Neanderthal 'population bottleneck' around 110,000 years ago may have contributed to their extinction

Neanderthals may have been headed toward their demise much earlier than experts previously thought, new research suggests.

In the study, published online Feb. 20 in the journal Nature Communications, researchers discovered that around 110,000 years ago, our closest human relatives experienced a "population bottleneck" that decimated their genetic diversity.

A population bottleneck occurs when there is a sudden reduction in genetic variation within a species. Bottlenecks in a species can be brought on by a number of processes, such as climate change, hunting or genocide. The end result of a bottleneck may be a population that is sickly or on the verge of extinction.

The scientists identified the bottleneck by analyzing changes in the shape of the Neanderthal inner ear over time.

When they analyzed the inner ears of Neanderthal skulls, they discovered that there was an abrupt decline in variation in this bone in skulls dated to the beginning of the Late Pleistocene, signaling a significant change in the Neanderthal skeleton.

Typically, comparisons among ancient DNA samples help researchers pinpoint when bottlenecks happened. But in this case, the team used the reduced variation in the Neanderthals' ear bones as a proxy. They focused on the semicircular canals, a set of bony tubes in the inner ear that are fully formed at birth. During life, these canals are filled with fluid, helping to maintain balance and detect head movements, such as shaking or nodding. Because semicircular canals are evolutionarily "neutral" — because their variation does not affect a person's survival — tracking subtle changes in the canals over time can shed light on the size and diversity of a past population.

Related: Did we kill the Neanderthals? New research may finally answer an age-old question.

Using CT scans, the researchers examined the semicircular canals of 30 Neanderthals from three time periods: 13 from the site of Sima de los Huesos in Spain that were dated to 430,000 years ago, 10 from the site of Krapina in Croatia that were dated to 120,000 years ago, and seven "late" Neanderthals from France, Belgium and Israel that were dated to 64,000 to 40,000 years ago.

This analysis revealed that the group of late Neanderthals had significantly lower variation in their inner ear bones than the earlier groups did, which led the researchers to conclude that a genetic bottleneck event occurred more recently than 120,000 years ago.

"By including fossils from a wide geographical and temporal range, we were able to capture a comprehensive picture of Neanderthal evolution," study co-author Mercedes Conde-Valverde, a biological anthropologist at the University of Alcalá in Spain, said in a statement. The reduction in diversity between early and late Neanderthals "is especially striking and clear, providing strong evidence of a bottleneck event," she said.

The findings fit in well with previous discoveries about Neanderthals, such as evidence of population turnover that negatively affected European Neanderthals' numbers. But it is unclear if the same pattern holds for southwestern Asian Neanderthals, like those who lived at Shanidar in Iraqi Kurdistan, the researchers wrote in their study, since their skulls were not available for analysis.

Neanderthal quiz: How much do you know about our closest relatives?