The Neanderthal population, which thrived across Eurasia for more than 250,000 years, faced a significant drop in genetic diversity approximately 110,000 years ago. Recent research identifies this phenomenon—a genetic bottleneck— as a pivotal moment that drastically diminished their numbers, resulting in reduced variability in both their physical characteristics and genetic makeup.

Although the extinction of Neanderthals around 40,000 years ago is well established, this earlier population decline points to challenges in their survival long before they vanished completely.

Scientists examining both fossil records and DNA data have discovered that this sharp population decrease significantly influenced the trajectory of Neanderthal evolution on Earth.

Evolution Shaped by Survival Challenges

For many years, researchers believed that Neanderthals evolved in a slow and steady manner, maintaining stable populations. However, new studies indicate that their evolutionary history was surprisingly complex and unstable.

Fossil evidence from varied periods shows that early Neanderthals possessed notable genetic and morphological diversity, displaying a wide array of physical traits that likely enhanced their adaptability in different environments throughout Europe and Asia.

However, a significant event took place about 110,000 years ago, resulting in a steep decline in the diversity that had previously characterized Neanderthal populations. This indicates a severe reduction in the number of individuals, potentially including entire groups.

Fossil data suggests that from 430,000 to 120,000 years ago, Neanderthal diversity actually increased before experiencing a substantial collapse. This loss may have had lasting effects on their capacity to adapt to future environmental challenges.

Insights from the Inner Ear

To gain insight into this population decline, scientists investigated the bony labyrinth—a portion of the inner ear involved in balance and movement—found in Neanderthal fossils. This method offers a novel means to examine genetic variation when DNA samples are too degraded for conventional analysis.

The research uncovered a distinct trend: earlier Neanderthals, such as those from Krapina, Croatia (approximately 120,000–130,000 years old), demonstrated significantly more variation in their inner ear structures compared to later, so-called “classic” Neanderthals.

This aligns with genetic data suggesting that a bottleneck event severely impacted their population. Interestingly, these findings also question the notion that Neanderthals began their existence with inherently low genetic diversity.

Fossils from Sima de los Huesos, Spain (approximately 430,000 years old), representing pre-Neanderthal ancestors, exhibited similar levels of diversity to early Neanderthals. This indicates that Neanderthals began their existence as a more genetically diverse species, later experiencing significant loss of that variation.

What Caused the Decline?

The precise reasons behind this genetic bottleneck remain unclear, but various theories have emerged. One potential cause could be climate change. Extreme periods of cold may have severely depleted food supplies, leading to a downturn in Neanderthal populations.

Another theory considers the impact of competition with other hominin species, particularly the Denisovans, who lived in overlapping regions. Although Homo sapiens were not yet prevalent in Eurasia at this time, interactions with other hominin groups might have intensified resource competition.

Some scholars propose that disease outbreaks may have played a crucial role, with sudden epidemics ravaging isolated Neanderthal communities and leaving only a small remnant behind.

Additionally, these findings call into question earlier assumptions that Neanderthals had experienced a prior genetic bottleneck at the origin of their lineage.

Evidence from fossil morphology contradicts this notion, as early Neanderthals exhibited a diversity level comparable to that of their pre-Neanderthal ancestors, suggesting that they initially possessed a greater degree of genetic variation than previously recognized.