The subject of the success of early anatomically modern humans (AMH) and the decline and eventual extinction of the Neanderthal has been a subject of heated debate now for decades. Each find results in a new theory which in turn gives birth to many counter theories and claims, and the circular argument moves slowly on towards some form of protracted resolution. While there may be a growing assemblage of fossils and artefacts to aid in the interpretation of prehistory, they are still a finite and incomplete representation of a vast period of time. It would be arrogant presumption to assume a definitive answer could be given for the ultimate cause of this model of success versus decline, however as I will attempt to show, the discipline is moving ever closer to understanding the complex nature of this popular conundrum. Through an examination of the effects of colonization, interaction and direct competition, as well as a process of assimilation of Neanderthal peoples by AMH, this essay aims to illustrate some of the theories surrounding the ultimate dominance of the AMH and the extinction of the Neanderthals.
Colonization, Interaction & Competition
Figure 1. Dispersal routes of the earliest AMH populations across Europe.
The initial expansion into Europe by early AMH populations is considered to be 43 – 42 kyr cal. BP (fig. 1), however due to the scarcity of physical evidence to support this hypothesis and direct dating suggesting a range of 41 – 39 kyr cal BP, a considerable gap has been left in the archaeological record (Higham et al. 2011, 521). Further study of human maxilla fragments from Kent’s Cavern, UK, via radiocarbon accelerator mass spectrometer produced a date of 36.4 – 34.7 kyr cal. BP, however an application of Bayesian analysis upon unfiltered bone collagen significantly altered this date to 44.2 – 41.5 kyr cal. BP (fig. 2), suggesting an extensive and swift expansion of early AMH populations across the whole of Europe more than 40 kyr ago (Higham et al. 2011, 521). Largely seen as the site with the clearest evidence of a transitional intersection between Neanderthals and early AMH populations, the Levant forms the central staging ground for expansion into Europe (Shea 2003, 173). Fossil remains and lithic assemblages found at Taburn and Skhul caves represented what Shae refers to as “a single population that was in the throes of an evolutionary transition” towards further specialised Neanderthal and AMH groups (Shae 2003, 173). It is suggested that this reflects the effects of steadily increasing sociocultural and environmental stress upon the Neanderthal populations, resulting in an evolution into early modern humans (Shae 2003, 173). While some Levantine lithic technologies are associated more with Neanderthals than AMH as well as the reverse, there is nothing to clearly define the tools or their manufacturing techniques as belonging solely to one group (Shae 2003, 175). They are no different from the vast majority the simple flake tool assemblages distributed throughout the European Middle Palaeolithic and African Middle Stone Age (Shae 2003, 175).
Figure 2. Bayesian age model for Kent’s Cavern
In Mellars et al.’s study of the interstratification of Neanderthal and AMH occupation of Châtelperron in central France, previous research and refuted claims were reassessed against current theoretical models to determine whether there was any degree of chronological overlap and coexistence between the two populations (Mellars et al. 2007, 3657 – 3662). By separating the stratified layers of Aurignacian and Châtelperronian material, it became possible to accept that some level of coexistence, exchange and hybridization or admixture was totally plausible (Mellars et al. 2007, 3662). A cultural exchange and transmission of ideas and abstract constructs appears to have taken place in sites such as Châtelperron and Uluzzian, with expressions of personal ornamentation previously associated with Neanderthals, being displayed among early AMH populations (Zilhão 2006, 183). This was likely facilitated by the fact that the early AMH immigrants would have found the indigenous Neanderthal populations to share a similar level of cultural and technological achievement as well as cognitive ability (Zilhâo 2006, 191 – 193). Based on current evidence, it is impossible to rule out interbreeding between Neanderthal and AMH populations (Mellars 2004, 464). The rapid disappearance of “distinctively Neanderthal patters of mitochondrial DNA, as well as the distinctive anatomical features of Neanderthals,” could reflect the results of competition between the two species, with the AMH having the advantage due to the complexities of their technological and organisational advancements (Mellars 2004, 464). As Banks et al. states the steady decline of Neanderthals “was not due to climate change or a change in adaption, but rather concurrent AMH geographic expansion appears to have produced competition that led to Neanderthal extinction (Banks et al. 2008, 1)”. Or as Shae states, “this rivalry was probably most intense, and its conclusion predetermined in our favour during the Middle Palaeolithic in the Levant (Shae 2003, 185).”
Assimilation, Hybridization or Replacement?
The ‘Recent African Origin’ model, also referred to as the ‘Out of Africa’, ‘African Replacement’, or ‘Replacement’ model, is one of two modes of theory against which the origins of AMH are assessed (Stringer 2002, 563). The alternative is the ‘Multiregional Evolution’ model, which is also known as ‘Regional Continuity’ model (Stringer 2002, 563). Where the former suggests a long-term replacement of indigenous populations by the AMH out of Africa as they spread throughout Eurasia, the latter examines patterns of gene flow throughout Africa and Eurasia due to AMH populations that evolved independently in these regions interacting over time (Stringer 2002, 563 – 564). Alternatives, such as the ‘Hybridization and Replacement’ and ‘Assimilation’ models, are variations on the aforementioned theories, with Hybridization allowing for varying degrees of hybridization between migratory AMH and indigenous pre-modern humans, and Assimilation emphasising a combination of factors from ‘Out of Africa’ and ‘Multiregional Evolution’ in a direct response to variable selection pressures and morphological change (Stringer 2002, 563 – 564). Green states that, “the analysis of the Neanderthal genome shows that they are likely to have had a role in the genetic ancestry of present-day humans outside Africa (Green 2010, 722).” The relatively minor percentage of genomes suggests a system for positive natural selection favouring those attributes that would benefit the development of early AMH (Green 2010, 722). The Environmental Genome Project revealed an element of Neanderthal admixture within the modern European gene pool of up to 5% (Plagnol 2006, 972). However, Villa and Roebroeks state that, “in 2010 a draft sequence of the Neanderthal nuclear DNA (fig 3.) provided clear evidence of interbreeding between Neanderthals and modern humans, estimating that Neanderthal inheritance makes up 1 – 4% of the genomes of people outside Africa (Villa & Roebroeks 2014, 6).” That figure has since been revised again due to the sequencing of Neanderthal material from the Altai Mountains, which lowers the percentage to as little as 1.5 – 2% (Villa & Roebroeks 2014, 6).
Figure 3. Table illustrating the ratios of Non-African to Neanderthal matches, with AN (ancestral nonmatch), DN (derived nonmatch), DM (derived match), and AM (ancestral match)
As Green argued previously, Villa and Roebroeks consider natural selection as a probable contributing factor for the favouring of specific Neanderthal genes in the development of AMH populations (Villa & Roebroeks 2014, 6). These selected genes may have allowed the early AMH to adapt to their new environments, but conversely would have contributed to “male hybrid sterility” in the populations through a lack of tolerance for specific genetic alleles, causing a further reduction in the percentages of surviving Neanderthal DNA (Villa & Roebroeks 2014, 7). This gene flow was likely to have occurred between AMH and Neanderthal populations approximately 50,000 years ago, when the two groups would have met one another in Eurasia (Villa & Roebroeks 2014, 6). Additionally there have been various fossils that have been identified as being “transitional”, i.e. reflecting some form of Neanderthal and anatomically modern human (AMH) hybridization, in Europe to date, however due to variations on an individual scale as well as by gender or age, it becomes exceedingly difficult to make a clear and concise judgement (Stringer & Günter 1994, 420). Stringer and Günter state that, “once again we must reiterate the points that the effects of hybridization could mimic those of in situ evolutionary change and that an attempt should be made to separate plesiomorphous from amorphous characters in comparisons of Neanderthal and early modern humans (Stringer and Günter 1994, 420). As both AMH and late Neanderthal fossils reflect anatomical characteristics contemporary with one another, it can be argued that there was a certain degree of genetic admixture between the two groups in Europe (Villa & Roebroeks 2014, 6). The most striking example of this has recently come out of Manot Cave (fig. 4), Western Galilee, Israel (Hershkovitz et al. 2015, 1). The discovery of a partial calvaria in Manot 1, appears to represent what can only be described as the first hybrid Neanderthal / AMH fossil evidence of mutual occupation of the Levant in the late Middle of Neanderthal and early modern humans (Stringer and Günter 1994, 420). As both AMH and late Neanderthal fossils reflect anatomical characteristics contemporary with one another, it can be argued that there was a certain degree of genetic admixture between the two groups in Europe (Villa & Roebroeks 2014, 6). The most striking example of this has recently come out of Manot Cave (fig. 4), Western Galilee, Israel (Hershkovitz et al. 2015, 1). The discovery of a partial calvaria in Manot 1, appears to represent what can only be described as the first hybrid Neanderthal / AMH fossil evidence of mutual occupation of the Levant in the late Middle and Neanderthals establish the southern Levant as the likely centre for initial interbreeding between the two species (fig. 5) (Hershkovitz et al. 2015, 3). However, while discoveries such as the Manot 1 calvaria are impressive, as Stringer and Günter rightly point out, while such features can be interpreted clearly, others may simply resemble Neanderthals in some aspects, but are quite different from them when elements such as facial shape and structure are considered such as the African fossils of Irhoud or Ngaloba (Stringer & Günter 1994, 422). Therefore it is important to utilise the full spectrum of tools at ones disposal to ensure the best possible identification of any fossil remains before constructing a theory about their specific origin or typology.
Large and effective AMH populations dispersed rapidly throughout most of Europe and assimilated Neanderthal groupings in a “limited if widespread” fashion, through mass exploitation of land, resources and technological invention (Richards and Trinkaus 2009, 16037 – 16038). Zilhão states that, “rapid absorption of the Neanderthals is consistent with the size imbalance between the two gene reservoirs and further supports significant levels of admixture (Zilhão 2006, 183).” However, it is possible that this was limited to specific zones or regions and entire Neanderthal groupings could have gone extinct without any form of hybridization or genetic admixture (Zilhão 2006, 183). This notion is further supported by the data which reflects a low genetic diversity among Neanderthal communities, which could be indicators for a contracting population (Villa & Roebroeks 2014, 7). If there was a disparity in numbers between the newly arrived AMH and existing Neanderthal populations, then the absorption and assimilation of the Neanderthal would have been rapid (Villa & Roebroeks 2014, 7). Stringer states that, “evidence shows that local extinctions can occur when a species’ range is contracting (fig. 6) (Stringer 2012, 1317).” This can also have an effect on a species’ evolution, reinforcing differences between isolated populations only allowing for admixture when refugia expands to allow for favourable conditions (Stringer 2012, 1317). This process of contraction and expansion would likely serve as a catalyst for the interbreeding between Neanderthals and AMH during the Late Pleistocene in Eurasia allowing for hybridization to occur during distinct climatic and biogeographical phases (Stringer 2012, 1320). As Villa & Roebroeks states, “the Neanderthal demise appears to have resulted from a complex and protracted process including multiple dynamic factors such as low population density, interbreeding with some cultural contact, possible male hybrid sterility and contraction in geographic distribution followed by genetic swamping and assimilation by the increasing number of modern immigrants (Villa & Roebroeks 2014, 7).” Despite this, Finlayson et al. states that the “Neanderthals survived in isolated refuges well after the arrival of modern humans in Europe,” up to at least 28 kyr BP as illustrated by the fossil remains found in Gorham’s Cave, Gibraltar (Finlayson et al. 2006, 850).
While many archaeologists and anthropologists have supported the idea that Neanderthals were somehow technologically, socially and cognitively inferior to the AMH populations, and therefore that led to their decline, current evidence suggests otherwise. It has become increasingly clear that this antiquated notion no longer holds any weight, and that Neanderthal communities were equally as complex upon the arrival of early AMH immigrants. It can be further argued that in many cases the similarities between Neanderthals and early AMH populations far outweigh the differences, therefore it should be the biological variations that separate the two species, not some sort of imagined superiority complex that places the AMH above all. Despite this, however, the AMH was the more successful species. While the two species may have been similar, the vast number of AMH versus Neanderthal, and their higher level of organisation meant that they were able to spread throughout Europe at an incredible speed, making the Neanderthal communities contract as they encroached upon or simply absorbed their lands. While this likely contributed to the hybridization seen within fossil samples, it is evident by the low percentage of Neanderthal DNA that these were limited occurrences caused either through low population numbers or time-limited events. This is represented through changes to refugia, declining genetic diversity, Neanderthal / AMH admixture and morphological changes seen throughout the Middle Palaeolithic. The ‘Assimilation Model’ as an explanation for the success and dominance of the AMH populations over their Neanderthal counterparts is becoming increasingly supported by advances in sampling genetic data and recent fossil finds. The ‘Assimilation Model’ works well here because it can be argued that Neanderthal decline was not simply down to a singular factor, but instead a myriad of complex and interconnected events and influences which led to their temporal extinction.
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