Tag: PLOS ONE

Inbreeding, Small Populations, and Demographic Fluctuations Alone Could Have Led to Neanderthal Extinction

Credit: Petr Kratochvil (CC0)
Neanderthal man

Neanderthal extinction could have occurred without environmental pressure or competition with modern humans

Small populations, inbreeding, and random demographic fluctuations could have been enough to cause Neanderthal extinction, according to a study published November 27, 2019 in the open-access journal PLOS ONE by Krist Vaesen from Eindhoven University of Technology, the Netherlands, and colleagues.

Paleoanthropologists agree that Neanderthals disappeared around 40,000 years ago—about the same time that anatomically modern humans began migrating into the Near East and Europe. However, the role modern humans played in Neanderthal extinction is disputed. In this study, the authors used population modelling to explore whether Neanderthal populations could have vanished without external factors such as competition from modern humans.

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Using data from extant hunter-gatherer populations as parameters, the authors developed population models for simulated Neanderthal populations of various initial sizes (50, 100, 500, 1,000, or 5,000 individuals). They then simulated for their model populations the effects of inbreeding, Allee effects (where reduced population size negatively impacts individuals’ fitness), and annual random demographic fluctuations in births, deaths, and the sex ratio, to see if these factors could bring about an extinction event over a 10,000-year period.

The population models show that inbreeding alone was unlikely to have led to extinction (this only occurred in the smallest model population). However, reproduction-related Allee effects where 25 percent or fewer Neanderthal females gave birth within a given year (as is common in extant hunter-gatherers) could have caused extinction in populations of up to 1,000 individuals. In conjunction with demographic fluctuations, Allee effects plus inbreeding could have caused extinction across all population sizes modelled within the 10,000 years allotted.

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The population models are limited by their parameters, which are based on modern human hunter-gatherers and exclude the impact of the Allee effect on survival rates. It’s also possible that modern humans could have impacted Neanderthal populations in ways which reinforced inbreeding and Allee effects, but are not reflected in the models. 

However, by showing demographic issues alone could have led to Neanderthal extinction, the authors note these models may serve as a “null hypothesis” for future competing theories—including the impact of modern humans on Neanderthals. 

The authors add: “Did Neanderthals disappear because of us? No, this study suggests. The species’ demise might have been due merely to a stroke of bad, demographic luck.” 

Ancient Egyptians

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In ancient Egypt, Sacred Ibises were collected from their natural habitats to be ritually sacrificed, according to a study released November 13, 2019 in the open-access journal PLOS ONE by Sally Wasef of Griffith University, Australia and colleagues.

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Egyptian catacombs are famously filled with the mummified bodies of Sacred Ibises. Between around 664BC and 250AD, it was common practice for the birds to be sacrificed, or much more rarely worshipped in ritual service to the god Thoth, and subsequently mummified. In ancient sites across Egypt, these mummified birds are stacked floor to ceiling along kilometers of catacombs, totaling many millions of birds. But how the Egyptians got access to so many birds has been a mystery; some ancient texts indicate that long-term farming and domestication may have been employed.

In this study, Wasef and colleagues collected DNA from 40 mummified Sacred Ibis specimens from six Egyptian catacombs dating to around 2500 years ago and 26 modern specimens from across Africa. 14 of the mummies and all of the modern specimens yielded complete mitochondrial genome sequences. These data allowed the researchers to compare genetic diversity between wild populations and the sacrificed collections.

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If the birds were being domesticated and farmed, the expected result would be low genetic diversity due to interbreeding of restricted populations, but in contrast, this study found that the genetic diversity of mummified Ibises within and between catacombs was similar to that of modern wild populations. This suggests that the birds were not the result of centralized farming, but instead short-term taming. The authors suggest the birds were likely tended in their natural habitats or perhaps farmed only in the times of year they were needed for sacrifice.

The authors add: “We report the first complete ancient genomes of the Egyptian Sacred Ibis mummies, showing that priests sustained short-term taming of the wild Sacred Ibis in local lakes or wetlands contrary to centralised industrial scale farming of sacrificial birds.”

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Citation: Wasef S, Subramanian S, O’Rorke R, Huynen L, El-Marghani S, Curtis C, et al. (2019) Mitogenomic diversity in Sacred Ibis Mummies sheds light on early Egyptian practices. PLoS ONE 14(11): e0223964. https://doi.org/10.1371/journal.pone.0223964

Funding: Human Frontier Science is acknowledged for financial support in the form of a grant to DL, SI, BH, and EW(RGP0036/2011). SW thanks Griffith University for a PhD scholarship. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing Interests: The authors have declared that no competing interests exist.