Neanderthal Evolution Reveals a Surprising Lack of Bottleneck Event
A recent study, conducted by researchers from the Universitat Autònoma de Barcelona and the Universidad de Alcalá, challenges the long-held belief of a significant bottleneck event in Neanderthal evolution.
Neanderthal. Image credit: Trustees of the Natural History Museum, London.
Understanding Neanderthals
Neanderthals are among the best-documented pre-modern humans in the fossil record, offering insights into their morphology, genetics, behavior, and culture. Dr. Alessandro Urciuoli and colleagues highlight the recent molecular-clock-based analyses which place Neanderthals and Denisovans diverging from the modern human lineage approximately 765,000 to 550,000 years ago, or even earlier based on morphological evidence.
The Past and Present of Neanderthal Evolution
While genetic divergence times are now well understood for the broader lineage, scientists continue to debate the relationships among European Middle Pleistocene populations, Middle, and Late Pleistocene Neanderthals, as well as the evolutionary processes leading to a fully developed ‘classic Neanderthal’ morphology.
One significant challenge in this field is understanding the mosaic morphology of Middle Pleistocene specimens, often referred to as the ‘muddle-in-the-middle,’ from which Neanderthals are believed to have evolved.
Researchers Analyze Semicircular Canals
In this new study, researchers focused on the morphological diversity within the structure of the semicircular canals, responsible for our sense of balance. This approach provided a novel perspective on Neanderthal evolution.
The analysis included two exceptional fossil collections: one from the Sima de los Huesos site in Spain (dated to 430,000 years ago), one of the largest samples of pre-Neanderthals, and another from the Krapina site in Croatia (dated to 130,000 to 120,000 years ago).
New Insights from Diverse Samples
By studying a wide range of Neanderthal fossils, researchers were able to map the full spectrum of Neanderthal development. Dr. Mercedes Conde-Valverde from the Universidad de Alcalá emphasized that the reduction in morphological diversity observed between the Krapina sample and classic Neanderthals suggested a possible bottleneck event.
However, Dr. Urciuoli and his team were surprised to find that pre-Neanderthals from Sima de los Huesos shared similar levels of morphological diversity with early Neanderthals from Krapina. This finding contradicted the prevailing hypothesis of a bottleneck event at the beginning of the Neanderthal lineage.
The Implications of This Research
The study’s findings, published in Nature Communications, present a new narrative on Neanderthal evolution. Instead of a dramatic reduction in diversity, these pre-Neanderthals exhibited a consistency in their developmental stages that challenges the existing theories.
These insights are crucial for a deeper understanding of Neanderthal development and their place in human evolutionary history. The excavation and study of diverse fossil samples continue to offer a window into an otherwise enigmatic period of pre-human species interaction.
Read the full study in Nature Communications for more technical details.
A. Urciuoli et al. 2025. Semicircular canals shed light on bottleneck events in the evolution of the Neanderthal clade. Nat Commun 16, 972; doi: 10.1038/s41467-025-56155-8
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