"This town ain't big enough for the both of us," says ranch foreman Nick Grindell to lawman Tim Barrett in the 1932 film The Western Code. Biologists know the principle well: Two animal species can rarely occupy the same niche. The same, it seems, goes for human populations. A new study of Neandertal and modern human sites in the south of France concludes that the moderns so greatly outnumbered their evolutionary cousins that Neandertals had little choice but to go extinct.
For more than 100,000 years, Neandertals had Europe all to themselves. Then, beginning roughly 40,000 years ago, modern humans—Homo sapiens—began migrating into the continent from Africa. Although researchers debate how long the Neandertals hung around, these ancient humans probably did not survive much longer than 5000 years. Just why they disappeared is also a matter of contention, but most experts agree that H. sapiens was able to outgun its rival in either direct or indirect competition for food and other resources.
Some genetic studies, based on both modern and ancient DNA sequences, have suggested that modern human population growth quickly outstripped that of Neandertals, but estimating population levels from these kinds of data is very difficult and inexact. So Paul Mellars and Jennifer French, archaeologists at the University of Cambridge in the United Kingdom, decided to look directly at the archaeological evidence for the presence of both groups in the region where the most excavations have taken place: southwestern France, including the lush Dordogne region, as well known for its prehistoric sites as for its wine and foie gras.
Mellars and French tapped into a comprehensive database of all Neandertal and H. sapiens sites in a 75,000-square-kilometer region that Pierre-Yves Demars of the University of Bordeaux had previously compiled. They looked at three prehistoric cultures and time periods: the Late Mousterian, from 55,000 to 44,000 years ago, associated with Neandertals; the Châtelperronian, from 44,000 to 40,250 years ago and also associated with Neandertals; and the Aurignacian, from 40,250 to 35,000 years ago and associated with modern humans.
For each of these three periods, the duo calculated three different proxies for population numbers: The number of sites, the size of each site, and what the authors term the "intensity of occupation," indicated by the number of stone tools found and the amount of meat consumed as estimated from the bones that Neandertals and modern humans left behind.
The differences were marked. Whereas more than 100 years of excavations in the region have revealed 26 Late Mousterian and 37 Châtelperronian sites, modern human sites number 147. Moreover, while the size of the prehistoric occupations—which included caves, rock shelters, and open-air sites—overlapped between Neandertals and modern humans, the H. sapiens sites were an average of two times larger. Finally, the intensity of occupation—the numbers of stone tools and animal bones found per square meter—was nearly twice as great in modern human sites compared with Neandertal sites.
In their report published online today in Science, Mellars and French argue that each of these three demographic indicators can be considered independent measures of population numbers and should be multiplied together to estimate population differences. When they did so, the team concluded that modern humans came to outnumber Neandertals by a factor of nine or 10.
"These data imply that numerical supremacy alone must have been a powerful if not overwhelming factor" that allowed moderns to outcompete Neandertals, the team writes.
Chris Stringer, a paleoanthropologist at the Natural History Museum in London, calls the study "the most comprehensive test so far of the idea" that moderns came to outnumber Neandertals in Europe. And Jean-Pierre Bocquet-Appel, a paleodemographer with the French national research agency CNRS in Paris, agrees with the authors that their three proxies for population can be treated as independent variables and thus multiplied together; but even if they are only added together, Bocquet-Appel says, moderns would still have outnumbered Neandertals by a factor of more than five.
As for why sheer numbers would have given moderns such an evolutionary advantage, Bocquet-Appel says that both H. sapiens and Neandertals would have been competing to hunt the same animals, such as cattle, deer, and horses. "It's a general law of ecology that two species cannot live in the same niche."
But James O'Connell, an anthropologist at the University of Utah in Salt Lake City, cautions that the study assumes Neandertals and moderns used their sites in the same way, whereas studies of modern hunter-gatherers indicate that site size and intensity of occupation can vary according to cultural factors that do not necessarily correspond to population numbers. For example, the site areas occupied by some Australian hunter-gatherers is "an order of magnitude greater" than that for the San people of the Kalahari, O'Connell says.