The Slippery Slope to Extinction

The Slippery Slope to Extinction

Neanderthals had bigger brains, sharper vision and were better adapted to the environment than homo sapiens. How did we replace them as Eurasia’s apex predator?

By Toby Lester

In the summer of 1856, in the Neander Valley, just east of Düsseldorf, a group of German miners entered a cave and began doing what miners do: They dug. To their surprise, they unearthed pieces of what appeared to be a very old human skeleton. Word of the discovery soon reached a local naturalist, who brought it to the attention of the anatomist Hermann Schaaffhausen. Might the good professor be able to take a look at the bones and date the skeleton?

Schaaffhausen was startled by what he found. The skull differed distinctly from modern human skulls. Not only that, the skeleton appeared to have been buried tens of thousands of years ago, well before modern humans arrived in Europe, at a time when mammoths and woolly rhinos roamed the continent. Schaaffhausen speculated that this Neanderthal Man (Neanderthal is German for Neander Valley) must have belonged to a prehistoric race of humanlike savages. When the geologist William King studied the evidence not long afterward, he agreed and in 1863 gave this race a name: Homo neanderthalensis.

Darwin had published “On the Origin of Species” just four years earlier, and for advocates of his new theory of evolution, the Neanderthals were a godsend. Here, it seemed, was the “missing link”: a barrel-chested hominid with a prominent brow, broad nose and thrust-forward jaw who helpfully bridged the gap between apes and human beings. To King, among others, one thing seemed certain about this new species: It manifested “simian peculiarities” and could not engage in moral or religious thought. Neanderthals were brutes.

That’s what most people still think. But recent discoveries are forcing a radical revision of this conception. In the past decade, evidence has emerged that Neanderthals painted pictures, adorned themselves with shells and feathers, created ochre and other pigments, buried their dead and had the capacity for speech—all advanced cognitive behaviors once thought to be uniquely human. They also seem to have built boats and sailed around the Mediterranean for 50,000 years before modern humans took to the sea. “They were sapient people,” the Paleolithic archaeologist João Zilhão has said. “That is probably the implication of the last decade of results.”

DNA evidence suggests that Neanderthals and modern humans diverged from a common ancestor in Africa about 350,000-400,000 years ago. Between 300,000 and 200,000 years ago, the fossil record suggests, Neanderthals gradually moved out of Africa and into parts of the Middle East, western Asia and Europe, where for a very long time they thrived. They used fire, cooked meat and vegetables, made tools and perhaps played musical instruments. They had bigger brains and sharper vision than we do; they were stronger than we are; they could tolerate cold temperatures better. Some had blond or red hair and pale skin. They were an evolutionary success story. Then, about 42,000 years ago, they entered a steep decline and eventually went extinct, along with many of the large animals they had shared the continent with for some 200,000 years.

What happened? Scientists have been debating that question ever since Neanderthal Man was discovered, and in “The Invaders: How Humans and Their Dogs Drove Neanderthals to Extinction,” the anthropologist Pat Shipman proposes a new answer that, intriguingly, has to do with . . . the whites of our eyes.

We didn’t evolve from Neanderthals. For hundreds of thousands of years the two species evolved separately—Neanderthals in Eurasia, modern humans in Africa—and only met up again about 50,000 years ago in Eurasia. Until recently, most paleoanthropologists assumed that by about 40,000 years ago Neanderthals and modern humans were coexisting in some parts of Europe and continued to do so until as late as 24,000 years ago, a remarkably long period for such strikingly similar predators.

In this scenario, two main factors led to the demise of the Neanderthals. The first was the Campanian Ignimbrite, a massive volcano eruption that occurred between 40,000 and 39,000 B.C., near modern-day Naples, and brought on a period of volcanic winter, which forced the Neanderthals into tiny isolated refuges on the Iberian Peninsula, where ultimately they died out. The second factor was the greater intelligence and resourcefulness of modern humans, who, the theory goes, simply outsmarted Neanderthals in the competition for habitat and food.

Ms. Shipman, a genial and authoritative guide to a complex field, carefully surveys the latest archaeological and genetic research, including the dramatic 2014 sequencing of the full Neanderthal genome, and rules out a dominant role for either of these factors. The Neanderthals, she notes, survived many periods of abrupt climate change prior to the eruption, so why should this one have been any different? They’d had some 200,000 years to adapt to the ups and downs of the Eurasian climate, in fact, so from an evolutionary perspective they would have been better able to survive the change than their African cousins. As for the intelligence question: Given what we now know about the cognitive abilities of Neanderthals, the theory that they were simply outsmarted by modern human beings just doesn’t hold up. Something else happened.

A possible explanation began to emerge early in 2013, when a team of archaeologists at Oxford University, using newly refined radiocarbon-dating techniques, re-examined the evidence at 11 Neanderthal sites on the Iberian Peninsula. These sites, dated to roughly 24,000 years ago, were the sole basis for the theory that Neanderthals had coexisted in Europe for millennia with human beings. But the new study pushed that dating back some 15,000 years and concluded that the Neanderthals died out between 41,030 and 39,260 years ago—quite possibly before the Campanian Ignimbrite and just a few thousand years after the arrival of modern human beings. People, not climate change, would seem to have triggered the demise of the Neanderthals. But how, if Neanderthals were as smart as we were and better adapted to their environment?

In search of an answer, Ms. Shipman turns to the relatively new field of invasion biology: the study of invasive species and their ecological impact. When we think of invasive species, we think of such plants and animals as kudzu, the zebra mussel and giant African snails, all of which are ranked among the world’s 100 most pernicious on the Global Invasive Species Database. But Ms. Shipman points out a problem with the database: It makes no mention of people, whom she calls “the most invasive species that has ever lived.”

This isn’t a rogue notion. Ms. Shipman cites a 2005 study by two ecologists titled “Fifty Millennia of Catastrophic Extinctions After Human Contact,” which provides abundant evidence for what its authors call “a global pattern of human arrival followed by faunal collapse and other ecological changes.” Decimating other species and radically altering our environment, in other words, is what we do. Whenever we’ve moved into new environments, moreover, we’ve always visited a special devastation on apex predators, who play a critical role in maintaining the integrity of ecosystems. Remove a well-established predator or add a new one and you often get what’s called a trophic cascade: the unraveling of an entire ecosystem.

Ms. Shipman cites a very recent example. A century or so ago, we Americans killed off the wolves living in Yellowstone National Park, because they threatened our livestock. In the absence of wolves—the apex predator of the ecosystem—elk proliferated and devoured the aspen saplings in the area. This changed the nature of the forest, depriving beavers and birds of their habitat and food. Without the ponds made by beavers, and without the dispersal of seeds by birds, fewer succulents grew in the spring, which denied grizzly bears food they needed upon emerging from hibernation. And on and on: a trophic cascade.

Something very similar happened soon after modern humans arrived in Europe, Ms. Shipman argues. In the space of just a few thousand years, as we spread through the region, we killed off the apex predators: first the Neanderthals and then, over time, cave bears, cave hyenas, lesser scimitar cats, dholes, mammoths and woolly rhinos, among other animals. How did we manage this? According to Ms. Shipman, we enlisted the help of dogs.

Until recently, this would have been a laughable idea. That’s because the generally agreed-upon date for the initial domestication of wolves (which is what dogs are, evolutionarily speaking) used to be 18,000 to 14,000 years ago, long after the demise of the Neanderthals. But Ms. Shipman reports that in 2009 researchers developed a technique for distinguishing the craniums of wolves and dogs, which they then used to date an early dog skull to roughly the era when the spread of humans to Europe seems to have led to the extinction of Neanderthals and other apex predators. Other similarly early dog craniums have since been identified—but only at human, not Neanderthal, sites.

Ms. Shipman devotes the final third of her book to exploring a fascinating range of evidence—genetic, archaeological, anthropological—that provides substantial support for this theory. She never proposes that the alliance of humans and dogs alone led to the extinction of the Neanderthals. In all likelihood, she writes, the mere presence of humans, a competitive new predator in the Eurasian ecosystem, was an important stressor, as were climate change and perhaps even infectious diseases brought by humans from Africa. But the domestication of dogs, she suggests, significantly tipped the balance: “The unprecedented alliance of humans with another top predator (wolf-dogs or a kind of wolf) may have been the final stress that pushed Neanderthals and many other species down the slippery slope toward extinction.”

So how did humans manage to domesticate wolves while their Neanderthal cousins, so similar in so many ways, did not? Here Ms. Shipman gets imaginative. Modern humans, she writes, have recently been shown to be the only extant primates whose irises are surrounded by white scleras—the whites of our eyes. We’re also the only primate to have eyelids that expose much of our scleras. What evolutionary advantage could this have possibly given us? “The white scleras and open eyelids,” she proposes, “make the direction of a person’s gaze highly visible from a distance.” Having white scleras allowed us to communicate subtly at a distance among ourselves and with our new best friend, dogs, a biological advantage that may have made all the difference as we competed for prey with Neanderthals—who, if they were like every other primate we know of today, had dark scleras.

Most animals, including apes and wolves, don’t make eye contact with humans; nor do they gaze at faces for long. Dogs, on the contrary, are excellent gaze-followers, a trait that scientists believe we selectively bred into them during their domestication. Once we had teamed up with dogs, we were unstoppable. In effect, Ms. Shipman writes, we had our first “living tool”: a domesticated fellow-predator who helped us run faster, see and smell better, ward off our enemies, guard our kills more effectively, and, in general, devote our energy to other kinds of social activity. The Neanderthals—not to mention the other big mammals we seem to have killed off after reaching Eurasia—didn’t stand a chance.

Ms. Shipman admits that scientists have yet to find genetic evidence that would prove her theory. Time will tell if she’s right. For now, read this book for an engagingly comprehensive overview of the rapidly evolving understanding of our own origins. And allow yourself to be tickled by the thought that, when it comes to what tipped the balance in our struggle for survival with the Neanderthals, the eyes might indeed have it.

—Mr. Lester is the author of “The Fourth Part of the World” (2009)
and “Da Vinci’s Ghost” (2012).