The Invaders

How Humans and Their Dogs Drove Neanderthals to Extinction

Pat Shipman

• Publisher: Belknap Press
• Year: 2015
• ISBN:
  9780674425408
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

A Times Higher Education Book of the WeekApproximately 200, 000 years ago, as modern humans began to radiate out from their evolutionary birthplace in Africa, Neanderthals were already thriving in Europe—descendants of a much earlier migration of the African genus Homo. But when modern humans eventually made their way to Europe 45, 000 years ago, Neanderthals suddenly vanished. Ever since the first Neanderthal bones were identified in 1856, scientists have been vexed by the question, why did modern humans survive while their closest known relatives went extinct?"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."
—Toby Lester, Wall Street Journal "Are humans the ultimate invasive species? So contends anthropologist Pat Shipman—and Neanderthals, she opines, were among our first victims. The relationship between Homo sapiens and Homo neanderthalensis is laid out cleanly, along with genetic and other evidence. Shipman posits provocatively that the deciding factor in the triumph of our ancestors was the domestication of wolves."
—Daniel Cressey, Nature

 

• 1 •

AND HE IS US

Most books about science do not open by declaring that some of the most revered scientists in the world have made a big mistake, but here goes: They’re wrong. Although it is almost universally acknowledged in the scientific community that invasive species are a terrible problem—altering ecosystems, causing extinctions, lessening biodiversity—there is a glaring omission in our catalog of invasive species. You can check for yourself by going to the website http://en.wikipedia.org/wiki/List_of_globally_invasive_species, where you will find a list of the 100 worst invasive species, drawn from the Global Invasive Species Database. That list is maintained by the Invasive Species Specialist Group of the International Union for the Conservation of Nature (IUCN). In other words, some of the most knowledgeable, clever, concerned, and well-informed people in the world are worrying about invasive species and the ways in which they are changing our world. They are compiling data, making lists, and documenting impacts of invasive species.

What species are on the list? A number of the names will be familiar to most people interested in conservation, ecology, or whole organism biology. There are invasive mammals such as house mice, Carolina gray squirrels, and brushtail possums; birds like the mynah and the English starling; plants including kudzu, purple loosestrife, and prickly pear; insects such as the malaria-carrying Anopheles mosquitoes, Asian gypsy moths, and red fire ants; mollusks including zebra mussels, apple snails, and giant African snails; and numerous others—cane toads, Nile perch, brown tree snakes, Dutch elm disease, and the chytrid frog fungus. The list goes on, and it is extraordinary for the breadth of organisms represented and the extent of the geographic distribution of these creatures. But there is one telling and very serious gap in such lists.

The most invasive, most environment-altering species of all—the one that has contributed to or directly caused the extinctions of thousands of species and the alteration of almost every habitat you can think of—is not on the list. If you go to the Global Invasive Species Database (http://www.issg.org/database/welcome/) and search the entire list, not just the 100 worst offenders, and type in “Homo sapiens,” our species, the following answer comes up: “No invasive species currently recorded.”

Why not?

Maybe because we—we humans, Homo sapiens—are the ones writing the list and we do not want to face our culpability. As a wise and eponymous possum in Walt Kelly’s cartoon, “Pogo,” said in a 1971 Earth Day poster, “We have met the enemy and he is us” (see Figure 1.1).¹

So he is—so we are.

I maintain that humans are the most invasive species that has ever lived. From a modest evolutionary beginning in Africa about 200,000 years ago, our kind has spread across the entire world, invading geographic region after geographic region, settling in to exploit new habitats until we can now be found living on every continent. We live in the sweltering tropics, in the far, cold north, on tops of mountains and in deep valleys, on islands and continents and island continents, in deserts, in rain forests, in temperate forests, and in open environments or closed ones. We do not live under water, except in artificial habitats like submarines, but many humans live in boats or on floating villages in lakes and rivers. We have become established in nearly every habitat on earth. It is an awful and awe-inspiring record.

Figure 1.1. Using the characters from his comic strip Pogo, Walt Kelly designed this poster for Earth Day 1971 to show the destruction humans have wreaked on our planet.

The adaptability, cunning, and technology that make our enormous geographic distribution possible also mean that our invasive success is unparalleled by any other species. We adapt ourselves to an incredibly broad range of habitats, lifestyles, diets, and climates. We adapt by the recording and sharing of knowledge through our exceptional language skills. We do this not only biologically but also through the invention and use of cultural buffers, like clothing, fire, shelter, the capture and transport of water, and the planting of crops. Some of these can be considered tools—items constructed to be used—and others are behaviors. We also create and use tools that have allowed us to circumvent the long process of evolutionary change as a way of expanding our resources and skills. For example, we have not evolved sharp, cutting teeth for slicing up food; we have invented first stone tools, then metal ones, and various implements to facilitate cutting made from almost every substance imaginable.

We have even “invented” and “manufactured” living tools by domesticating other animals, by controlling their breeding until their genes produce the attributes we want. Then—because a domestic species and a domesticating species form a kind of covenant or pact—we are able to borrow some of the domesticated species’ anatomical and behavioral skills, such as their superior eyesight and hearing, their speed in locomotion, their tremendous power, their cutting teeth, quick paws, and deadly claws, and their intensely specific sense of smell. This arrangement seems to work to our advantage, but I caution that a nuanced view is more realistic. Living tools like horses, dogs, cats, cattle, or pigs are not passive but active participants. If they do not want to and agree to work with us, they do not work to our advantage at all. Domestication is a continually negotiated agreement between two species, not an enslavement of one by the other.² And some species flatly refuse to be domesticated.

Even though the domestication of plants has involved less negotiation, what such plants have given us has been essential to our worldwide success as a species and our ever-increasing population. It has not been a one-way street, however. Plants have benefited from their domestication by increased fertility, larger seeds, protection from animals, or simple boons like watering.

There has been a serious price for our success in worldwide terms. Modern humans, Homo sapiens, have decimated millions of acres of once-productive land until the soil has eroded into the seas; this practice continues. We have cut down or burned vast areas of forest and woodland and prairie that once produced oxygen to replenish our atmosphere as well as fruit, leaves, roots, and nuts to feed ourselves and many other creatures. We have single-handedly accounted for the pollution, poisoning, and drying up of innumerable water sources around the world, thanks to our insatiable and growing needs, our toxic chemicals, and our enormous piles of debris. More than all this, we have contributed to the extinction of more species than we can calculate.

I am not alone in laying the blame at our own doorstep. In 2005, ecologists David Burney and Tim Flannery wrote a review article called “Fifty Millennia of Catastrophic Extinctions after Human Contact.”³ The title tells most of the story. Time after time, in place after place, humans have spread into new areas, and shortly after our appearance, numerous other species have gone extinct. As Burney and Flannery maintain, there is “a global pattern of human arrival followed by faunal collapse and other ecological changes … without known exception.”⁴ Think about that: a global pattern of faunal collapse. In many cases, the now-extinct parts of the fauna included many large mammals, birds, or reptiles. Large animals are more vulnerable to extinction because they reproduce more slowly than small animals, thus making the loss of any reproductive-age adults more serious. And larger animals need larger ranges or territories to live in, so they are more vulnerable to habitat loss, too. Since Paul Martin started writing about this pattern at length in 1967, the phenomenon has been called “megafaunal extinction.”⁵ The devastation we have wrought does not stop with animals but includes plants, too. In another recent article, an impressive team of ecologists writes: “Modern extinctions are largely being caused by a single species, Homo sapiens, and … from its onset in the late Pleistocene, th[is] … mass extinction has been characterized by the loss of larger-bodied animals in general and of apex consumers in particular.… The loss of apex consumers is arguably humankind’s most pervasive influence on the natural world.”⁶ An apex consumer is one at the top of the heap, a species that directly or indirectly consumes all the nonpredatory biomass in the ecosystem. We are apex consumers, of course, and we have deliberately or unconsciously worked hard to eliminate any other apex species that might compete with us in every ecosystem we have entered.

In some historical instances, climate change disrupted ecosystems and initiated an apparent population decline in some large mammals—leaving them especially vulnerable to human activities—yet there can be little doubt that human invasion was a (but not necessarily the only) key factor in triggering extinctions on the large landmasses of Eurasia, Australia, and the Americas, in the circumpolar regions, on numerous islands, and on the island continent of Madagascar.⁷ I would not claim that the arrival of humans has been the only factor contributing to many extinctions, but a spate of extinctions following human settlement is the norm.

Apex predators are especially influential and powerful in shaping an ecosystem. Marine ecologist Robert Steneck of the University of Maine observes, “Studies from widely divergent ecosystems all found a single predator can control the distribution, abundance, body size and species diversity in the system.”⁸ The human role in extinctions is easiest to prove on small islands where the limits of resources vital to the survival of the indigenous species are more obvious. But even on a continental basis, there is good evidence of the human role in the loss of mammoths and mastodons, of aurochs and wild horses, of many larger Madagascan lemurs, of dodos in Mauritius, and of moas in New Zealand. We humans have helped kill off amazing giant marsupials, enormous predatory birds and lovely passenger pigeons, giant ground sloths, woolly rhinos, fearsome dire wolves and saber-toothed cats, as well as other species that managed to survive changing climates—until humans came along. Human predation is not the sole factor in all of these extinctions, but we cannot escape the conclusion that we have played a crucial role in them.

This book looks at a particularly pivotal point in our history, the time during which the last species of nonhuman hominin (Neanderthals) went extinct. I argue here that Neanderthal extinction was triggered by the appearance of modern humans in Neanderthals’ geographic range; simply put, humans are a supremely well-adapted invasive species, and we behaved exactly like one during this extinction event. I have been pleased to find other anthropologists expressing similar views, and I try to give full credit here to others when credit is due.⁹

Why our close cousins the Neanderthals died out has puzzled paleoanthropologists ever since Neanderthals were first recognized as a species in 1856. As more fossils, modern research, and new techniques of analysis have proliferated, the extinction of a species with such familiar attributes—with the ability to make fire, to manufacture and use tools, to cooperate socially, to take down large mammals, to use symbols, art, and communication to at least some extent—has continued to seem enigmatic. But once you understand what makes a successful invasive species and which factors determine its impact on the ecosystem it invades, you will puzzle no longer.

• 2 •

HERE WE COME, READY OR NOT

What exactly is an invasive species? Perhaps the simplest definition is that it is a species that moves into a new geographic region where it has not previously (historically) lived. The species that are invaded—those that are indigenous to an area without human intervention—are considered native. An endemic species is a subset of native ones, comprising those that have evolved in a particular area and are found only there. An invader is neither of these; it is a species that does not “belong” in the region. An invader is nonindigenous, nonnative, nonendemic, alien, and very often disruptive to the ecosystem.

The U.S. government legally defines an alien species as being “with respect to a particular ecosystem, any species, including its seeds, eggs, spores, or other biological material capable of propagating that species, that is not native to that ecosystem.”¹ By this same executive order, an invasive species is defined as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.”² What this means is that a species that is not disruptive to the ecosystem or harmful to humans is not technically considered invasive. This is a somewhat shortsighted and anthropocentric point of view.

How is a species’ invasion different from a geographic expansion? The distinction lies in a question of timescale, distance, and influence.³ Expanding a species’ territory by a few miles probably will have a negligible impact on the ecosystem as a whole. Expanding a species’ distribution from one habitat or ecosystem or continent to another—across a mountain range or other formidable geographic barrier—is considered an invasion, as this change is more likely to have far-reaching effects.

Ecosystems are complex entities, crisscrossed and bound together by webs of cooperation, symbiosis, and mutual dependencies. The close relations among species within an ecosystem are stretched, warped, and complicated by competition, mutually exclusive needs, complementary needs, and the uncertainties of coexistence. Dropping a wholly new organism into the mix can disrupt the functioning of the...