Living on the edge: Was demographic weakness the cause of Neanderthal demise?

Abstract

The causes of disappearance of the Neanderthals, the only human population living in Europe before the arrival of Homo sapiens, have been debated for decades by the scientific community. Different hypotheses have been advanced to explain this demise, such as cognitive, adaptive and cultural inferiority of Neanderthals. Here, we investigate the disappearance of Neanderthals by examining the extent of demographic changes needed over a period of 10,000 years (yrs) to lead to their extinction. In regard to such fossil populations, we inferred demographic parameters from present day and past hunter-gatherer populations, and from bio-anthropological rules. We used demographic modeling and simulations to identify the set of plausible demographic parameters of the Neanderthal population compatible with the observed dynamics, and to explore the circumstances under which they might have led to the disappearance of Neanderthals. A slight (<4%) but continuous decrease in the fertility rate of younger Neanderthal women could have had a significant impact on these dynamics, and could have precipitated their demise. Our results open the way to non-catastrophic events as plausible explanations for Neanderthal extinction.

Fig 1. Spatial distribution and location of the 3 Neanderthal subpopulations.

Southern Europe (labeled A in green), Northern Europe (labelled B in yellow), and Eastern Europe (labeled C in purple) according to [61]. The full demographic model we used to simulate Neanderthal population dynamics was composed of three sub-models corresponding to each of the identified sub-populations. We included a migration parameter (noted ψ) to allow for individuals to move from a sub-population to another.

Fig 2. Simulated population trajectories of the Neanderthals over 10,000 yrs.

Lines color correspond to the three subpopulations of Neanderthals in Europe (see Fig 1: subpopulation A in green, B in yellow and C in purple) and in black to total population. Dotted red line shows the MVP (minimum viable population). The top-panel histogram displays the distribution of the time at extinction of the whole Neanderthal population. Right panel gives the proportion of simulated trajectories that hit the threshold population size of 5,000 under which the population was considered as extinct, e.g. the quasi-extinction probability. We present results of median of the 10,000 simulations for scenarios where the overall Neanderthal population never goes extinct (Fig 2A Parameters used in the simulation are shown in Table 1 “Survival”), disappears in 10,000 yrs (Fig 2B Parameters used in the simulation are shown in Table 1 “Demise in 10,000 yrs”), 6,000 yrs (Fig 2C Parameters used in the simulation are shown in Table 1 “Demise in 6,000 yrs”) and 4,000 yrs (Fig 2D Parameters used in the simulation are shown in Table 1 “Demise in 4,000 yrs”).

Fig 3. Simulated trajectories for the Neanderthals overall population and for the 3 sub-populations with reduced survival.

Parameters used in the simulation are shown in Table 1 “Survival”, reducing young infants survival by 0.4% (Fig 3A) or reducing adult survival by 10% (Fig 3B).