michigan. “In conservation, there is no comparing a 10-million-year-old tree or turtle species with a decades-old strain of insect or plant.”
Bull agrees that speciation and extinction don’t cancel each other out. “If we only use number of species as a way of measuring progress that someone makes on conservation, then we’re missing a load of other important considerations,” he says. “We cannot replace something lost with something gained when it comes to nature.” Human-driven speciation may turn out to be a calling card of the Anthropocene. But no matter how many species of underground mosquitoes humanity inadvertently creates, they won’t make up for what it destroys.
