What Happened to the Neanderthals?

The last appearance date of Neanderthals is commonly cited as ca. 30 thousand years ago (ka). This date follows the emergence of modern humans in Europe by several millennia, but our understanding of the exact timing and duration of this interval is obscured by the limitations of our dating methods. For example, peaks in atmospheric radiocarbon production during this time result in a large degree of uncertainty in the relevant radiocarbon dates (Conard & Bolus 2008). The two species may have coexisted in Europe for up to ten millennia, and possibly came across each other during this time, although the duration of this coexistence is debated, as is contact between the two (e.g., Finlayson 2000, Pinhasi et al. 2011). The question of what may have happened during these encounters and what the role of the early modern humans could have been in the Neanderthal extinction, have been the subject of intense discussion and a focal point in Neanderthal research.

The Neanderthal disappearance is viewed by some as a true extinction. Others however, contend that Neanderthals did not become extinct, but instead were assimilated into the modern human gene pool. The fossil record is ambiguous on this point: a few European Upper Paleolithic modern human specimens have been proposed as potential Neanderthal-modern human hybrids, but this interpretation has been questioned (e.g., see Smith 2005, Harvati et al. 2007). Analysis of Neanderthal and Upper Paleolithic modern human mitochondrial DNA shows no indication of interbreeding (e.g., Ghirotto et al. 2011). However, recent research on Neanderthal nuclear DNA has found evidence for limited admixture: a small portion (up to ~4%) of the genomes of non-Africans so far examined may derive from Neanderthals, suggesting that interbreeding probably occurred in the Near East during the earliest dispersal of modern humans out of Africa, but prior to their arrival in Europe (Green et al. 2010). Demographic modeling of admixture combined with territorial expansion, however, indicates that this level of introgression would be produced under very low (<2%) interbreeding rates and strong barriers to reproduction between Neanderthals and modern humans, arguing against assimilation (Currat & Excoffier 2011). Pending the completion of the Neanderthal genome and ancient DNA analyses of early modern Europeans dating to the Upper Paleolithic, and following the recent discovery of a third possibly coexisting species from Denisova cave (Krause et al. 2010), it is premature to conclude that the currently observed level of admixture constitutes assimilation. Regardless of this small contribution to the modern human gene pool, Neanderthal populations across Europe vanished abruptly in the fossil record, and several scenarios have been proposed to account for this observation. Most invoke a degree of competition, either direct or indirect, with modern humans, or alternatively, deteriorating environmental conditions, as major factors.

Hypotheses advocating competition have proposed several possible modern human competitive advantages. These include technological advances, such as 1) better clothing and shelter, 2) improved hunting techniques and more diverse subsistence strategies, which included the consumption of birds and fish, 3) social differences, such as larger group sizes and more elaborate social networks among modern humans, and 4) demographic factors, possibly including differences in birth and mortality rates or in interbirth intervals of the two species (see references in Harvati 2007). Indeed important differences have been found between Neanderthals and modern humans in their life history and demography, including faster growth and possibly shorter life expectancy in Neanderthals (see Harvati 2007, Smith et al. 2010), as well as a much higher population density among Upper Paleolithic modern humans compared to Neanderthals (Mellars & French 2011).

The relevance of climate in this debate was until recently discounted, as Neanderthals disappeared in Oxygen Isotope Stage 3 (OIS 3) when conditions were thought to be relatively stable (Stringer et al. 2003). Some recent hypotheses, however, consider climatic instability during the millennia building up to the last glacial maximum to have been a driving force in the Neanderthal extinction. One model postulates that habitat degradation and fragmentation occurred in the Neanderthal territory long before the arrival of modern humans, and that it led to the decimation and eventual disappearance of Neanderthal populations. In this view modern humans would have arrived in areas previously occupied by Neanderthals after the latter were already extinct, and the two species would never have met in Europe (Finlayson 2000). A similar model considers the Neanderthal demise as only one of the many Late Pleistocene megafauna extinctions caused by the loss of an environment with no modern analogue (Stewart 2005). Support for a significant climatic effect comes from recent detailed palaeoclimatic records, according to which OIS 3 was dominated by much more unstable climatic conditions than previously thought (van Andel & Davies, 2003) and may have been precipitated by unusually intense volcanic activity (Golovanova et al. 2010). Modeling of climatic stress (defined as the indirect effects of environmental change) based on these new data found two stress peaks at ~65 and ~30 ka, the second appearing to be more prolonged and severe than the first, and possibly related to the Neanderthal extinction (Stringer et al. 2003). This may have been precipitated by the coeval eruption. However, as Neanderthals had survived previous cold phases, it is difficult to accept climate change as the sole reason for their demise. Furthermore, no association has been found between proposed dates for the last Neanderthal appearance and major climatic events, suggesting that Neanderthals did not become extinct following a catastrophic climatic event (Tzedakis et al. 2007). If climate played a significant role, therefore, it would be a more complex one, perhaps involving environmental deterioration in combination with the advent of modern humans, and therefore with increased competition for limited resources. In this view it is the interaction between the effects of fluctuating climate and environment and of competition with modern humans that would have led to the eventual Neanderthal extinction.