"Competitive exclusion" and the extinction of Neandertals: should we believe it?

I’ve been out of e-mail range for the past week. In the meantime several people e-mailed me this new paper:

Neanderthal Extinction by Competitive Exclusion
Background: Despite a long history of investigation, considerable debate revolves around whether Neanderthals became extinct because of climate change or competition with anatomically modern humans (AMH).
Methodology/Principal Findings: We apply a new methodology integrating archaeological and chronological data with high-resolution paleoclimatic simulations to define eco-cultural niches associated with Neanderthal and AMH adaptive systems during alternating cold and mild phases of Marine Isotope Stage 3. Our results indicate that Neanderthals and AMH exploited similar niches, and may have continued to do so in the absence of contact.
Conclusions/Significance: The southerly contraction of Neanderthal range in southwestern Europe during Greenland Interstadial 8 was not due to climate change or a change in adaptation, but rather concurrent AMH geographic expansion appears to have produced competition that led to Neanderthal extinction.

OK, so should we believe it?

The authors are out to test the idea that climate killed the Neandertals (also covered by me in 2007, not to mention “The unbearable hotness” from last week).

The authors confine their analysis to a simple question: Were the European ecologies of the later times of Neandertal existence compatible with those that existed slightly earlier? If the climate deterioration is insufficient to explain the range reduction of late Neandertal sites, then we need some other factor. Competition with the non-Neandertal population would be a logical hypothesis, in this event.

Competitive exclusion is not a new concept applied to Neandertals. The novel element in the paper is its inclusion of paleoclimate models to support the hypothesis that Neandertals and modern humans actually would have competed in the same niche.

The authors applied an optimization algorithm to data. The data included:

  1. Paleoclimate predictions for small areal units within Europe for three time periods between 43,000 and 35,000 (calibrated) years ago.

  2. The locations and dates of archaeological sites dating to these periods, whether Mousterian, Châtelperronian, or Aurignacian. The first two are assumed Neandertal, the third modern human.

The algorithm tries to find shared paleoclimate features among the locations represented by archaeological sites. Once these are found, the algorithm finds other areas that fit the same paleoclimate parameters as those where archaeological sites were found. In other words, it is an attempt to determine the total ecological range of the populations represented by the sites.

For example, here are the results for the earliest of the three time periods, H4. This is a comparison of the maps of Europe for both the Mousterian-Châtelperronian (left) and Aurignacian (right) archaeological samples:

H4 paleoclimate predictions for Neandertals and modern humans

In this map, red areas are those predicted to be suitable for habitation by Mousterian-Châtelperronian (left) and Aurigacian (right) populations, respectively. One thing stands out: they have almost identical ecological tolerances. Modern humans were not using different ecological zones than Neandertals. The analysis of the later periods shows that the modern humans were not exploiting climate changes at the expense of Neandertals.

The key graph of the paper shows that during the latest time period, near 35,000 years ago, the paleoclimate models predict a very large area of Europe would have been suitable for Neandertal habitation – at least, if their habitation were constrained only by climate. But the Neandertal sites during that time period are restricted to a very small area. So some additional factor is required. The authors promote the hypothesis that the important factor was the population growth of modern humans.

Now, should we doubt the results? I think this is a good test of the hypothesis that climate change killed the Neandertals. It didn’t. They survived through an entire glacial cycle before 40,000 years ago. Without some other factor, they would still be here today.

The paper is stronger than many that have tried to make a similar argument – that climate couldn’t have killed various extinct megafauna. In large part, that is because both the American megafaunal disappearances and the entry and growth of human populations coincided with a period of rapid climate change. In the time frame of the last Neandertals, there were important climate changes, but the paleoclimate models indicate that these changes weren’t enough to make Europe uninhabitable for either humans or Neandertals.

The paper is not a test of Neandertal genetic extinction. It takes Neandertal population disappearance as a given. Models that involve gene flow or cultural exchanges between Neandertals and other populations are not part of this paper’s scheme.

In this sense, the assumption that the archaeological industries can be analyzed with methods developed for species seems questionable. The paper acknowledges this issue:

Our assumption is that human adaptive systems, defined here as the range of technological and settlement systems shared and transmitted by a culturally cohesive population within a specific paleoenvironmental framework, can be considered to operate as a species with respect to their interaction with the environment. This does not imply, however, that human adaptive systems necessarily remained stable over time, as might be the case with animal species occupying narrow and stable niches. Humans can change their adaptive systems rapidly through technical and social innovations in response to environmental change. We know, however, that this was not the case during the late Middle and Upper Paleolithic, periods during which specific human adaptive systems spanned a number of climatic events. Thus, the method described in this study is particularly relevant for addressing issues of human adaptive system stability and eco-cultural niche stability (Banks et al. 2008:2).

But Neandertals during the span from 43,000 to 35,000 years ago were adopting various Upper Paleolithic technological elements. That seems to contradict the assumption that the “technological and settlement systems…operate as a ‘species’.” Instead, it seems to indicate that these systems changed significantly across the time frame modeled in the paper. I don’t think that observation weakens the hypothesis that modern and Neandertal populations may have competed in the same ecological niche. If the Neandertals were using the same technical elements, it probably reinforces the hypothesis of competition.

But I think it is important to bear in mind what the paper tested. The analysis rejects the hypothesis that climate was sufficient to drive a range restriction of Mousterian and Châtelperronian. It doesn’t provide additional information about the mode of such restrictions. With that in mind, I admire the paper and see some useful additional work that might be tackled with similar methods.

References:

Banks WE, dErrico F, Peterson AT, Kageyama M, Sima A, et al. (2008) Neanderthal Extinction by Competitive Exclusion. PLoS ONE 3(12): e3972. doi:10.1371/journal.pone.0003972