Between 35,000 and 45,000 years ago, Neanderthals in Europe and Asia were replaced by the first modern humans. Why and how this transition occurred remains somewhat controversial. New research from the journal Science suggests that sheer numbers may have played a large role in modern humans' eventual takeover; archeological data shows that early populations of modern humans may have outnumbered Neanderthals by more than 9 to 1.

Two archaeologists from Cambridge University analyzed data from the Aquitaine region of southern France, which has Europe’s highest density of sites from this era, and one of the most complete archeological records. They used data from three time periods that encompassed the transition between Neanderthals and modern humans: the Mousterian and Chatelperronian eras, during which Neanderthals lived, and the Aurignacian period, which was dominated by modern humans. By examining differences between land use during these time periods, the researchers hoped to determine whether population dynamics played a role in the transition between these two hominins.

Because of the difficulties in estimating long-ago populations, the researchers used a few different proxies for population sizes and densities. They analyzed the number of occupied sites in each era, the size of these sites, and the accumulation rates of stone tools and animal food remains. Through these proxies, the researchers could get good estimates of population dynamics during the transition from Neanderthals to modern humans in Aquitaine.

From the Mousterian to the Chatelperronian era, there was very little increase in the number of rock-shelter sites. There were about 26 sites occupied in the Mousterian era, and 31 in the Chatelperronian period, suggesting that the Neanderthal population was not growing quickly. However, there were about 108 sites occupied by modern humans in the Aurignacian period. The increase is similar for occupied open-air sites. Adjusted for time scales, these figures suggest that, between the last Neanderthal-dominated era and the first era dominated by modern humans, the population numbers and densities increased by a factor of about 2.5.

A similar trend was seen in the sizes of occupied areas, with the Neanderthal sites averaging less than 200 square meters, while several of the modern human sites reached up to 600 square meters. From the size differences of the sites, the researchers estimate population increased up to 3 times as the Neanderthal-dominated era ended and modern humans occupied their sites.

Finally, the accumulation of stone tools and animal remains tells a similar story: modern humans were far more numerous than the Neanderthals they replaced. The densities of stone tools and animal food remains skyrocketed between the Chatelperronian and Aurignacian eras—according to these differences, the modern human population probably outnumbered the Neanderthals by a factor of about 1.8.

Each of these statistics, taken alone, tells only part of the story. Since these archaeological proxies was developed independently, the estimations can be looked at cumulatively to get a better idea of the different population sizes. When evaluated as a whole, these estimations show that the population size and densities of modern humans may have been more than 9 times those of the Neanderthals around the time of the population's transition. It’s very likely that a numerical advantage that large played a significant role in modern humans’ dominance over their earlier counterparts.

While the study did not directly address the features that gave modern humans a population advantage, the authors suggest that it was probably due to a combination of factors such as improved food storage, an increase in social cohesion, and the potential for trade and the exchange of goods.

Science, 2011. DOI: 10.1126/science.1206930 (About DOIs).