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\section{Discussion}
In this paper we present the
hitherto first model of potential Neanderthals distribution across Europe during the Last Interglacial (Eemian, MIS 5e, $\sim$130 ka BP). We have applied a robust state-of-the-art modeling approach to take advantage of a sparse set of archaeological records and a palaeoclimatic simulation to produce
the our model. We analyzed
the this model using regression-tree based statistical methods to assess how environmental variables shaped habitat suitability at continental and local scales, and assessed to what extent our preliminary hypothesis
on regarding the abiotic drivers of
Neanderthals Neanderthal distribution matched the results of the model.
The papers by \citet{Richter2005}, \citet{Richter2006}, \citet{Wenzel_2007}, and \citet{Gaudzinski2011} are among the most relevant
ones recent contributions discussing the distribution and
survival strategies adaptations of
Neanderthals during their last years. \textit{Homo neanderthalensis}. These papers provide a valuable review on Eemian Neanderthal sites (many of them used to calibrate our model), and offer a valuable discussion on whether or not the dense forests present in Central Europe during MIS 5e were suitable for Neanderthals due to the lack of large herds of mammals (especially \citet{Gaudzinski2011}). Since our model is focused on abiotic drivers, we cannot contribute to the discussion of whether or not dense forests were
a harsh an unsuitable environment for Neanderthals. But according to our results, the
central and north North European
plains occupied Plain covered by dense forests during the Eemian were
already relatively cold and flat, resulting in low habitat suitability values. This does not mean that we consider abiotic factors to be more relevant in shaping Neanderthals distribution; we are simply adding new information that may help to formulate new questions about this topic. To disentangle the relative effect of climate, topography, and the presence of dense forests
over Neanderthals across the Neanderthals' northern
range edge would require a reliable vegetation simulation (as in \citet{Allen20102604} or \citet{Gaillard2010483}),
that which is not currently available.
Two of the papers mentioned above, (\citet{Wenzel_2007} and \citet{Richter2006}) discuss the use of beaches to obtain
seafood. marine foods; more recently, Cohen et al. (insert: Cohen, K.M., MacDonald, K., Joordens, J.C.A., Roebroeks, W., Gibbard, P.L., 2012. The earliest occupation of north-west Europe: a coastal perspective, Quaternary International 271, 70-83.) also consider coastal areas - many of which are now submerged or have been dramatically altered by the many trans- and regressions of the Late Pleistocene - as highly suitable if not optimal spaces for European hominids. A
few new handful of recently discovered Eemian sites found in Italy and
Spain Spain, for instance, are located by the sea, and our model shows that habitat suitability was very high across most parts of the Mediterranean, Atlantic, Adriatic, Aegean, and
the Black Sea coasts.
Actually, In fact, the
distribution of habitat suitability values
interpreted along the associated with these newly
available found sites
challenges challenge the
idea notion of Neanderthals as a
central-European central European species. Our model
shows suggests that the presence records in northern Germany could
have been represent the tail of
Neanderthals distribution, Neanderthal distribution during MIS 5e, while abiotic conditions were close to
optimum optimal along the coastlines. Our findings support the idea of Eemian Neanderthals being a species mainly thriving in warm
coastlines, coastline habitats, able to exploit the resources provided by the sea \cite{Stringer200814319, Colonese201186}
and as well as probably small game living near coastal habitats \cite{Blasco2014}.
This suggestion is in line with earlier suggestions regarding the importance of coastal habitats in human evolution and dispersal (insert: Bailey, G., 2004. World prehistory from the margins: the role of coastlines in human evolution. Journal of Interdisciplinary Studies in History and Archaeology 1 (1), 39-50.). In addition, our rigorous modeling Neanderthal abiotic tolerance ranges suggests that this hominid species shows no preference for cold conditions.
\paragraph{Hypothesis on the drivers of Neanderthals distribution}
According to
our the hypothesis proposed
hypothesis, here, the northern edge of
the Neanderthals' distribution should be controlled by cold winters due to
the low availability of small and big game, and cold stress. Our model confirms that minimum temperature of the coldest month influences habitat suitability at the continental scale, but the
local scale local-scale analysis
shows suggests that the low habitat suitability of places within and beyond the northern edge (for example: Southern Baltic coast, Jutland peninsula, Southern Sweden, Southern Finland) can be explained by a combination of cold winters and plain slopes.
We
hypothesized further hypothesize that hot and dry summers may have shaped the southern limit of Neanderthals distribution due to heat stress and low summer productivity
resulted resulting from drought, but also that this effect could have been buffered by
the nearness of proximity to the sea
in the along Mediterranean coastlines. The analysis of our model showed that southern localities with summer temperatures higher than 34 ºC (2, 3, 17, 26, 27) consistently presented low habitat suitability, while habitat suitability was generally very high
in the along Mediterranean coastlines (and
the southern coast of the Black Sea), with summer temperatures between 29 and
34ºC. 34 ºC.
Finally, our hypothesis puts emphasis on the importance of
having a proper topographic
setting factors at the local scale: diverse enough to promote a high availability of ecological niches, and
hence hunting
resources as a result, and gathering resources, but not too rough to fulfill the high mobility requirements of Neanderthals. The analysis of the model at the continental scale reveals that slope is the third most important predictor, with an optimum between 3 and 7 degrees. At the local scale, optimum slope coefficients resulted in high habitat suitability
in for localities along the Mediterranean coast, while an excess of slope reduced habitat suitability
(Adriatic (e.g. on the Adriatic coast,
the Aegean Islands,
and the Cantabric coast of Iberia).
In summary, our model supported the main points of our hypothesis (with the exception of the effect of summer drought
in along the southern limit), but even provided further information about the complex interaction between temperature and topography to define habitat suitability for Neanderthals during MIS 5e.
\paragraph{Implications for Neanderthal dispersal}
The habitat suitability model presented in this study provides guidelines
to interpret for interpreting the potential effect of climatic and topographic barriers
over for Neanderthals dispersal. Mountain ranges like the Alps and Pyrenees
were probably
were strong barriers during the Eemian (and during the early Weischselian, according to \citet{Andel}) that could have offered an opportunity to be crossed during the summer \cite{Richter2006}, but a rough topography could still have been an issue for the mobility requirements of Neanderthals. Extensive
flat plains dominated by a continental
climate climatic regime (Pannonian
plain, Plain, North European
plains) Plain) could
have also
have prevented
Neanderthals successful Neanderthal migration, especially during
summers summer in the
south South and
winters winter in the
north. North.
Our model shows at least six suitable geographical regions, more or less isolated from each other:
i) the Atlantic and Mediterranean coasts of the Iberian Peninsula;
ii) France and Western Germany, limited by the Alps, the
north North European
plains Plains and the
sea; sea (???); iii) the British Islands,
but Neanderthals although, interestingly, Neanderthal presence
during the Eemian has
despite concerted efforts not been confirmed there
during the Eemian \cite{Penkman_2011};
iv) Italian peninsula, limited by the Po Valley and the Alps;
v) the Peloponese and Aegean Islands, limited by the sea (but see \citet{Richter2005}, \citet{Broodbank_2006} and \citet{Ferentinos_2012}, for a discussion about the possibility of seafaring by Neanderthals), but without any reliable Neanderthal material attributed to MIS 5e; and
vi ) the coasts of Anatolia, Lebanon, and Israel.
In the long term, the compartmentalization of Neanderthal populations
suggested by these relatively isolated niche pockets could
have have, especially together with low population densities, led to
repeated local extinctions (insert: Bocquet-Appel, J.-P. and Degioanni, A., 2013. Neanderthal Demographic Estimates. Current Anthropology 54 (S8), S202-S213.; Premo, L.S. and Hublin, J.-J., 2009. Culture, population structure, and low genetic diversity in Pleistocene hominins. Proceedings of the National Academy of Sciences 106 (1), 33-37.
Premo, L.S. and Kuhn, S.L., 2010. Modeling Effects of Local Extinctions on Culture Change and Diversity in the Paleolithic. PLoS ONE 5 (12), e15582.) and genetic differentiation, as shown in \citet{Fabre_2009} where the authors studied genetic variation within twelve samples of mitochondrial DNA obtained from Neanderthal samples dated between 100 and 29 ka BP (but according to \cite{Higham_2014} the latter date may be more probable around 40 ka BP). They defined two genetic demes within our study area (see Fig. 2 in \citet{Fabre_2009}: a northern region that comprises the North of the Iberian Peninsula, Eastern France, Germany and the rest of
central Central Europe; a southern region comprising the East of the Iberian Peninsula,
the South of France,
the Italian Peninsula, and the Balkans. These demes do not
completely readily fit with our habitat suitability model: there seems to be connectivity between
Eastern the eastern and
Western western parts of the Iberian Peninsula due to a corridor of suitable habitat
at along the south of Pyrenees, and between
Western western and
Eastern France, that show eastern France characterized a large extension of
well connected well-connected suitable habitat. But both models agree in the important effect of the Alps as the main geographic barrier shaping the separation between ecologically suitable areas and genetic demes. A detailed analysis of habitat suitability maps along with cultural and genetic evidences can help to understand the causes for gradients of cultural and genetic change across large territories.
Likwise, Ruebens (2013: Ruebens, K., 2013. Regional behaviour among late Neanderthal groups in Western Europe: A comparative assessment of late Middle Palaeolithic bifacial tool variability. Journal of Human Evolution 65 (4), 341-362.) could identify several culturally distinct regions, each characterized by distinct tools shapes and production methods. As an information transmission system culture is, at the population level, arguably sensitive to forces similar to genetics (insert: Lycett, S.J. and von Cramon-Taubadel, N., 2015. Toward a “Quantitative Genetic” Approach to Lithic Variation. Journal of Archaeological Method and Theory 22 (2), 646-675.
Lycett, S.J., 2015. Cultural evolutionary approaches to artifact variation over time and space: basis, progress, and prospects. Journal of Archaeological Science 56 (0), 21-31.) Like the above genetic analyses, however, Ruebens' work was also focused on the periods after MIS 5e, albeit with important implications for the interpretation of our abiotic ecological factors.
\paragraph{Limitations}
The modeling approach presented in this paper has certain limitations, most of them
caused by grounded to the input
data. data or lack thereof. Neanderthal sites attributed to MIS 5e are scarce in Europe, and their distribution is spatially biased due to a differential availability of areas suitable to preserve
the remains, like such remains (e.g. travertines, caves, lake basins or beach
deposits, deposits) across Europe \cite{Richter2006}. The chronology of the remains, or more precisely, the method used to define their chronology, is another important source of
uncertainty (if not the most). uncertainty. There are very different methods to assign dates to archaeological remains beyond the
range of radiocarbon
range, and dating, each
one of them has associated with its own problems, and usually wide confidence intervals
(except tephras), that can be even - wider
at times than the
focus period
in focus here (the Eemian interglacial lasted for around 10,000 years). Therefore, and despite our effort to diminish chronology errors as much as possible by filtering dubious dates, there is
an the obvious risk of including data points belonging to periods different than the Eemian that may affect the outcome of the model.
One solution would be to weight Weighting the presence records according to their
uncertainty, uncertainty may offer one analytical solution to this problem, but
it was one that lies beyond the scope of
this paper to test the
potential advantages of this approach. present paper. Adding new sites through new fieldwork or improved dating protocols present another solution in the long term.
\paragraph{Conclusions}
Our model shows the highest habitat suitability values along coastal areas with mild summers, while the Central European sites, which have dominated our view about the Neanderthal life style during the Eemian, showed low habitat suitability and were interpreted to belong to the distribution tail. Therefore,
most many current interpretations of
the knowledge we have about Neanderthals Neanderthal lifestyle during MIS 5e may not
represent accurately
represent this species preferred ecotone and lifestyle, i.e. the technological adaptations required to survive in coastal
habitats, and further habitats. Further research
would be is needed to investigate the cultural and technological differences between Neanderthal populations inhabiting habitats with
such contrasting ecological features.
The methodology presented here to analyze the habitat suitability model is novel, and has proven useful to obtain an insight into the abiotic
processes factors driving habitat suitability at the local scale in different regions across the study area. This methodology can help us to move beyond
the "pretty map", simply placing dots on maps, and convert SDMs into a source of valuable information
that can help to interpret for interpreting the ecological conditions at
Pleistocene archaeological sites.
...
