In the fossil deposits of Spain and Croatia lies a truth that has waited millennia to be revealed. An international team of researchers has discovered evidence of a devastating evolutionary bottleneck that hit Neanderthals between 130.000 and 50.000 years ago. The research, which analyzed the inner ear anatomy of dozens of fossils, reveals how this population underwent a drastic reduction in its genetic diversity: an event that may have contributed decisively to their extinction. The discovery challenges established theories and opens new scenarios on the history of our closest evolutionary relatives.
The study of the bottleneck
The analysis was conducted by a team led by Alessandro Urciuoli e Mercedes Conde Valverde, respectively of the Universitat Autònoma de Barcelona and University of Alcalá. The researchers took a special look at the semicircular canals, structures in the inner ear responsible for our sense of balance. The choice of this anatomical part is not accidental: these tiny canals can tell a lot about the morphological diversity of a population.
The research, published on Nature Communications. (I link it to you here), analyzed two exceptional collections of fossils. The first comes from the site Chasm of the bones in Spain, dating back to 430.000 years ago, which represents the largest available sample of pre-Neanderthals. The second collection comes from the site of krapina in Croatia, dated between 130.000 and 120.000 years ago. This comparative approach has allowed us to outline an unprecedented evolutionary picture. And what came out of it?
The precipice of diversity
The results of the comparative study were nothing short of surprising. The morphological diversity of the semicircular canals in classical Neanderthals was significantly lower than that of their predecessors. This finding aligns perfectly with previous paleogenetic studies, suggesting a significant evolutionary bottleneck in the history of this species.
The reduction in diversity observed between the Krapina sample and classical Neanderthals is particularly striking and clear, providing compelling evidence for a bottleneck event.
commented the Dr. Conde-Valverde. This decline in diversity may have made Neanderthals more vulnerable to environmental changes, potentially contributing to their extinction.
Yet another surprise from the past
A particularly interesting aspect of the research concerns the pre-Neanderthals of Sima de los Huesos. These ancient hominids showed a level of morphological diversity similar to that of the early Neanderthals of Krapina, challenging the common idea of an upstream, and not downstream, bottleneck. In other words, at the origin of the Neanderthal evolutionary line. Urciuoli emphasizes the importance of this discovery:
We were surprised to find that the pre-Neanderthals from Sima de los Huesos showed a similar level of morphological diversity to the early Neanderthals from Krapina. This calls into question the common assumption of a bottleneck event at the origin of the Neanderthal lineage. Rather, this bottleneck occurred in a second phase, with somewhat dramatic features.
Evolutionary implications
The research sheds new light on the complex evolutionary history of Neanderthals. The divergence between Neanderthals, Denisovans and the modern human line is now dated to between 765.000 and 550.000 years ago, or even earlier according to morphological data. The Neanderthal It diverged shortly thereafter, as evidenced by genetic and morphological evidence from Middle Pleistocene humans from Sima de los Huesos.
The debate about the relationships between the European populations of the Middle Pleistocene and the Neanderthal populations of the Middle and Late Pleistocene, however, continues. The mosaic morphology of the Middle Pleistocene finds, the so-called “muddle-in-the-Middle”, that phase of overlap and complex interaction between different human species from which Neanderthals are believed to have evolved, remains an open topic of discussion in the scientific community.
Bottleneck, new awareness
The results of the Spanish analysis open new perspectives for the understanding of human evolution. A late bottleneck may have played a crucial role in the fate of Neanderthals, making them potentially more vulnerable to climate change and competition with theHomo sapiens. The research also highlights the importance of studying seemingly minor anatomical features, which can reveal valuable information about our evolutionary history.
The discovery reminds us of how complex and multifaceted human evolution is, and how much we still have to learn from our ancient relatives. The mystery of the Neanderthals continues to unravel, one fossil at a time, revealing a story of adaptation, resilience, and ultimately vulnerability.
