HUMANS are natural invaders, the mammalian equivalent of Burmese pythons, cane toads and Asian carp. Our species came from Africa and invaded Europe about 50,000 years ago. Perhaps surprisingly, this invasiveness may explain why we have outlasted our last close relatives, Neanderthals, by tens of thousands of years. I believe that the key to our success as invaders lies in our partnership with a weapon with a wagging tail: the domestic dog.
When early modern humans first entered Europe, Neanderthals had been living there for roughly 250,000 years. They knew the terrain and ecosystem intimately. They shared many of our physical and behavioural traits, such as large brains, specialised abilities for making tools and fire, and methods for hunting the same large game. Genetically, Neanderthals were so much like us that we interbred, albeit rarely. Yet the evidence is very clear that we thrived during the period of overlap,while Neanderthals went extinct. Why?
Climate instability has been a favourite candidate. Severe fluctuations from warmer and wetter to colder and drier, and back again, started about 45,000 years ago. Until recently, Neanderthal sites appeared to show a pattern of progressive retreat to more southerly, milder locations, ending with their extinction about 27,000 years ago. However, improvements in radiocarbon dating have dramatically undercut this interpretation. All well-dated Neanderthal sites have been found to be at least 39,000 years old and no southerly shift through time is evident. Besides, we faced the vacillating climate at the same time as the Neanderthals and they had survived similar cold periods before our arrival. Some new factors were at work.
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Our presence was one element. Did we force Neanderthals into extinction? Yes, but not through violence or killing 鈥� once a preferred hypothesis. We were simply better at hunting than they were.
鈥淒id we force Neanderthals into extinction? Yes, but not through violence or killing鈥�
A model for understanding how one invasive predator might outcompete a similar rival comes from the reintroduction of wolves to Yellowstone National Park in the US. Though wolves were integral to that ecosystem for millennia, they were wiped out there by settlers by about 1920. The effects of removing the wolf were striking. Coyotes formed larger, more wolf-like packs, while elk populations soared, changing the vegetation close to rivers by eating young trees and shrubs. Pronghorn antelope populations dropped as more coyotes preyed on their offspring; beavers disappeared from the park and songbirds declined in number.
Reintroducing just 31 wolves in the mid-1950s transformed the ecosystem again. Wolves targeted their closest competitor, killing coyotes in confrontations over carcasses and consuming enough prey to hinder their survival. Coyotes avoided areas favoured by wolves and shifted to smaller prey. Coyote packs fragmented and their overall population declined sharply. More pronghorns survived; elk herds diminished; and riverine vegetation came back, encouraging the return of beavers and songbirds.
In a similar way, Neanderthals bore the brunt of our invasive impact. They might have abandoned areas where their rivals were numerous, as coyotes did, but unlike them did not shift to different prey. Wolves dominate coyotes by sheer size and power. We humans dominated through our diverse hunting skills. One advantage is that we had projectile weapons, while Neanderthals had only handheld or muscle-powered weapons. Distance killing exposed us to far fewer risks and expended less energy. More food for less work meant more energy for reproduction. The second advantage is that, at about the time of the demise of the Neanderthals, we 鈥渋nvented鈥� dogs.
We did not set out to create dogs. There were no other domesticated animals at the time and, until recently, no one thought domestic dogs appeared until about 15,000 years ago. In 2009, a team led by my Belgian colleague began investigating ways to tell dogs apart from wolves using statistical methods. These two canids are so similar that they can and do interbreed; no simple genetic or physical trait distinguishes them. However, a complex analysis of skull shape reliably separates wolves from both modern dogs and from the accepted prehistoric wild dogs. Analysing additional fossil canid skulls, the team recognised a group of ancient dog-like animals intermediate in shape between wolves and prehistoric dogs. I call them wolf-dogs, not because I believe they were hybrids, but because deciding which group they belonged to is not easy.
Whatever wolf-dogs were, they were different from contemporary wolves. Chemical analysis of their bones shows their diets differed from those of humans or wolves at the same sites. Wolf-dog mitochondrial DNA differs from that of any other canid and is very primitive compared with that of other modern and fossil dogs and wolves.
The oldest wolf-dog yet identified (there are now more than 40) is an astonishing 36,000 years old, much older than expected for a domesticated animal. How long would domestication take when no one could possibly know how to do it? The oft-cited silver fox farm experiment, conducted in Siberia, produced a domesticated fox in 40 generations. However, the original foxes were not wild 鈥� their ancestors had been in captivity for 50 years 鈥� and they were caged, so only those chosen by experimenters bred. These conditions were not like those in the first domestication, which probably took thousands of years of trial and error. If it did, then wolf-dog domestication started before Neanderthals went extinct. The telltale evidence of early domestication should lie in behaviour. Dogs travel with us, bond with us, cooperate with us, and change our lives. They are our best friends. Wolves are not.
The prediction that the presence of wolf-dogs would coincide with a behavioural shift in humans is borne out by the archaeological record. All known wolf-dogs occur in sites created by humans, not Neanderthals. The sites themselves are extraordinary and contain the , though mammoths were previously rare in archaeological sites. Some were clearly hunted, their bones butchered, skinned and charred. The sites include hearths, tools and huts built from mammoth bones. Though top predators are always rare in ecosystems, wolf remains at these sites are so abundant that they must have been targeted. Their luxuriant fur would be useful in near-Arctic conditions, and territorial wolf-dogs 鈥� like wolves and dogs today 鈥� would probably not tolerate the presence of any other canid.
Even if wolf-dogs were poorly domesticated, cooperating with them would have offered huge advantages. We gave them food, shelter and protection. They provided faster pursuit of prey and the ability to track by smell. They could surround and harass large prey until they tired, making our long-distance weapons more successful and saving wolf-dogs from risk. Territorial wolf-dogs defended the carcasses, the camp and us, enabling us to have longer-term settlements close to the kill site. Wolf-dogs enlarged our ecological niche, enabling us to outcompete Neanderthals.
This hypothesis still requires elaboration and testing. Not everyone accepts the evidence that wolf-dogs were domesticated so very early, or that they were instrumental in pushing Neanderthals into extinction. But I think that the combination of humans and dogs was unstoppable. Since then, we and our best friends have invaded other continents and initiated the sixth global extinction. Working with humans, dogs are deadly weapons.
This article appeared in print under the headline 鈥淗ow we hounded out the Neanderthals鈥�
