How much was Climate a contributing factor in the Neanderthals’ demise?

Between 250,000 and 30,000 years (kya) ago, Homo neanderthalensis roamed the plains of Western and Central Eurasia (Kuhn & Stiner, 2006, p. 956). Homo sapiens, on the other hand, were in Southern China >80 kya (Liu et al, 2015) and Europe by 40-45 kya. What makes Neanderthal extinction so much more widely discussed than any other extinction is that it occurs during a period of many possible-contributing factors. Here we have a large brained, culture-using, bipedal hunting hominin that disappears from existence after a possible <10 kya co-habitation with Humans (Conroy & Pontzer, 2012, p. 535), during a major climatic upheaval. The Neanderthal demise has spawned multiple papers on the subject, and although climate would have greatly affected the Neanderthal’s environment, whether or not it led to their demise is hotly debated.

Neanderthals disappeared during Oxygen Isotope Stage 3 (OIS-3; Stewart et al, 2003, p. 221). It lasted from 60-27 kya with cold temperatures that would abruptly transition to a warming period (known as Dansgaard-Oeschger events) with mild conditions, and quickly return to cold conditions in as little as a few decades (Van Meerbeeck et al, 2009, p. 33). Heinrich events – massive ice surges into the North Atlantic Ocean from the Laurentide Ice Sheet during certain stadials – also contributed to rapid climate change (Van Meerbeeck et al, 2009, p. 33). This caused the environment to fluctuate back and forth between forest and open habitats (Finlayson & Carrión , 2007, p. 214). Extinctions of multiple species are marked during this period (Stewart, 2005, p. 38). The Last Glacial Maximum (LGM) also occurs between 21-19 kya and holds the record of the largest ice sheets of the last glacial (Van Meerbeeck et al, 2009, pp. 33-34). During OIS-3, Neanderthals are found to retreat westwards and southwards, their numbers dwindling towards the LGM (Stewart et al, 2003).

Neanderthals post-cranially fit a cold-adapted form (Holliday, 1997) and may therefore be viewed as adapted to the climatic conditions of the Pleistocene (Burke, 2012, p. 230). Yet over generations, Neanderthals are found to migrate North during warmer periods (Skrzypek et al, 2011, p. 481) and contract southwards during colder phases. Once in Eurasia, Humans follow the same pattern (Olszewski, 2006). These contractions may have been backwards migrations or local extinctions (Skrzypek et al, 2011, p. 481). Although Neanderthals had been surviving these cold conditions previously, factors such as variations in vegetation type, which in turn may have affected growth and mortality in herbivores, could have led to famine among Neanderthal populations (Jiménez-Espejo et al, 2007, p. 846). Trinkaus (1995, p. 137) reinforces this idea with higher frequencies of dental enamel hypoplasias found in later Neanderthal populations, indicating that during their development they were less buffered by their parents nutritionally. This, however, cannot account as the decisive extinction factor as Neanderthal populations appear to have been too small for food-shortage to be a major component (Sørensen, 2011, p. 18), and large mammalian fossils found on both Human and Neanderthal sites suggests that competition was not a factor great enough to stop them hunting the same game (Stewart, 2004, p. 187).

What cannot be ignored is that the Neanderthal demise occurred contemporaneously with the arrival of Humans. Adding to this is the fact that Neanderthals persist in Iberia and Gibraltar later than any other Neanderthal populations and then shortly go extinct once Humans move in (Banks et al, 2008). The environment acting as a barrier to Human colonisation is argued to be a strong factor in the persistence of Neanderthals in southern Iberia. With Humans only sweeping in once the climate caused a favourable change in this area (Sepulchre et al, 2007).

The annihilation of the Neanderthals by Humans is usually referred to as ‘rapid’ (Adler et al, 2008, p. 832), but in terms of a hominin lifespan, 10,000 years is far from quick. There are also no injuries on Neanderthal skeletons that could be inferred as violence from Humans (Sørensen, 2011, p.25) and little evidence of coexistence (Mousterian and Aurignacian artefacts being found in separate stratigraphic layers; Valet & Valladas, 2010, p. 3888), although genetic evidence showing that we interbred would mean there was some contact. Direct competition from Humans is usually termed the ‘overkill hypothesis’, whereby human predation is blamed for multiple species’ extinction, even though there is little evidence in the archaeological record (Pettitt, 1999, p. 220), only that both were in the same area. In fact, much of Neanderthal replacement is inferred from the appearance of the Aurignacian, even though it is typically impossible to tell how many occupation events took place in a given archaeological layer (Pettitt 1999).

Although there is no evidence for interspecies warfare, numerous theories have been proposed for how we outcompeted Neanderthals. One argument is that the OIS-3 had much stronger winds, as well as occurring much more abruptly than any other OIS event. Humans were capable of creating complex clothing required for this period, whereas Neanderthals developed this ability too late, a result of their biological cold adaptations delaying the need for such technology (Gilligan, 2007). Accordingly, bone awls and eyed needles are not found until the Upper Palaeolithic (Kuhn & Stiner, 2006, p. 960), however bone awls are associated with both Humans and Neanderthal assemblages (Froehle & Churchill, 2009, p. 108), while eyed needles are made much later at 35-30 kya (Froehle & Churchill, 2009, p. 108). Thus, if eyed needles are an allegory for tailored clothing, Humans do not create them until 10 kya of cold climate occupation. Wales (2012, p. 793) contends that while Neanderthals didn’t go extinct from mass hypothermia, non-tailored clothing is another disadvantage to the Neanderthal checklist of failures. Langbroek (2001, p. 124) argues, however, that the lengthy departure of the Neanderthals to extinction altercated to be due to their cognitive inferiority is a poor hypothesis that leads the debate down a “blind alley”.

Other hypotheses that cite Humans as behaviourally superior, range from, Lieberman’s (1992) view that the Neanderthal’s inability to produce the full range of language that Humans can, was the genetically isolating mechanism that led to their extinction. Kuhn & Stiner’s (2006) idea that the division of labour (as well as trade; Horan et al, 2005) as a Human autapomorphic trait gave them a demographic advantage over the Neanderthals. While others (Froehle & Churchill, 2009; Hockett & Haws, 2005) argue that it was Human’s greater dietary breadth that allowed us to increase and expand our numbers greatly to the considerable misfortune of the Neanderthals. In this respect, climate is not a factor at all, rather Humans possessed something Neanderthals did not, and allowed Humans demographic advantage.

Some theories combine demographic advantage of Humans with Neanderthal’s adaptation to the cold climate of Europe. In this view, the energy required by Neanderthals to sustain cold resistance coupled with high metabolism would reduce their fertility compared to Humans (Steegmann et al, 2002). Zilhão’s (2001, 72) paper concurs with this, positing that warmer climate species usually have higher fertility compared to their colder climate counterparts, allowing Humans to outbreed and interbreed with the scattered populations of the Neanderthals. The Neanderthals were therefore either eradicated or genetically swamped (meaning that the Neanderthal species merged with the larger Human population via interbreeding; Smith et al, 2005). In this respect, Neanderthals and Humans are seen to be unable to coexist together, both species increasing their populations until a critical point when Humans gain the advantage (Kuhn & Stiner, 2006, p. 961).

Throughout hominin history however, our ancestors have been sharing the planet with each other. Being the only hominin species alive now might predispose us to believe we cannot coexist with any other, an opinion that harks back to a time in anthropology prior to the discovery of multiple hominin species in the same locality (Stewart, 2004, p. 186). As for the possibility of Humans genetically swamping Neanderthals, a higher admixture rate than 1-4% would be expected as well as different admixture rates in different localities. Instead, the admixture rate is found equally among all Eurasians (Green et al, 2010, p. 721). Demographically-speaking, it has been indicated through lack of distant sources for Mousterian production that Neanderthals did have small territory ranges (Harvati , 2010, p. 371) and therefore small groups. Although, Jiménez-Espejo et al. (2007, p. 845) argue that displacement of an earlier, peripheral species by a new species has been found to be repeatedly inadequate, however their assertion is not fully explained.

Towards the end of the OIS-3 the extinction of the straight-tusked elephant, Merck’s rhino, European dung-beetle and Neanderthals have been recorded, along with a decrease in numbers of cave bear and smaller carnivore. Do we blame this on the colonisation of Humans or take the parsimonious explanation of abrupt climate and environmental shifts together with a decrease in temperature and carrying capacity that culminated in the LGM? (Stewart, 2005, p. 43) It is possible to argue that the real reason Neanderthals did not survive was simply because they weren’t as flexible as Humans in their response to climatic change (Olszewski 2006), yet evidence also points to climatically-forced local extinction of Human groups (Shea, 2008; Bradtmöller et al, 2012). Since extinction evidence is mainly based on tool finds, it could be argued that what we see as tool variation could be multiple extinction and replacement episodes of Aurignacian populations.

The first dates obtained for Aurignacians does not equal the first Aurignacians, nor do the last dates of Mousterian mean the last Neanderthals, only that the Neanderthals were high enough in population size to be detected in the archaeological record (Finlayson et al, 2004, p. 1206). Mellars & French (2011) use French site artefact density to show that Aurignacians were at higher densities compared to Châtelperronian and Mousterian sites. While they use this as evidence to argue that Human numerical supremacy was a powerful tool for demographic and territorial competition, they never explore the idea that Neanderthals may have lived in comparatively smaller groups. Indeed, the human teeth found at Daoxian provides us with evidence that although Humans successfully spread throughout Asia, Neanderthals and climatic factors may have prevented Human dispersal into Europe until Neanderthal’s decline (Liu et al, 2015, p. 4; Finlayson & Carrión, 2007, p. 219). This hypothesis may also be applied to Iberia too. These may be more parsimonious theories than invoking a completely new set of integrated novel capacities without studying underlying sub/cognitive processes (de Waal & Ferrari, 2010, p. 202).

It has sometimes been questioned that if Neanderthals had survived other glacial periods, why not the later stage of OIS-3? (Sørensen, 2011, p. 22) But it is important to remember they were not the only megafauna to meet their demise during this period (Stewart, 2005, p. 40). This period experienced greater climatic instability due to the Campanian Ignimbrite eruption at 40 kya, coinciding with Heinrich Event 4 that caused an extremely cold climatic phase (Sørenson, 2011, p. 25; Fitzsimmons et al, 2013).

Hublin & Roebroeks (2009) suggest that because Neanderthals were top carnivores their carrying capacities would have to be quite low, which would be in accordance with small group sizes. When the climate caused habitat changes, the northern Neanderthal populations would become extinct, while Southern territories would be maintained (Hublin & Roebroeks, 2009). During an upturn in climate, Southern populations would then expand outwards into Northern climes, beginning the whole process again, leading to repeated bottlenecks (Hublin & Roebroeks, 2009). Fragmentation of populations would lead to low genetic variability – reminiscent of other carnivore species of the same latitudinal range (Hublin & Roebroeks, 2009, pp. 507-508) – and possible inbreeding. In conservation, inbreeding is associated with high extinction rates as there is reduction in overall fitness of the population due to a decrease in heterozygosity resulting in an increased susceptibility to environmental changes, predators, pests and a decrease in reproductive rates (Dennell et al, 2011, p. 1516).

An overall decrease in genetic diversity is viewed from South to North in other Pleistocene species, although there were Northern refugium also (Hofreiter & Stewart, 2009). Finlayson’s (2008) paper sees a fragmentation of Neanderthal populations, first from continental areas and finally to coastal strongholds. This is in agreement with Pettitt (1999) and Skrzypek et al. (2011) which discusses a temporarily settled Central Europe where Neanderthal extinction could have started and radiated outwards.

What appears to be happening, is that as Neanderthals either migrate out or go extinct in areas, a niche is left open for Humans to eventually occupy long after Neanderthal vacation (Gilligan, 2007, p. 500; Langbroek, 2001, p. 129). This idea suggests Neanderthals were indirectly preventing Humans entering Europe in the more temperate periods of early OIS-3 (Langbroek, 2001, p. 130). This also explains why Humans were in Northern Spain by 40 kya but not Southern Iberia until well after 30 kya (Gilligan, 2007, p. 500), when Neanderthals finally vanished. In turn, Human occupation of a given area may have then prevented Neanderthal recolonisation (Langbroek, 2001, p. 134). In this view, Humans were not superior to Neanderthals nor outcompeted them, simply Neanderthal lost their footing during a harsh climate, allowing Humans to gain an eventual advantage.

In truth, understanding the Neanderthal demise is problematic because among other things there is an overemphasis on artefacts and possible biased interpretations of data (Jiménez-Espejo et al, 2007, p. 837) based on the paradigm of the author (Stewart, 2005, p. 43). There is also no modern equivalent of the environment that the Neanderthals went extinct in, creating flawed reconstructions (Stewart, 2005, p. 44). However, from the research reviewed here, it appears that climate did play a large role in the Neanderthal’s demise causing extinctions of both hominin species, ultimately isolating Neanderthal breeding populations and allowing Humans to prevent Neanderthal recolonisation. If we want a closer analysis of why many species went extinct yet we survived, we need to look for a less “special” emphasis on Human survival and a more impartial approach.

References:

  1. Adler, D. S., Bar-Yosef, O., Belfer-Cohen, A., Tushabramishvili, N., Boaretto, E., Mercier, N., Valladas, H. & Rink, W. J. (2008). Dating the demise: Neanderthal extinction and the establishment of modern humans in the southern Caucasus. Journal of Human Evolution, 55, 817-833. doi: 10.1016/j.jhevol.2008.08.010
  2. Bradtmöller, M., Pastoors, A., Weninger, B. & Weniger, G.C. (2012). The repeated replacement model – Rapid climate change and population dynamics in Late Pleistocene Europe. Quaternary International, 247, 38-49. doi: 10.1016/j.quaint.2010.10.015
  3. Banks, W. E., d’Errico, F., Townsend Peterson, A., Kageyama, M., Sima, A., Sánchez-Goñi, M.F. (2008). Neanderthal Extinction by Competitive Exclusion. PLoS ONE 3 (12), 1-8. doi: 10.1371/journal.pone.0003972
  4. Burke, A. (2012). Spatial abilities, cognition and the pattern of Neanderthal and modern human dispersals. Quaternary International, 247, 230-235. doi: 10.1016/j.quaint.2010.10.029
  5. Conroy, G. C. & Pontzer, H. (2012). Reconstructing Human Origins: A Modern Synthesis. Third Edition. London: W. W. Norton & Company Ltd
  6. de Waal, F. B. M. & Ferrari, P. F. (2010). Towards a bottom-up perspective of animal and human cognition. Trends in Cognitive Sciences, 14 (5), 201-207. doi: 10.1016/j.tics.2010.03.003
  7. Dennell, R. W., Martinón-Torres, M. & Bermúdez de Castro, J. M. (2011). Hominin variability, climatic instability and population demography in Middle Pleistocene Europe. Quaternary Science Reviews, 30, 1511-1524. doi: 10.1016/j.quascirev.s009.11.027
  8. Finlayson, C. (2008). On the importance of coastal areas in the survival of Neanderthal populations during the Late Pleistocene. Quaternary Science Reviews, 27, 2246-2252. doi: 10.1016/j.quascirev.2008.08.033
  9. Finlayson, C. & Carrión, J. S. (2007). Rapid ecological turnover and its impact on Neanderthal and other human populations. Trends in Ecology and Evolution, 22 (4), 213-222. doi: 10.1016/j.tree.2007.02.001
  10. Finlayson, C., Pacheco, F. G. & Vidal, J. R. (2004). Did the moderns kill off the Neanderthals? A reply to F. d’Errico and Sánchez Goñi. Quaternary Science Reviews, 23, 1205-1216. doi: 10.1016/j.quascirev.2003.12.016
  11. Froehle, A. W. & Churchill, S. E. (2009). Energetic Competition Between Neanderthals and Anatomically Modern Humans. PalaeoAnthropology, 96-116
  12. Fitzsimmons, K. E., Hambach, U., Veres, D. & Iovita, R. (2013). The Campanian Ignimbrite Eruption: New Data on Volcanic Ash Dispersal and Its Potential Impact on Human Evolution. PLoS ONE, 8 (6), 1-13. doi: 10.1371/journal.pone.0065839
  13. Gilligan, I. (2007). Neanderthal extinction and modern human behaviour: the role of climate change and clothing. World Archaeology, 39 (4), 499-514. doi: 10.1080/00438240701680492
  14. Green, R. E., Krause, J., Briggs, A. W., Maricic, T., Stenze, U, Kircher, M., Patterson, N., Li, H., Zhai, W., Hsi-Yang Fritz, M., Hansen, N. F., Durand, E. Y., Malaspinas, A.S., Jensen, J. D., Marques-Bonet, T., Alkan, C., Prüfer, K., Meyer, M., Burbano, H. A., Good, J. M., Schultz, R., Aximu-Petri, A., Butthof, Höber, A. B., Höffner, B., Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E. S., Russ, C., Novod, N., Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Z., Gušic, I., Doronichev, V. B., Golovanova, L. V., Lalueza-Fox, C., de la Rasilla, M., Fortea, J., Rosas, A., Schmitz, R. W., Johnson, P. L. F., Eichler, E. E., Falush, D., Birney, E., Mullikin, J. C., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D., Pääbo, S. (2010). A Draft Sequence of the Neanderthal Genome. Science, 328, 710-722. doi: 10.1126/science.1188021
  15. Harvati, K. (2010). Neanderthals. Evolution: Education and Outreach, 3, 367-376. doi: 10.007/s12052-010-0250-0
  16. Hockett, B. & Haws, J. A. (2005). Nutritional ecology and the human demography of Neanderthal extinction. Quaternary International, 137, 21-34. doi: 10.1016/j.quaint.2004.11.017
  17. Hofreiter, M. & Stewart, J. (2009). Ecological Change, Range Fluctuations and Population Dynamics during the Pleistocene. Current Biology, 19, 584-594. doi: 10.1016/j.cub.2009.06.030
  18. Holliday, T. W. (1997). Postcranial Evidence of Cold Adaptation in European Neanderthals. American Journal of Physical Anthropology, 104, 245-258
  19. Horan, R. D., Bulte, E. & Shogren, J. F. (2005). How trade saved humanity from biological exclusion: an economic theory of Neanderthal extinction. Journal of Economic Behaviour & Organisation, 58, 1-29. doi: 10.1016/j.jebo.2004.03.009
  20. Hublin, J. J. & Roebroeks, W. (2009). Ebb and flow or regional extinctions? On the character of Neanderthal occupation of northern environments. Comptes Rendus Palevol, 8, 503-509. doi: 10.1016/j.crpv.2009.04.001
  21. Jiménez-Espejo, F. J., Martínez-Ruiz, F., Finlayson, C., Paytan, A., Sakamoto, T., Ortega-Huertas, M., Finlayson, G., Iijima, K., Gallego-Torres, D. & Fa, D. (2007). Climate forcing and Neanderthal extinction in Southern Iberia: insights from a multiproxy marine record. Quaternary Science Reviews 26, 836-852. doi: 10.1016/j.quascirev.2006.12.013
  22. Kuhn, S. L. & Stiner, M. C. (2006). What’s a Mother to Do? The Division of Labour among Neanderthals and Modern Humans in Eurasia. Current Anthropology, 47 (6), 953-981. doi: 10.1086/507197
  23. Langbroek, M. (2001). The trouble with Neanderthals. Archaeological Dialogues, 8 (2), 123-151
  24. Lieberman, P. (1992). On Neanderthal Speech and Neanderthal Extinction. Current Anthropology, 33 (4), 409-410
  25. Liu, W., Martinón-Torres, M., Cai, Y.J., Xing, S., Tong, H.W., Pei, S.W., Sier, M.J., Wu, X.H., Edwards, R.L., Cheng, H., Li, Y.Y., Yang, X.X., Bermúdez de Castro, J. M. & Wu, X.J. (2015). The earliest unequivocally modern humans in southern China. Nature, 000, 1-17. doi: 10.1038/nature15696
  26. Mellars, P. & French, J. C. (2011). Tenfold Population Increase in Western Europe at the Neanderthal-to-Modern Human Transition. Science 333, 623-627. doi: 10.1126/science.1206930
  27. Olszewski, D. I. (2006). “Van Andel,T.H. and W. Davies (Eds.) — Neanderthals and Modern Humans in the European Landscape During the Last Glaciation.” PalaeoAnthropology, 116-119
  28. Pettitt, P.B. (1999). Disappearing from the world: An archaeological perspective on Neanderthal extinction. Oxford Journal of Archaeology, 18 (3), 217-240
  29. Sepulchre, P., Ramstein, G., Kageyama, M., Vanhaeren, M., Krinner, G., Sánchez-Goñi, M.F. & d’Errico, F. (2007). H4 abrupt event and late Neanderthal presence in Iberia. Earth and Planetary Science Letters 258, 283-292. doi: 10.1016/j.epsl.2007.03.041
  30. Shea, J. J. (2008). Transitions or turnovers? Climatically-forced extinctions of Homo sapiens and Neanderthals in the east Mediterranean Levant. Quaternary Science Reviews, 27, 2253-2270. doi: 10.1016/j.quascirev.2008.08.015
  31. Skrzypek, G., Wiśniewski, A. & Grierson, P. F. (2011). How cold was it for Neanderthals moving to Central Europe during warm phases of the last glaciation? Quaternary Science Reviews 30, 481-487. doi: 10.1016/j.quascirev.2010.12.018
  32. Smith, F. H., Janković, I. & Karavanić, I. (2005). The assimilation model, modern human origins in Europe, and the extinction of Neanderthals. Quaternary International 137, 7-19. doi: 10.1016/j.quaint.2004.11.016
  33. Sørensen, B. (2011). Demography and the extinction of European Neanderthals. Journal of Anthropological Archaeology 30, 17-29. doi 10.1016/j.jaa.2010.12.003
  34. Steegmann Jr., A. T., Cerny, F. J. & Holliday, T. W. (2002). Neanderthal Cold Adaptation: Physiological and Energetic Factors. American Journal of Human Biology, 14, 566-583
  35. Stewart, J. R. (2004). Neanderthal-Modern Human Competition? A Comparison between the Mammals Associated with Middle and Upper Palaeolithic Industries in Europe during OIS 3. International Journal of Osteoarchaeology 14, 178-189. doi: 10.1002/oa.754
  36. Stewart, J. R. (2005). The ecology and adaptation of Neanderthals during the non-analogue environment of Oxygen Isotope Stage 3. Quaternary International 137, 35-46. doi: 10.1016/j.quaint.2004.11.018
  37. Stewart, J. R., van Kolfschoten, T., Markova, A. & Musil, R. (2003). Neanderthals as Part of the Broader Late Pleistocene Megafaunal Extinctions? In T. H. van Andel & W. Davies (Eds.), Neanderthals and modern humans in the European landscape during the last glaciation: archaeological results of the Stage 3 Project (pp. 221-231). Oxford: Oxbow Books
  38. Trinkaus, E. (1995). Neanderthal Mortality Patterns. Journal of Archaeological Science, 22 (1), 121-142. doi:10.1016/S0305-4403(95)80170-7
  39. Valet, J.P. & Valladas, H. (2010). The Laschamp-Mono lake geomagnetic events and the extinction of Neanderthal: a causal link or a coincidence? Quaternary Science Reviews, 29, 3887-3893. doi: 10.1016/j.quascirev.2010.09.010
  40. Van Meerbeeck, C.J., Renssen, H. & Roche, D.M. (2009). How did Marine Isotope Stage 3 and Last Glacial Maximum climates differ? – Perspectives from equilibrium simulations. Climate of the Past 5, 33-51. Retrieved from clim-past.net/5/33/2009
  41. Wales, N. (2012). Modelling Neanderthal clothing using ethnographic analogues. Journal of Human Evolution, 63, 781-795. doi: 10.1016/j.jhevol.2012.08.006
  42. Zilhão, J. (2000). The Ebro Frontier: A Model for the Late Extinction of Iberian Neanderthals. In Stringer, C., Barton, R. N. E. & Finlayson, C. (Eds.), Neanderthals on the edge: 150th anniversary conference of the Forbes’ Quarry discovery, Gibraltar (pp. 111-121). Oxford: Oxbow Books

 

Featured Image:

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s