In part 1, I described how I responded to an interesting question about the extinction of the Neanderthals. My correspondent was perplexed. Although he could see how competition, disease, interbreeding, and hunting might have reduced the population of the Neanderthals appreciably, he didn’t see how any of these forces could have driven them to extinction. It’s a big planet, after all, and various hominids had managed to coexist on it for a long time. What was so different 40,000 years ago, when Neanderthals died out (at least in Europe)?
Well, the arrival of Homo sapiens seems to have had consistent effects on lots of species, not just Neanderthals. There was a wave of extinctions in northern Eurasia around 45,000 years ago, just as H. sapiens seem to have arrived, and a general trend of large animals going extinct shortly after the arrival of H. sapiens globally (staggered in time, but always matching the arrival of humans with tools and dogs and fire, and probably new diseases and pests). It’s unlikely that the last mammoth, moa, elephant bird, or wooly rhinoceros was killed by a human (nor, more recently, the last passenger pigeon, giant auk, or Steller’s sea cow). But most populations operate right at the edge of survival, especially those with large bodies. Large bodies require lots of food, which means they need lots of room to hunt. A change in the climate that cuts prey populations even marginally will mean it takes more energy to gather the day’s meal (and that the day’s meal will need to be larger). That means more exposure to predators, and less reserves to rely on if an individual gets sick. It means that competition with a new rival will be more intense, too. As that all happens, population density will drop, meaning that the search for mates gets harder and costlier. It gets harder and harder to keep up, and populations start declining. At some point, all those factors will combine to mean that a species is doomed to extinction, even if the last member doesn’t die for decades or even millennia. (David Quammen’s The Song of the Dodo is an extended exploration of these issues, and Elizabeth Kolbert’s The Sixth Extinction, which won a Pulitzer last year, addresses many of them, too).
Think of this sort of competition like people sinking wells for water. The first person to sink a well might go down 10 feet and get enough water to function. Sometimes the well runs dry if they use too much or if there’s a drought, but eventually the water flows back. As long as all their neighbors sink wells to 10 feet, too, everyone’s in the same situation. When new neighbors can sink a well to 20 feet, things change. When the first people’s well sucks dry, the newcomers can still draw water. That means it might take longer for the first person’s well to recover, and it might run dry more often. There might not be enough water for irrigation at key moments. That reduces how much food the first person can raise to eat or sell by a few percent, undercutting their livelihood over the long run without threatening starvation or dehydration right away. It may also mean they can’t afford to sink a new well, leaving them doomed, even though they could keep operating for a few years.
It’s possible that Neanderthals were on their way out before anatomically modern humans arrived; it’s also possible that the arrival of humans precipitated changes which led to the Neanderthal extinction. Again, that may have involved direct conflict, but more likely it was mediated through competition for food and other resources. In the harsh environment of northern Eurasia, such competition would likely have been fierce. Indeed, the sparse population density that my correspondent originally mentioned is evidence for that competition: modern human populations are able to live densely because we aren’t reliant on the same scarce food supplies (and Neanderthals would surely have lived more densely if they could have). Given that intense pressure, a bout of disease, or an episode of violent conflict, or other brief moments of additional pressure against Neanderthals may have been enough to set them on an inevitable course toward extinction.
Again, I’m not an anthropologist, so I’m relying mostly on my broader understanding of extinction dynamics rather than a deep knowledge of the body of research on this question. But I hope that it helps explain how forces like hunting, disease, interbreeding, or food competition could lead to extinction even if any one of them could not kill every single Neanderthal.