IntroductionThe transition from the Middle Paleolithic (MP; ca. 300 – 44 ka BP) to the Upper Paleolithic (UP; ca. 44 – 14.7 ka BP) was a period of environmental fluctuation and of change within the Homo species. Neanderthal populations were declining, while Homo sapiens populations were increasing and spreading throughout Eurasia. Moreover, these events were occurring in the context of climatic fluctuations (i.e., Glacial Periods), and two ice sheets covered large areas of Eurasia during this period. The climatic fluctuations had a profound impact on local vegetation and environments. Through this change, Homo spp. were limited to both where and when they could move across the landscape; furthermore, the locations that they could settle may have become increasingly limited as population sizes grew at the beginning of the Upper Paleolithic.Neanderthals first appeared in Europe about 250 kya. Neanderthals had been extremely successful in coping with climatic fluctuations in Eurasia for about 200,000 years (Callegari et al. 2013). They would have endured the Saalian/Riss Glaciation, the Last Interglacial (LIG), and the Weichselian/Wu?rm glacial maximum (Gilligan et al. 2007). The LIG generally brought on warmer temperatures, which are similar to temperatures in modern day, less precipitation north of the Alps, and more precipitation south of the Alps. Neanderthals’ stocky bodies and relatively short forearms and lower legs provided a way to regulate thermal energy (Collard and Cross 2017; Gilligan et al. 2007; Holliday 1997; Trinkaus 1981). With these physical attributes, Neanderthals were able to endure cooler and more seasonal climates (Holliday 1997; Trinkaus 1981). Further, regional climates may have had a major impact on “bio-behavioral evolution”. Several researchers have attempted to evaluate the effects of climate on fitness in modern groups. O’Connell (2006) suggested that Neanderthals may have had biological responses to climatic fluctuations.Anatomically modern Homo sapiens (AMHs) began dispersing out of Africa around 107–95 kya, then subsequently around 90–75 kya and 60–47 kya (Timmermann and Friedrich 2016). Around the time that the Neanderthals’ population was declining, Europe was being repopulated by AMHs who had a major dispersal from Africa and the Near East around 44.2 ka BP (also known as the Aurignacian period). So, how and why did Neanderthals become extinct, while AMH’s populations were beginning to grow and thrive? Therefore, the central question of this research is – how have paleoclimate and paleoenvironmental reconstructions been used in the debate on Neanderthal extinction? The two leading hypotheses are that AMHs out-competed Neanderthals, thus eventually replacing them, and climate change led to Neanderthal extinction. Further, some researchers suggest that climate change was closely connected to changes in demography. Due to climate extremes, Neanderthal populations migrated and became isolated due to patchiness of their environmental niche. As the landscape became less patchy, Neanderthals were not as successful in comparison to AMHs.BackgroundLate Middle PaleolithicDuring MIS-5a–5c (105–74 kya), the temperature was relatively warm across Europe (Jochim 2002a; Van Andel et al. 2003b). Further, Neanderthals were well-suited to cooler periods, such as the Weichselian-Würmian glaciation (ca. 75–10 kya), and to other milder instances of climate (McKee et al. 2015:41; Van Andel et al. 2003a). During the late MP, the MIS-4 (ca. 66–59 kya BP) period was a Glacial Maximum, which the temperature was relatively colder than in the previous “Transitional Phase” (Van Andel et al. 2003b:33; Table 1). However, by the end of the MP, another climatic event began known as MIS-3 (ca. 59–44 kya BP), which was known as a “Stable Warm Phase.” Van Meerbeeck and colleagues (2011) also show from several proxies that around 50 kya there was a warmer period, which is recorded in the Greenland Stadials (GS) 15–14. The population levels and site-use appeared to be much lower than that of the late Aurignacian period (c. 38–29 ka BP), which was populated more by AMHs. The carrying capacity for Neanderthals was below the threshold as local resources were stable. Neanderthals had moved south during MIS-4, then moving back when conditions became favorable again, and by the time they were able to start moving back, AMHs had begun populating parts of Europe (Mu?ller et al. 2011). Neanderthals seem to stay fairly local, perhaps within a 20-km radius of their sites (Conard 2012). For example in southwestern Germany, chert assemblages consisted of Jurassic chert which is a local chert source, because most of the chert is within at least 20 km, if not much closer. However, within this radius, the different groups moved fairly frequently and did not stay in one place for too long, as most of the artifact densities for each context are very minimal. Neanderthals generally had a narrower diet breadth compared to AMHs (Banks et al. 2008). Neanderthals subsisted predominantly on reindeer and horse with some ibex as well as a variety of other smaller fauna such as hare, fish, and birds (Conard 2011, 2012; Riehl et al. 2014).Upper Paleolithic From the beginning of the Aurignacian period (ca. 44.2 kya BP) to the end of the Gravettian period (ca. 32 kya BP), six rapid climatic fluctuation events (also known as Dansgaard-Oeschger (DO) events) were recorded through global paleoclimate proxy records around Greenland, where the annual air temperatures increased by 15 degrees Celsius within a few decades (Jochim 2002b; Van Andel et al. 2003b). These fluctuations would have had profound impacts on the local flora and fauna. During the last third of the MIS-3 (ca. 37–27 kya BP), the biomes in Europe fluctuated as a result of cooler and warmer periods (Miller 2009; Riehl et al. 2014). During warmer periods, the biomes trended more towards temperate grassland with some forest niches; whereas, during cooler periods, the biomes trended more towards steppe environments (or an immense area that mostly consists of grassland with no forests or trees). Boreal forests usually reside at a particular range of latitudes, which are referred to as climatic timberlines; for the early MIS-3, they resided between 48 to 50 degrees North (Van Meerbeeck et al. 2011). At the beginning of the Aurignacian (ca. 44.2 kya BP), evidence suggests that the landscape in western Europe was an interglacial tundra with a partially steppe-environment; therefore, some woody species were present across the landscape. Palynological data reveals that some boreal components were present, which was probably of the conifer group. Throughout the Aurignacian, the temperatures became increasingly cooler. However, around 36 kya BP, the temperatures may have actually increased, making the winters and summers warmer. especially given the presence of Hippophae rhamnoids (sea buckthorn) (Huijzer and Vandenberghe 1998). This also has been recorded on a local scale at Hohle Fels cave through geoarchaeology and micromorphology which shows a warmer and moister environment compared to previous periods (Miller 2009). Throughout the Gravettian (ca. 34.4–32 kya BP), the climate trends towards cooler conditions because wood vegetation decreases with a slight increase in shrub components in the area (Riehl et al. 2014). Evidence also suggests that a cooler and drier climate began around 31 kya BP. The botanical remains show that only a few species of trees were available in the area and with a steppe-like environment, it becomes evident that it was not a forested region but had sparse tree coverage. Leading up to the MIS-2 (or the Last Glacial Maximum (LGM)), the interstadials and stadials became increasingly cooler. Around 49 degrees North in latitude, Europe was mostly tundra – consisting of both trees and shrubs; furthermore, it was in close proximity to the major tree line, making it extremely susceptible to even small climatic fluctuations. Therefore, the susceptibility of vegetation to climatic changes and fluctuations in most of Europe, at least north and east of the Iberian Peninsula, might be a reason for the movement of Neanderthals into smaller patches and particularly being located within the southern Iberian Peninsula.Preferred Climate and Environments by NeanderthalsNeanderthals may have thrived in particular climates and environments in Europe. However, the values of different climate and environmental variables are difficult to ascertain. Further, the Neanderthal niche may have changed over time in Europe. During the Last Interglacial (130–116 kya, also MIS 5e), Neanderthals were living mostly in cool temperate and warm temperate and mesic zones (Nicholson 2017). They may have had a stronger preference for the warm temperate and mesic zone. The number of observed sites is 5.8 times greater than the number of expected sites, especially given that the warm temperate and mesic zone makes up a small percentage of the total land area in Europe (Nicholson 2017:Table 4). So, Neanderthals preferred somewhere between warm and cool temperate climates, and in more treed areas (Benito et al. 2017; Finlayson and Carrión 2007; Nicholson 2017; Papagianni and Morse 2015:172–179). In addition, hominin species, like Neanderthals, probably had the capacity to make clothing, even if it was very simple (Gilligan et al. 2007; White 2006). Some scholars suggest that the ability to make tools, like scrapers, are a sufficient precursor to having the ability to make clothes. Therefore, if Neanderthals had the ability to make clothes, then they could survive slightly cooler conditions, even outside of the climate that they preferred. Generally, Neanderthals do not settle in more northern latitudes, particularly above 55° north (Gilligan et al. 2007). In colder regions, Neanderthals would have needed ‘complex clothing’ and greater than three layers of clothing, which is probably unlikely for Neanderthals. Neanderthal sites are generally not found north of 55° latitude. However, when sites are found north, the sites are usually dated to periods with slightly warmer conditions. AMHs had a diverse toolkit, which is considered to be of “Mode 4 technologies” and consists of items like bone awls (Gilligan et al. 2007). Therefore, they would have had the ability to make more ‘complex clothing’, which means the clothing could have more than one layer, be a better fit, and provide more protection (Gilligan et al. 2007:Table 1).Alternatively during interglacial periods, Neanderthals would have needed to be able to adapt to the increase in temperature. Nicholson (2017) explains that Middle Eastern Neanderthal groups may have interbreed with the European populations, which would have provided traits that were better adapted to warmer temperatures. These warm adaptations could have been important in a warm temperate and mesic climate regime, as there would have been less thermoregulation stress.Climate ModelsReconstructions of paleoclimates and paleoenvironments allows for a greater understanding of the past, particularly of the effects of climate-driven variability on past populations. Paleoclimatology is the study of past climates using proxies, such as tree rings (dendrochronology), pollen, ice sheets, glaciers, and cave deposits, from the environment. These proxies are sensitive to climatic variability and can be processed to reveal time signatures, such as the rings of a tree.Two types of climate models have been used for large-scale and global reconstructions. First, General Circulation models (GCMs) simulate climate based on variables such as solar radiation, atmospheric pressure, and carbon dioxide (Van Meerbeeck 2010:35). Many of these types of variables go into the GCM. Further, many of the GCM variables are derived from other models, such as atmospheric dust content forcing. Climate forcings are variables that are internal or external to the climate system that forces the climate to change (Bloom 2017; Lord Grey School 2017). External climate forcing would occur outside Earth’s system, such as the earth’s orbit or incoming solar radiation. Internal climate forcing would be within earth’s system, such as volcanic activity and greenhouse gas content. GCMs can be incredibly complex. Some variables in a GCM, such as insolation, are based on astronomical theory and orbital parameters. For example, insolation at any point on earth and at any time can be calculated (Berger and Loutre 1991; Van Meerbeeck 2010:48). Some researchers use GCMs to form the base of their models. For example, Nicholson (2017) modeled paleoclimate zones, which used a GCM as a component in the model (Otto-Bliesner et al. 2006), from ca. 130–116 kya to understand Neanderthal distributions in relation to climate and environment (Nicholson 2017). Nicholson constructed the paleoclimate zones based on a previous model that defined 125 paleoclimate zones for the world (Metzger et al. 2013). Metzger and colleagues (2013) made the zones using 42 variables such as precipitation, maximum temperature, aridity, and other indices. Therefore, the use of multiple models from multiple studies makes reproducibility nearly impossible for most researchers.Second, a Species Distribution Model (SDM) is slightly less complex than GCM. A SDM consists of variables such as soils, pollen, fauna, and glacial that record changes in the environment and climate over time (e.g., Tarasov et al. 2013). Then, a modern analog, such as weather station climate value, is related to the past data to translate the past data into climate values, such as average temperature or precipitation. For example, the modern analog Technique (MAT) approach uses a proxy approach, where a group of fossil pollen can be matched to a similar group of modern pollen (Overpeck 1985; Viau et al. 2006, 2012; Whitmore et al. 2005; Williams and Shuman 2008). Since modern climate data can be linked to a modern pollen location, then the associated-modern climate can also be matched to the fossil pollen that matched. For a larger scale and more complex SDM, Huijzer and Vandenberghe (1998) created the Multi-Proxy Approach (MPA), where they reconstructed the climate and characterized the warmer and cooler periods from 74–13 kya. Other researchers also develop complex models for reconstructing the climatic context during the period of the Neanderthals (e.g., Tzedakis et al. 2007).Use of Climate and Environmental Models in the Neanderthal Extinction DebateResearch has been conducted to situate and explain the extinction of Neanderthals and the emergence of new Homo spp. cultures around the defining time of the Middle to Upper Paleolithic, which has usually been coupled with climatic information and demography (Müller 2011; Tzedakis et al. 2007). Conard (2011:224) states thatWith major changes in the climate and environment across Europe, could that change have been the leading cause of Neanderthal extinction?Scholars have mostly either argued for or against climate change as the leading cause in the extinction; however, a more parsimonious option has recently been suggested by Kolodny and Feldman (2017). Many researchers have argued that Neanderthals were outcompeted by AMHs. For example, Banks and colleagues (2008) did climate simulations using GCMs and reconstructed eco-cultural niches. An eco-cultural niche is a range of environmental conditions that a species can exist in without significant changes. Their hypothesis was that either Neanderthals would have had to contract during changing climate or that there was competition with AMH populations. During the three periods modeled of climate change, Neanderthals have a broader distribution. However, the authors focus on niche breadth, which provides the diversity of abiotic conditions under which a species can maintain a population. The Pre-Heinrich event 4 (pre-H4) (43.3–40.2 kyr cal BP) and Heinrich event 4 (H4) (40.2–38.6 kyr cal BP) have a similar trend; however, Greenland Interstadial 8 (GI8) (38.6–36.5 kyr cal BP) shows a major difference from pre-H4 and H4. AMHs’ niche breadth and distribution broadened, while Neanderthals narrowed. Therefore, Banks and colleagues (2008) concluded that competition led to Neanderthal extinction. On the Iberian Peninsula, d’Errico and Goñi (2003) show that poorly dated and correlated records have led to false conclusions that climate change was the reason for extinction. However, the authors use pollen-rich deep sea core data to describe the Iberian Peninsula, particularly around H4. The colder climate led Neanderthals to contract to southern Iberia, a desert-steppe-like environment, where they persisted for a while longer until AMHs spread to southern Iberia. The authors conclude that competition from AMHs led to the replacement of Neanderthals.Other scholars have argued that climate change was the driving force behind Neanderthal extinction. For example, Finlayson and Carrión (2007) evaluated the pollen record from three sites. The data show differences in assemblages of plant species between warmer and cooler conditions (Finlayson and Carrión 2007:217). Around 31–27 kya, more woodlands existed in southern Iberia; then after 27 kya, the temperature returned to cooler conditions and a more steppic environment. Ultimately, the spread of modern humans and the decline of Neanderthals coincides with climate-driven changes of the ecology. AMHs were also living on physiographical boundaries, which may account for their wider diet breadth and ability to cope with changing conditions. The distribution of Neanderthal sites slowly diminishes over time and moves south and west (Finlayson 2008). Neanderthals had several “stronghold” locations, which eventually diminished before extinction. Other scholars focused on the direct effects of climate change on Neanderthals. For example, Stringer and colleagues (2003) examined the relationship between climatic stress and extinction. Climatic stress is the (Stringer et al. 2003:235). They used a pollen record from Italy and oxygen isotope data from Greenland to calculate climatic stress. They conclude that the increasing climatic stress led to the extinction of Neanderthals. Further, the larger Heinrich events may have disrupted hominin behavior, and contributed to the extinction of Neanderthals (Burke et al. 2017).Kolodny and Feldman (2017) recently put forth a more parsimonious model through population simulations. The authors focus on the four forces of evolution (i.e., selection, mutation, gene flow, and genetic drift). They conclude that migration and random species drift may have been the contributing factor to Neanderthal replacement. However, this explanation does not negate the effects that climate or competition may have had on Neanderthals; however, migration and drift provide a more parsimonious model.Similarly, Cucart-Mora and colleagues (2018) used an agent-based model to simulate interactions between Neanderthals and AMHs in Iberia. They also allowed for agents in the model to become hybrid agents, the offspring of a Neanderthal and AMH. They concluded from the simulations and archaeological data that there was a (Cucart-Mora et al. 2018:21). The authors suggest that differences in birth and death rates and mobility between Neanderthals and AMHs can account for the Neanderthal extinction.After periods of climatic instability, the environment was conducive to the increase of other industries, like the Aurignacian, and to promoting change among hominin behavior (Jiménez-Espejo et al. 2007; Potts 2013). For example, Jiménez-Espejo and colleagues (2007) used a single marine record to describe the climate in Southern Iberia. They showed a positive correlation between Barium excess and climatic instability. After periods of instability, the populations of other industries were able to increase, predominantly AMHs. Therefore, climate is part of the equation and linked to changes in adaptation.One way to adapt to climatic change is to innovate and make technological advances. For example, Bocquet-Appel and Tuffreau (2009) showed that there are some technological responses to changes in macroclimates among AMHs. Further, AMHs may have had a greater capacity for innovation (Klein 2008). However, since Neanderthals maintained small numbers, they may have been limited in their technical creativity. Ultimately, Neanderthals may have been in a Malthusian trap, due to the hunter-gatherer lifestyle, which led to low carrying-capacity and a demographic equilibrium (Bocquet-Appel and Tuffreau 2009; Malthus 1926; Wood 1998). Neanderthals were generally carrying heavier weaponry (i.e., Mousterian) that was better for ambushing and larger fauna; whereas, AMHs carried lighter tools and could carry out more strategic hunting (Finlayson and Carrión 2007).Finally, the teeth of Neanderthals and AMHs can provide some insights into variability in tool technologies. El Zaatari and colleagues (2016) show that occlusal molar microwear can help scholars to understand how Neanderthals adapted by eating more abrasive foods when in forested areas then not as much in colder temperatures, perhaps eating more meat. Neanderthals also were more heavily processing bones during colder periods (Hodgkins et al. 2016). However, AMH’s microwear complexity stays relatively the same leading up to Neanderthal extinction; the authors (El Zaatari et al. 2016) attribute this to AMHs’ ability for technological innovation.Discussion and ConclusionScholars have used a variety of climate and environmental models to argue for and against climate change as being the main reason for Neanderthal extinction. In some cases, climate and environmental models can be very complex and very difficult to reproduce. A researcher would need a lot of time and expertise to create, use, or reproduce a GCM or SDM; however, a GCM would be more difficult due to the internal and external forcing parameters. Furthermore, several of the models discussed above use additional models in conjunction with a GCM or SDM, further adding to the complexity. One way that scholars have sought to ground-truth the larger climate models is to examine local records for a study area (e.g., Jiménez-Espejo et al. 2007) and making sure that the local record is well-dated in comparison to larger datasets or ice cores, such as the radiocarbon climatostratigraphic approach (Tzedakis et al. 2007). One issue with Stringer and colleagues (2003) is that they are only using two locations, Italy and Greenland, to make generalizations about all Neanderthals in Europe. This does not take into account small changes across space, nor the microclimatic factors that occur at local scales. Despite that some of the climate and environmental models can be complex, climate change should be discussed in relation to Neanderthal extinction. Climate potentially when and where Homo spp. could move and the available resources. Thus, the evidence suggests a combination of competition, climate change, demographic factors, and migration and random species drift as contributing to Neanderthal Extinction. However, the weighting of each factor could vary greatly across time and space. One way forward from the complexity is to study these processes on a local scale and consider the local paleoclimatic and paleoenvironmental data.The evidence suggests that the movement of AMHs, who have a wider diet breadth, into Eurasia were better able to handle climatic fluctuations. AMHs would not have needed to move too far to consume similar foods, particularly since they were residing on sharp physiographical boundaries (Finlayson and Carrión 2007). AMHs also had more diverse hunting technologies, especially compared to Mousterian artifacts, and strategies, such as hunting at a distance versus ambushing.Neanderthals had been very successful with their strategy of a narrower diet breadth and occasionally eating more abrasive foods (El Zaatari et al. 2016). However, since they were slightly less flexible and were more limited on where to move when climate changed, they were outcompeted by AMHs. Their reproductive success rate may have also suffered during this later period. Since Neanderthal populations were likely smaller, drift may have had a greater effect. Therefore, it is likely that multiple interacting causes, including climate, led to the Neanderthal extinction.