How Did The Decline Of Neanderthals Occur?

The Neanderthals were a human population found on a vast territory that extended from the Atlantic ocean to Siberia and from England to the Near-East. From their origin 400,000 years ago in Europe until their disappearance 40,000 years ago, they survived and expanded their territory in spite of the great climatic fluctuations that took place on our planet, varying between cold periods (during the glacial epochs) and more temperate climates (during inter-glacial epochs).

Scientific debate has focused on the disappearance of this population for a number of years. Even if scientific opinions are not unanimous, the disappearance of Neanderthals is almost always set in relation to the arrival and expansion in Europe of Homo sapiens. A number of hypotheses have been proposed for this, above all: 1) The competition for resources that would have been disadvantageous to Neanderthals, especially during periods of great climatic change (influenced by huge volcanic eruptions); 2) The appearance of new illnesses, even of epidemics, which supposedly drastically reduced the Neanderthal population; and 3) Direct conflicts between the populations in which Homo sapiens had the advantage.

These different explanations for the disappearance of the Neanderthals, however, do not take into account archeological findings which show that the disappearance of this population was spread out over thousands of years, during which there were genetic exchanges between Neanderthal and Homo sapiens. Today’s human populations retain traces of Neanderthal DNA.

This study of the disappearance of the Neanderthals published today in Plos One does not attempt to explain “why” the Neanderthals disappeared, but to identify “how” their demise may have taken place. This original approach is made on the basis of demographic modeling undertaken by two anthropologists (Anna Degioanni, Aix-Marseille Université LAMPEA, and Silvana Condemi, Aix-Marseille Université ADES), an evolutionary biologist (Christophe Bonenfant, Université Claude Bernard Lyon LBBE), and a statistician (Sandrine Cabut, CNRS, LAMPEA).

What is known is that the Neanderthals were present on a vast territory, that they were subdivided into at least 3 large groups, and that they were hunter-gatherers (who had hunter-gatherer characteristics, which, however, were not necessarily identical to those of hunter-gatherer sapiens). They were around 70,000 in Europe 50,000 years ago, and their disappearance was not brutal but occurred over a long period of time. According to some researchers, their demise took place up to 10,000 years ago, whereas for others it would have been a little faster, occurring over a period of 6,000 or even 4,000 years.

The photo of the Neandertal from La Chapelle-aux-Saints (France) is a courtesy of the Musée de l’Homme de Néandertal La Chapelle aux-Saints.

Thanks to our modeling, we were able to test different demographic parameters starting from different hypotheses. According to our first hypothesis, the parameters would have allowed the Neanderthals to maintain a constant population. However, since we know that the Neanderthals did disappear, this hypothesis is not likely, yet it is very important to identify what demographic values must be modified to account for their disappearance. We subsequently modified the values of the demographic parameters (eg fertility, mortality, and migration) to identify the values that brought about Neanderthal “disappearance” over a period of 10,000, 6,000, or 4,000 years.

Following thousands of simulations, these researchers show that neither the argument concerning the mortality of the youngest or the adults, nor the argument based on an epidemic or a cumulative demise (due for example to conflict) is capable of explaining their disappearance over the course of thousands of years.

By contrast, the disappearance of the Neanderthals may be explained by a very small reduction in fertility. These research show that this decrease did not concern all females but only the youngest (less than 20 years old).

In conclusion, the results of this study, thanks to a method of ecological modelization that makes it possible to compensate for the lack of reliable demographic data for Neanderthals, permit us to exclude both the conflict with Homo sapiens and the spread of epidemics as causes of Neanderthal disappearance. And, if this study does not indicate the causes of a probable reduction in fertility, a number of possibilities are proposed.