The Invaders: How Humans and Their Dogs Drove Neanderthals to ExtinctionBy Pat ShipmanThe Belknap Press of Harvard University Press, 2015The humans who lived in Europe during the Pleistocene 45,000 years ago faced a fierce world. Their populations had endured sweeping climate changes over the centuries, and they shared their landscape with animals as monstrous as anything the world had seen since the dinosaurs: massive cave bears, saber-toothed tigers, lions bigger than any in Africa today, cave hyenas, huge woolly mammoths, woolly rhinoceri, wolves, leopards, roving packs of dholes - a world as fearsome and strange as something out of a science fiction novel.And one of the strangest things about that world was those humans themselves, because they weren't us. They were squatter, heavier, hairier, much stronger. They had very large brains, very skillful hands, a strongly family-oriented culture, and considerable technological capabilities. They were a species of human we know as Homo neanderthalensis, the Neanderthals, and they'd lived and laughed and hunted and died for millennia in Eurasia, weathering radical climate shifts and game migrations, raising their young, caring for their infirm, and burying their dead.Things had been like that in Eurasia for hundreds of thousands of years. Then, roughly 40,000 years ago, everything began to change in what, anthropologically speaking, amounts to an eye-blink of time. Cave bears, saber-toothed tigers, mammoths, rhinos, lions, leopards, dholes … fierce as they were, they all vanished from the forests and steppes of Eurasia. And Neanderthal populations first drastically dwindled and then vanished as well, and now, in our time, for two hundred years, ever since the discovery of the first Neanderthal fossils, debate has raged as to what caused this catastrophic die-off. Did the climate shift too suddenly for adaptation or migration to keep pace? Did reproductively vulnerable species 'bottleneck' and enter irretrievable decline? Unlike with the extinction of the dinosaurs 66 million years ago, which was precipitated by a cataclysmic asteroid strike, the near-total extinction of the Pleistocene Eurasian species spectrum has no one prevailing theory to explain things.There is an overwhelmingly likely culprit, however. Scientists, much like Sherlock Holmes, try never to theorize in advance of the facts, but nevertheless, something happened to the continent of Eurasia right before all its megafauna – including its resident species of human being – disappeared.Modern humans happened. Homo sapiens arrived in Eurasia roughly 45,000 years ago, and very shortly thereafter, virtually every large species of prey animal and competing predator was gone. The patently obvious deduction is that Homo sapiens intentionally and methodically wiped out all those other species.But science teaches the invaluable habit of distrusting the patently obvious, and so retired anthropology professor Pat Shipman takes very little for granted in her endlessly fascinating new book The Invaders: How Humans and Their Dogs Drove Neanderthals to Extinction. In fact, she has her own hypothesis to put forward as to a key factor that hastened the Neanderthals' demise. Perhaps you spotted it in her book's subtitle.Shipman characterizes modern humans as the ultimate invasive species, flooding into a new ecosystem and radically destabilizing every aspect of it. In her clear-cut and pervasively (but perhaps not entirely intentionally?) ironic chapter “What Does an Invasion Look Like?”, Shipman illustrates the whole concept of invasive species with a modern example: the Greater Yellowstone Ecosystem and the much-smaller Yellowstone National Park inside it. The Park was officially designated in 1872, long before which the native inhabitants of the region, the Shoshone, Nez Perce, Crow, Cheyenne, and others, had been driven off or exterminated – at which point, Shipman writes, “The incoming white human settlers functioned as an invasive predator and promptly eliminated their chief remaining rivals, the wolves.” In 1915 the Federal Bureau of Biological Survey and its Division of Predator and Rodent Control sought to wipe out all large predators from federal lands, which in the case of wolves was largely accomplished by the 1930s.And as Shipman succinctly points out, “The removal of wolves, the indigenous apex predator, made a huge difference to the ecosystem.” Everything from elk population to foliage density was altered, and it all began to alter again when wolves were re-introduced to Yellowstone National Park in 1995-96:

Almost as soon as wolves were released in 1995, they began killing and driving away coyotes. It was as if the first item on their agenda was “get rid of coyotes,” as the ranchers' and settlers' agenda had been first “get rid of Indians” and then “get rid of wolves.” Quite simply, the wolves would not tolerate the presence of coyote rivals in their territories – and they were equally merciless with members of other wolf packs that strayed onto their turf.

And if modern humans and wolves sound formidable individually, imagine how much more formidable they'd be if they worked together. That's the heart of Shipman's hypothesis: that Homo sapiens, roughly ten thousand years after arriving in Eurasia, stumbled onto a crucial adaptation that had eluded all other apex predators before them: making an alliance with another apex predator. Somehow, modern humans forged a symbiotic relationship with wolves that quickly led to a kind of wolf-dog that was no longer entirely wild. Homo sapiens had domesticated the competition.Shipman theorizes that it was a feat Neanderthals couldn't match. “Whatever abilities modern humans used to capture and apparently domesticate wolves into wolf-dogs,” she writes, “were either unknown to Neanderthals or beyond their capabilities.” She even offers an intriguing possibility for what Homo sapiens' x-factor might have been: the whites of their eyes! The idea being that the white sclera surrounding the modern human iris greatly facilitates gaze-directed silent hunting – an obvious advantage in tracking prey – and that it's something certain kind of canids share to a greater degree than others (hence the centrality of bright-eyed wolf to the story rather than, say, omnipresent but black-eyed bush dog). Thus the tendency of modern dog-owners to stare meaningfully into their dogs' upturned faces might be vital in explaining how either the dog-owner or the dog is here at all.Modern humans, Shipman contends, in forming this kind of “unprecedented alliance with another species … created for ourselves an ability to borrow the traits of other species and use them to enhance our own survival in almost every habitat on the planet.” Wolves instinctively understood exactly the kind of hierarchical social structure humans already had, which made it that much easier for Homo sapiens to begin domesticating wolf-dogs and using them in ways present-day hunters will recognize: a pack of canines can detect prey long before humans can, and they can chase that prey farther and longer than humans can, and, crucially, they can keep that prey at bay and stationary until humans can arrive with their superior numbers and projectile weapons. The wolf-dogs would have realized in short order that in exchange for their instinctive distrust of hominins the arrangement would garner them more reliable kills. And the humans would have seen that the wolf-dogs were helping to secure more meat than they'd provide if they themselves were simply slaughtered. And so the 35,000-year-old partnership between humans and dogs began – in multiple genocides.It's a word Shipman never uses, although she holds no illusions about the ominous timing of the whole thing. Modern humans appeared, spread through the Neanderthals' range, eventually domesticated wolf-dogs, and shortly thereafter, the Neanderthals were gone. Good cautious scientific thinker that she is, Shipman makes no declarations, and she's careful to stipulate that drastic climate change also played a key role: it was the “synergy” between the advent of modern humans and a sudden shift in climate that tipped the balance against the Neanderthals. She even occasionally goes so far as to allow for the absurd possibility – mentioned also elsewhere in the literature of Neanderthal extinction – that there were no hard feelings:

People today are often frightened of strangers; how much more threatening would meeting another hominin species be? Limited resources and food competition would only heighten the fear. Possibly there was no conscious awareness of competition between Neanderthals and modern humans, but equally possibly, there was. At any rate, Neanderthals went extinct after the arrival of modern humans and possibly not long after.

The ideas are unfailingly thought-provoking, but there's a vein of contradiction running through a good deal of The Invaders, and it's hard not to suspect it's because the author is a member in good standing of Homo sapiens. The central contradiction is actually embodied in that Yellowstone example: the point isn't the white settlers and the wolves – it's the white settlers and the Indians. Where newly-introduced wolves are content merely to kill or drive off direct rivals for prey, Homo sapiens tends to be much more industrious when it comes to extirpation. There are wolves and coyotes in Yellowstone today. But all of Eurasia wasn't big enough for Homo sapiens and Neanderthals.The modern human propensity for genocide is aptly illustrated in the case of mammoths. Remains of the animals have been found in connection with many Neanderthal sites, but it's only with the advent of modern humans that the mammoth population as a whole begins to be affected. Paleontologists have discovered mammoth 'megasites' involving the remains of dozens of animals all in the same place – a disproportion, as Shipman notes, for which there was no precedent:

Modern humans disturbed the long-standing ecosystem with their arrival about 45,000 years BP, but starting about 32,000 years ago there was a second extraordinary change. From then until about 15,000 years ago, modern humans were killing and using mammoths in extraordinary numbers not seen in any Neanderthal sites.

Anyone even passingly familiar with the behavior of humans in the modern world – denuding entire ecosystems, lavishing waste and slaughter in all directions, moving one of the greatest mass extinctions in the history of life on Earth – will have no trouble at all understanding the ugly truths behind mammoth 'megasites.' But science, as noted, is inherently cautious, even when, as in this case, that caution ends up looking a little silly:

The use of mammoth resources does not prove mass killings, but sites with large numbers of mammoths need to be explained. Why would mammoths start dying in large numbers only after modern humans arrived? S. V. Leshchinsky and colleagues have suggested that woolly mammoths were stressed by nutritional deficiencies because of climate change, but why such stresses might become acute after the arrival of modern humans is unclear.

Shipman warns against a too-easy, overly-dramatic reading of the evidence; indeed, one of the scientists she interviews laughs at the “Ernest Hemingway” notion that Homo sapiens was responsible for wiping out the mammoths (and the cave bears, and the saber-toothed tigers, and the lions, and the rhinos, etc.). She reminds her readers that virtually no evidence has turned up in the fossil record to indicate direct modern human-Neanderthal violence, and she sticks to her own hypothesis: that the amazing, logic-defying partnership of modern humans and wolves, combined with a sudden climate change even more severe than usual, drove the already-strained Neanderthal population in Eurasia over the brink and into extinction.Many factors, an unprecedented inter-species cooperation, a quickly-changing environment, a robust new species out-competing a struggling old species. A very intriguing new take on an old question, this one featuring the basset hound currently snoring on your couch. A broad-based new hypothesis that most definitely doesn't go in for any Ernest Hemingway notions about one miswired, genocidal species wiping out anything bigger than a jackrabbit within a thousand miles in all directions.Perfectly reasonable, and it might even end up being true. Just don't mention it to the passenger pigeon.____Steve Donoghue is a writer and reader living in Boston with his dogs. He’s recently reviewed books for The Washington Post, The National, The Wall Street Journal, The Boston Globe, Historical Novel Review Online, and The Quarterly Conversation. He is the Managing Editor of Open Letters Monthly, and hosts one of its blogs, Stevereads.