On September 28, 2015, James Bristle and his friend were hard at work on his farm in the outskirts of Chelsea, Michigan. Perched in the driver's seat of a backhoe, Bristle was digging a trench for a new drainage pipe. The noise of his machine's engine and hydraulic shovel disrupted what was otherwise a serene soybean field. Racing to get the job done as dusk approached, Bristle had no idea that, with each payload of earth, he was digging closer to the remains of a lost world. About eight feet down, the shovel hit something. "We thought it was a bent fence post," Bristle told his local newspaper. "It was covered in mud."

The object turned out to be a massive bone, belonging to something not immediately recognizable. Word of the discovery quickly reached Daniel Fisher, a paleontologist at the University of Michigan. Within a few days Fisher and his team were deep in a 10-foot excavation hole (with an excited crowd of onlookers gazing over them). The exhumed carcass, the scientists confirmed, belonged to an extinct animal: a woolly mammoth that died over 11,000 years ago. The creature's strange burial, missing body parts, and the discovery of remnants of what may have been a stone axe all pointed to the likely source of the beast's demise: It had been butchered to death by humans.

Woolly mammoths once lumbered over an area that stretched from the rocky shores of Ireland, across Europe and the Siberian tundra, through Alaska, down to Kansas, and almost all the way to East Coast. They were amazing animals—large, social, and intelligent like elephants, with evolutionary adaptations that made them suited for the cold. They survived over 300,000 years, through warm and cold spells and all the other menaces that nature threw at them. But then, less than 4,000 years ago, they disappeared off the face of the Earth.

Ever since the turn of the 19th century, when the French naturalist Georges Cuvier first identified the woolly mammoth as a long-extinct creature, scientists have played the role of detective, solving the mystery of who or what murdered the mammoths. As it turns out (thanks to the discovery of fossils in the years that followed) mammoths weren't the only victims: There were mastodons, six-foot tall beavers, ground sloths the size of black bears, armadillos bigger than Volkswagen Beetles, American lions and cheetahs and camels, and saber-toothed tigers—to name just a few. Between 10,000 and 13,000 years ago, the Americas alone—once a zoo filled with enough wonders to put a modern African safari to shame—lost over 70 genera of large mammals.

For a long time, the prevailing theory pinned climate change as the culprit behind these disappearances. The end of the Ice Age, the argument goes, precipitated natural changes that killed these animals off. But why was it that big mammals died at greater rates? And why were the losses so heavily concentrated in the Americas? Beginning in the late 1950s, the legendary geoscientist and University of Arizona professor Paul Martin developed an answer to these questions that would shake the scientific establishment. The evidence, he believed, was not consistent with climate change. The truth was more ominous.

"The pattern just emerged. We weren't looking for it."

The late Martin, often photographed wearing suspenders and a hat reminiscent of Indiana Jones, became a giant in his field, especially after 1973, when he published a seminal paper in Science that included what came to be known as "the overkill hypothesis." Martin proposed that, around 13,500 years ago, when human colonizers, the so-called Clovis people, first crossed the Bering Land Bridge from Eurasia into Alaska, they "spread southward explosively, briefly attaining a density sufficiently large to overkill much of their prey." According to his theory, humans cruised through the American continents on a murderous spree that wiped out dozens of species.

The late Paul Martin. (Photo: Thomas R. van Devender/Wikimedia Commons)

The idea was controversial, to say the least, and many brushed it aside. But it opened up a debate that proved hard to settle. "The problem is that the fossil record rarely discloses the cause of mortality, much less of extinction," Martin explained in his memoir. Adding to his difficulties, sample sizes tend to be small in archaeology and paleontology (they have to dig up their data, site by site), and the two main culprits for extinction—the expansion of human beings into new habitats and climate change—occurred simultaneously, making a silver bullet hard to find.

But Martin kept at it. The truth, he reasoned, was buried just beneath our feet. With enough data from fossil bones, the timing of the extinctions, he argued, could be "used to map the spread of homo sapiens throughout the New World." If the data showed a pattern—humans arrive, animals die—his theory could be proven correct. All they had to do was keep digging.

Count Todd Surovell, director of the Frison Institute and professor of anthropology at the University of Wyoming, among those who were skeptical of the overkill theory. This fact proved slightly awkward for Surovell, because he knew Martin well. "I first met him around 1996 when I was a graduate student at the University of Arizona," he says. "I was sitting in an empty classroom waiting for one of our department seminars to begin when he came in and sat down next to me. He introduced himself. I was a bit star-struck." A bright-eyed graduate student who found himself sitting alone with a giant in his field, Surovell politely introduced himself in return. But he couldn't hold back.

"I don't believe in overkill," he confessed.

Martin, known for his gentle demeanor, responded to the new student with his signature folksy charm: "Well, I'm far out a limb, but I'm pretty comfortable out here," Martin said, by Surovell's recollection. It was an interaction that Surovell would never forget.

Over the next seven years of Surovell's graduate education, he would visit Martin in his office, where the two would discuss the extinctions that occurred toward the twilight of the Ice Age. "He took me under his wing," Surovell says, despite the fact that Surovell, like many in his discipline, remained an open skeptic of Martin's theory.

Meanwhile, evidence was slowly piling up suggesting Martin was right after all, especially when it came to the woolly mammoths. In 1993, after discovering new fossils, scientists learned that woolly mammoths had, in fact, survived the end of the Ice Age. While they were wiped off the mainland, they continued living on Wrangel Island, 90 miles north of Siberia in the Arctic Ocean, and on St. Paul's Island, just south of Alaska. They died there of mysterious causes less than 4,000 years ago—long after the end of the Ice Age. "Both Wrangel Island and St. Paul's Island were apparently unknown to prehistoric hunters until thousands of years after the spread of Clovis hunters into North America," Martin said in an interview with American Scientist. But the debate still wasn't settled: Some argued it was possible these islands, so close to the North Pole, retained a climate and vegetation that sustained the creatures. "At times the dispute resembles the partisan passion of a hard-fought political campaign," Martin wrote in his memoir.

A little over a decade ago, Surovell, still in the opposition party, began researching the issue in earnest himself. His research started with the woolly mammoth's entire family, technically an "order," the proboscideans—a group that includes at least four other varieties of mammoth, as well as gomphotheres, the American mastodon, and modern-day elephants. "They could be found in hot, cold, wet, and dry environments over five continents," Surovell says. Though this vibrant group of creatures once filled the world, only elephants—now found in zoos and small swathes of sub-Saharan Africa and South Asia—are still living.

Researching the hunting patterns of prehistoric humans, Surovell and University of Wyoming associate professor Nicole Waguespack decided to compile a data set of all the records of proboscidean kill and scavenge sites closely associated with human artifacts, organizing them by date and then mapping them around the world. "The global pattern just emerged," he says. "We weren't looking for it." They published their findings in 2005, in the science journal PNAS. Their conclusion was crystal clear: "Humans disperse into new regions. They exploit proboscideans. Proboscidean suffer local extinction."

James Bristle (left) and Daniel Fisher. (Photo: University of Michigan)

"That was the turning point for me," Surovell says. He began becoming an overkill convert. "Of course, it wasn't just proboscideans that suffered extinction. A huge array of large bodied species did as well." In January 2016, Surovell and his colleagues published another paper in PNAS, this time testing Martin's overkill model of the Americas. They plugged in the radiocarbon data of extinct big mammals, from a large number of discoveries like the one unearthed on James Bristle's farm. Surovell and his team found that the extinctions began near Alaska, spread down the continental United States, and then continued through South America—just as Martin had predicted. "This finding is difficult to reconcile with a climatic explanation for extinction, unless one were to argue that climate change mimicked the direction and timing of human expansion across the New World," Surovell says.

As the debate rages on over which animals were the victims of overkill, the evidence keeps stacking up that humans were to blame for many of these disappearances, particularly woolly mammoths. Two weeks after exhuming the dead mammoth on James Bristle's soybean farm, Daniel Fisher and Michael Cherney released a study showing chemical evidence that, over a span of thousands of years, baby mammoths weaned from their mother's milk for increasingly shorter periods—a pattern, they say, that's observed in modern animals facing intense predation. "This shift to earlier weaning age in the time leading up to woolly mammoth extinction provides compelling evidence of hunting pressure and adds to a growing body of life-history data that are inconsistent with the idea that climate changes drove the extinctions of many large ice-age mammals," Cherney said.

With growing evidence that humans caused the extinction of the woolly mammoth and its cousins, an ongoing tragedy is put in an even more disturbing light: The only proboscideans that remain, elephants, are being slaughtered at an alarming rate. The global population has fallen from 1.2 million in 1980 to fewer than 500,000 today.

"It is the final chapter of a story that began around a million years ago," Surovell says.

Now it's machine guns and machetes, not Stone Age weapons, murdering elephants to supply a global demand for ivory that pays big bucks. The economic processes may look different, but the underlying causes remain the same. If the overkill theory is indeed correct, it would mean that proboscidean extinction has always been an economic problem, one that existed even before modern technology or mass-consumption, nation-states, industry, or capitalism.

This economic problem is more than just a moral tragedy. It's a collectively irrational outcome—a clear case of "the invisible hand" leading away from the outcome that would prove best for society as a whole. Hunters and poachers have done more than kill off animals and disrupt ecological systems with their overkill (which can cause economic damage); they also destroyed sources of calories, clothing, trinkets, and, to those who care, the joy of seeing these creatures in the wild (which also has economic value, in the form of zoos and eco-tourism). Humanity is objectively worse off as a result. But here's the thing: The creatures were a renewable resource. Had people shown some restraint, these animals would have re-stocked their population, providing society with all of the associated future benefits, hunting or otherwise.

Stone Age humans have some excuse for failing to reach this better outcome. As science writer Elizabeth Kolbert chronicles in her book The Sixth Extinction, mankind didn't even begin recognizing that animals could go extinct until well into the French Revolution.

"It is the final chapter of a story that began around a million years ago."

But we now have all the information we need. And the knowledge that creatures can go extinct, even with modern culture and institutions, may not be enough to save wild elephants. The problem cuts deeper. Even if the prehistoric hunters knew and cared about the consequences of their actions, an economic issue would have thwarted any coordinated effort to prevent overkill. It is the same economic problem that affects wild elephants today. It is known as the "tragedy of the commons"—the same issue that leads to deforestation and overfishing. It is an incentive problem that people face when they encounter open-access resources (meaning those you can't prevent other people from using): Why stop hunting when you have no assurances that others will also stop? Or, put more practically, why clean the sink when you know your roommate will throw his pasta sauce-stained bowl in there later? These poisonous incentives affect common resources like a plague, leading to their overuse and, in the case of animals, their overkill.

To be sure, this is also a story about evolution, which has not been on the side of proboscideans. While they are incredibly smart (they are known, for instance, to mourn their dead), they are big and slow—the perfect targets for a predator equipped with the right tools. They also take a long time to reproduce: Mother elephants remain pregnant for two years, and typically give birth to only one calf. But their greatest evolutionary disadvantage seems to be what makes them the most valuable to humans: They are meaty, with beautiful ivory tusks. Found grazing and drinking in open areas, they are lumbering sacks of economic value ready for the taking. When the creatures encountered a new apex predator, the homo sapien, the odds were stacked against them.

But the tragedy of the commons can be prevented. The standard economic solution to this problem is for the government to intervene in some way, creating some sort of regulatory scheme to coerce or incentivize people to act in the group's interest. Another solution, if possible, is to assign property rights. The logic is that, when people have personal control over resources, they will have incentives to see them last. In their paper, "Elephants," published in the American Economic Review in 2000, the Harvard economist Michael Kremer and Charles Morcom build a theoretical case that elephant extinction can be prevented either through tough government penalties (if the governments are credible enough as a threat to poaching), intervening in the ivory market to disincentivize hunting, or creating big sanctuaries (essentially nationalizing or privatizing the elephants).

Stone Age hunters, of course, lacked the institutional machinery for any of these solutions. But, as political scientist Elinor Ostrom found in her Nobel Prize-winning research, governments are not necessarily necessary for overcoming the economic dysfunction that people face when they encounter open-access resources. One of Ostrom's cases studies, for example, showed that fishermen in Nova Scotia developed informal rules and norms to prevent overfishing over the course of the 19th and 20th centuries. This case and others like it led Ostrom to argue that small groups can develop social norms to do what is in their collective best interest—even in the face of the poisonous economic incentives that can lead to the tragedy of the commons. In other words, people can evolve and do what's best for their community, even in the face of temptation and with the absence of the strong arm of the government.

But today, despite the hard work of people around the world—and government interventions, some of which may be paying dividends—elephants are still very much at risk of extinction. In order to have an Ostrom-style solution, or any solution, communities need the political will to overcome the tragedy. Stone Age hunters didn't have it. Neither do modern poachers, who often come from desperately poor communities. Economists have argued that conservation and other environmental causes are a "luxury good," like a flat-screen television or sports car—something people demand only once they get rich enough. That's why, they believe, issues like this only become a priority once societies develop. Could that change come too late for the elephants?

In a working paper released in December 2015, the economists Naima Farah and John R. Boyce find that the discovery and exchange of mammoth tusks is having a serious effect on the market for living elephant tusks. Since the collapse of the Soviet Union, they write, tusks from dead mammoths, found in the frozen Siberian tundra, have risen to account for as much as 20 percent of all ivory production. Crunching the numbers, they conclude, "Mammoth ivory trade may be saving elephants from extinction." In the long run, however, it may be too optimistic to believe that such a laissez faire solution can forestall wild elephant extinction.

The much-beloved Paul Martin had his own solution to the tragedy facing modern elephants. He advocated for "rewilding" them in America—shipping them over from Africa and Asia and letting them loose into the wild (or, at least, into much bigger private sanctuaries than the ones that exist now). In addition to saving the species (and returning proboscideans to an ancestral home), Martin believed this would provide an added benefit to ecosystems damaged by the creatures' disappearance.

Imagine herds of wild elephants roaming America, thriving with the protection of philanthropic or state institutions. It'd be quite the sight. More than that, it would prove that our species has evolved enough to overcome the tragedy that likely befell the woolly mammoth.

Lead Photo: A mural by Charles R. Knight depicting a herd walking near the Somme River. (Photo: Wikimedia Commons)