A new study revealed that small populations, inbreeding, and random demographic fluctuations could have been the reasons behind the Neanderthal extinction around 40,000 years ago, according to paleoanthropologists.

The paper was published on Wednesday on the open-access journal PLOS ONE by researchers from Eindhoven University of Technology, the Netherlands. Their findings exonerate humans from causing the Neanderthal’s disappearance, suggesting that “the species” demise might have been due merely to a stroke of bad, demographic luck.”

Neanderthals are extinct species of archaic humans who lived in Eurasia about 430,000 years ago. Scientists divide them into three subcategories: the early Neanderthals who lived between 430,000 and 130,000 years, followed by the classic Neanderthals who lived from 130,000 to 47,000 years, then the late Neanderthals who lived between 47 thousand to 28 thousand years.

Little was known about the genetic diversity of Neanderthals, or the relationship between them and modern humans, for which studies have long reported friction before the eventual disappearance.

The first fossil of a Neanderthal man was discovered in 1856 in the Neander Valley, Germany, and Neanderthals are the sister group of all modern humans. Previous studies have revealed similarities between the human genome and the Neanderthal genome, a move that has led to the identification of genetic traits of current humans.

The Neanderthal’s extinction happened 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.

Scientists remain puzzled by the sudden extinction of Neanderthals, some 40,000 years ago, but Krist Vaesen, associate professor of the philosophy of innovation at the department of Philosophy and Ethics, Eindhoven University of Technology told Daily News Egypt that his research suggests that scientists might have been too quick in attributing the demise of Neanderthals to the invasion of our species, Homo sapiens. 

“Relying on models from conservation biology, we conclude that the downfall of Neanderthals may have been the result of their small population size alone,” he said.

In the new study, the team used data from contemporary 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). 

After that, researchers simulated for their model, the effects of inbreeding Allee effects -which is a phenomenon in biology 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.

Vaesen further explained that the team used a set of mathematical and computer models from conservation biology to represent Neanderthal populations. And then let these populations undergo three processes that small populations are particularly vulnerable to: inbreeding, difficulties in mate-finding, and random fluctuations (in mortality and reproduction). 

“To our own surprise, we found that even if Neanderthals had been fully identical to modern humans, they faced, over time, a considerable risk of extinction, and this merely due to their small population size,” Vaesen added.

This suggests that no invasion of another species, let alone a superior species, was needed for them to vanish from the face of the earth. Neanderthals might simply have been unfortunate to have lived in small numbers.

The population model showed that inbreeding alone was unlikely to have led to extinction which has happened only in the smallest model population. However, reproduction-related Allee effects through which 25% or fewer Neanderthal females gave birth within a given year (as is common in modern 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.

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 modelling for demographic issues alone, the authors believe these models may serve as a “null hypothesis” for future competing theories—including the impact of modern humans on Neanderthals.

“We see the results of our study as a null hypothesis (no relation between the two measured subjects) against which other explanations need to be assessed. Regardless of whether external factors or resource competition played a role in the extinction of Neanderthals, our study suggests that any plausible explanation also needs to incorporate demographic factors as key variables,” said Vaesen.