Demographic explanations of neanderthal extinction: a reply to Currie and Meneganzin

In a recent paper, Currie and Meneganzin (Biol Phil, 2022, 37, 50) critically engage with a recent demographic explanation of the demise of Neanderthals (Vaesen et al. 2019). Currie and Meneganzin suggest that, contrary to how it is (supposedly) presented, Vaesen et al.’s explanation is not (and in fact, could never be) ‘stand-alone’, i.e., competition and environmental factors always interfere with demographic ones. Here I argue that Currie and Meneganzin misconstrue what the study in question does and does not purport to show. I conclude that, in the relevant sense, the explanation Vaesen et al. provide is a standalone demographic one.


Introduction
The demise of Neanderthals is a matter of ongoing controversy. Various explanations have been given for the extinction, and these explanations can be grouped into three categories: environmental, competitive and demographic. According to the first, Neanderthals went extinct due to environmental (e.g., climatic) change (Tzedakis et al. 2007;Finlayson 2008;Golovanova et al. 2010;Müller et al. 2011; but see Sørensen 2011, and the review by Villa and Roebroeks 2014). On competitive accounts, Neanderthals disappeared because they were inferior (cognitively, socially, technologically) to Anatomically Modern Humans (AMHs) in competing for the same resources (Marean 2005;Mellars 2005; Shea and Sisk 2010; Wynn and Coolidge 2011; but see Sørensen 2011, and the review by Villa and Roebroeks 2014). Finally, more recent, demographic explanations emphasize the role played by demographic factors, including migration (Kolodny and Feldman 2017), the disconnected-ness of Neanderthal sub-populations (Vaesen et al. 2019) and fertility (Degioanni et al. 2019).
In a recent paper, Currie and Meneganzin (C&M) critically engage with the latter three studies (Currie and Meneganzin 2022). They object to the fact that these studies, in contrast to how they (implicitly) present things, do not offer stand-alone demographic explanations. In fact, C&M contend that extinction events in general can never be accounted for in demographic terms alone. Their basic argument for this is that demography cannot be separated from environmental and competitive factors.
Although some of Currie and Meneganzin's charges may be correct, I argue that not all are; and in particular, Vaesen et al.'s demographic account remains unscathed.

Demography as not separable from competition and environment
C&M start by outlining their preferred account of explanation-one which I will also adopt throughout-namely a difference-making account. According to differencemaking accounts, some factor is a cause of some event on the condition that if an ideal intervention on that factor were to be made, the event would turn out differently. C&M do agree that demographic factors might be difference-makers, but insist that they are not independently manipulable-and hence that standalone demographic explanations are impossible. They illustrate their claim by assessing the three aforementioned studies (viz., Kolodny and Feldman 2017, Vaesen et al. 2019, Degioanni et al. 2019. Let us consider the study by Degioanni et al. to see how their argument works. According to the model of Degioanni et al., a small decrease (less than 4%) in the fertility of young Neanderthal women could lead to substantive decreases in Neanderthal population sizes, up to the point where Neanderthals would disappear. In this case, a drop in fertility is a difference-maker, but the cause of such a drop must be something else, that is, the ultimate cause is environmental or competitive. And, as C&M rightly point out, Degioanni et al. seem to acknowledge themselves that fertility decreases might track environmental change. Importantly, for there to be a sustained drop in fertility, such environmental change must also be sustained (or long-term). Accordingly, Degioanni et al. implicitly accept one of the explanations that their account was supposed to be an alternative to (viz., explanations in terms of long-term climatic change). C&M conclude that fertility decreases are manipulable, but not without also (first) manipulating environmental factors. 1 Vaesen et al.'s basic premise-one that C&M agree with-is that Neanderthals lived in small disconnected sub-populations. Such small populations are particularly subject to extinction, due to inbreeding, Allee effects, and stochasticity, factors which the authors define as follows:

Vaesen et al. not separable but separable
Inbreeding depression refers to the reduction in fitness of individuals that arise from matings between genetic relatives, matings thus that are more likely to occur in small populations. Inbreeding, which seems to have been common in Neanderthals (Prüfer et al. 2014, Ríos et al. 2015, Prüfer et al. 2017, Sikora et al. 2017, might lead to a lower fitness because it increases the chances of the expression of recessive, deleterious traits and because homozygotes often have a general disadvantage relative to heterozygotes. Harris and Nielsen (2016) estimate that, due to inbreeding, Neanderthals had at least 40% lower fitness than modern humans on average. Allee effects refer to the effects that population density has on reproduction and, thus, on population growth (Courchamp et al. 2008). At lower densities, the case we are concerned with here, growth rates might drop due to problems in mate-finding, and to several problems that highly cooperative species, such as Neanderthals, are particularly susceptible to, including low availability of helpers in cooperative hunting, defending kills from kleptoparasites, and allo-parenting (Courchamp et al. 2000). Finally, stochastic, annual fluctuations in births, deaths and sex ratio are more likely to place smaller populations on a trajectory towards extinction than bigger ones (Courchamp et al. 2008).
Based on models drawn from conservation biology, Vaesen et al. show that these three factors might indeed have been sufficient for Neanderthal populations to disappear-and this without competitive interactions with AMHs or sustained/long-term environmental change.
A pertinent distinction here is between competitive and environmental factors that have been claimed to be explanatory of the demise of Neanderthals-long-term environmental change (Tzedakis et al. 2007;Finlayson 2008;Golovanova et al. 2010;Müller et al. 2011) and an inferiority of Neanderthals in competition with AMHs (Marean 2005;Mellars 2005; Shea and Sisk 2010; Wynn and Coolidge 2011)-and those that have not been claimed to be so-e.g., short-term demographic fluctuations, short-term environmental fluctuations, and competition with conspecifics and non-human animals. It is only the former type of explanations that Vaesen et al. claim to offer an alternative to. While they might agree that inbreeding, Allee effects and stochasticity in the end relate to some competitive and environmental factors (inseparability), they can still rightfully maintain, as they do, that inbreeding, Allee effects and stochasticity are separable from the competitive and environmental factors that have often been regarded as explanatory for the demise of Neanderthals.

The sense in which sense Vaesen et al. provide a neutral model
C&M further contend that Vaesen et al. are mistaken in presenting their model as a neutral model. A neutral model is taken to provide a "(1) privileged/preferable explanation (inasmuch it represents a 'null' hypothesis) which is (2) both demographic and neutral." (C&M, p. 50). They dismiss (2) based on their inseparability claim: demographic explanations are not standalone, and always need to smuggle in environmental and competitive factors. Regarding (1), C&M submit that demographic models should not be preferred a priori, e.g., on grounds of simplicity, where simplicity means fewer causal factors. They point out that an explanation in terms of environment or competition alone would be just as simple and, further, that given that Neanderthal extinction is generally assumed to be due to a mixture of factors, it makes no sense to present demographic accounts as an alternative to competitive or environmental ones.
In response to C&M's arguments against (2), and as pointed out above, Vaesen et al.'s explanation is not neutral in a general sense, but neutral viz-à-viz the selective and environmental factors that the literature has proposed to be causally responsible for Neanderthal extinction. And, regarding (1), Vaesen et al.'s model should in fact be privileged as the most simple one, not because it involves fewer causal factors, but because it involves fewer contested causal factors. Indeed, we have seen that climatic accounts have been contested, and the same goes for accounts that trade on the competitive inferiority of Neanderthals. Vaesen et al. start from an assumption that isn't contested (also not by C&M)-viz., the assumption that Neanderthals lived in small disconnected sub-populations (see beginning of previous section)-and examine whether features of such sub-populations (viz., inbreeding, Allee effects and stochasticity) might be enough to drive these sub-populations to extinction.
Note that Vaesen et al. provide a 'how-possibly' explanation (see the 'might' in the title of their article). This implies that they are open to the possibility that, if indeed they were present, competition and sustained environmental change might have quickened the process of extinction. Such factors, for that matter, could be easily incorporated in Vaesen et al.'s models.
Despite their disagreement sketched above, C&M and Vaesen et al. do seem to agree on one, perhaps the most important, point. So let me conclude this short article by emphasizing it. At the end of their paper, Vaesen et al. write that "any plausible explanation of the demise also needs to incorporate demographic factors [viz., inbreeding, Allee effects and stochasticity] as key variables". C&M seem to acknowledge just as much when they state that "demographic factors can (and should!) play a role in explanation" (C&M p. 50). So, in what is arguably the most relevant respect, the divide between C&M and Vaesen et al. might not be so big after all.
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