Neanderthals, our closest extinct relatives, experienced a significant decline in genetic diversity around 110,000 years ago, a precursor to their eventual disappearance roughly 70,000 years later. This “bottleneck” event, previously suggested by DNA studies, has been further corroborated by a novel approach: analyzing the morphology of Neanderthal inner ears.
A recent study published in Nature Communications explored this phenomenon by examining the semicircular canals, tiny tubes within the inner ear responsible for balance. Led by paleontologist Alessandro Urciuoli from the Universitat Autònoma de Barcelona, the international research team compared these structures across different Neanderthal groups, spanning various time periods. Their findings confirm a substantial loss of genetic diversity, reinforcing the bottleneck theory.
Inner Ear Structure as a Genetic Proxy
The inner ear’s structure, particularly the semicircular canals, is fully formed at birth and governed by specific genes. This makes variations in these canals a valuable tool for studying evolutionary relationships and genetic differences between fossil specimens. According to Binghamton University anthropologist Rolf Quam, a co-author of the study, “The development of the inner ear structures is known to be under very tight genetic control…This makes variation in the semicircular canals an ideal proxy for studying evolutionary relationships between species in the past since any differences between fossil specimens reflect underlying genetic differences.” This novel approach offers a unique window into the genetic history of Neanderthals.
Comparing Neanderthal Groups
The researchers analyzed three distinct Neanderthal groups: pre-Neanderthals (Atapuerca fossils from Spain, approximately 400,000 years old), early Neanderthals (Krapina fossils from Croatia, around 130,000 years old), and classic Neanderthals from various regions and time periods.
The comparison revealed a marked decrease in morphological diversity of the semicircular canals in classic Neanderthals compared to their predecessors. This aligns with previous DNA analyses indicating a significant loss of genetic variation between early and classic Neanderthals. Co-author Mercedes Conde-Valverde, a paleontologist from the Universidad de Alcalá, noted, “By including fossils from a wide geographical and temporal range, we were able to capture a comprehensive picture of Neanderthal evolution. The reduction in diversity observed between the Krapina sample and classic Neanderthals is especially striking and clear, providing strong evidence of a bottleneck event.”
Unexpected Findings and Future Research
Interestingly, the study also yielded an unexpected result. While a loss of diversity was expected between pre-Neanderthals and early Neanderthals, the researchers found similar levels of morphological variation in both groups. This challenges the prevailing assumption of a bottleneck event at the very beginning of the Neanderthal lineage and warrants further investigation into early Neanderthal demographics and the potential impact of their divergence from our ancestors.
The Bottleneck’s Impact on Neanderthal Extinction
Neanderthals went extinct around 40,000 years ago, and the reasons remain complex. Interbreeding with Homo sapiens played a role, but the reduced genetic diversity and smaller population sizes likely made them more vulnerable to environmental pressures and less adaptable to change. While the exact cause of the bottleneck 110,000 years ago remains unclear, it likely contributed to their eventual demise.
Conclusion
This new research provides valuable insights into the demographic history of Neanderthals and confirms the occurrence of a significant genetic bottleneck. While not the sole cause of their extinction, this event likely weakened their resilience and played a role in their disappearance. The study also highlights the usefulness of inner ear morphology as a tool for understanding the genetic history of extinct hominins, opening up new avenues for future research.
