A study published in Nature Communications describes evolutionary changes in Neanderthal morphology through the analysis of their inner ear structures, known as the bony labyrinth. The research, led by paleontologists Alessandro Urciuoli from Universitat Autònoma de Barcelona and Mercedes Conde-Valverde from Universidad de Alcalá, provides insights into the population dynamics and genetic diversity of Neanderthals.
Urciuoli and his team investigated the evolution of the semicircular canals in the Neanderthal clade using diffeomorphometry (DMorph), an approach that allows direct comparison of continuous surfaces and is sensitive to changes in canal volumetric proportions. "We were surprised to find that the pre-Neanderthals from the Sima de los Huesos exhibited a level of morphological diversity similar to that of the early Neanderthals from Krapina," said Urciuoli, the lead author of the study, according to Phys.org.
The study focused on two collections of fossil humans: one from the Sima de los Huesos site in Spain, dated to 430,000 years ago, and another from the Croatian site of Krapina, dated to approximately 130,000 to 120,000 years ago. By analyzing these fossils, the researchers measured the morphological disparity in the semicircular canal and vestibule of Neanderthal clade groups, using it as a proxy for their underlying genetic diversity. They compared this data with a sample of classic Neanderthals of different ages and geographical origins.
"The reduction in diversity observed between the Krapina sample and classic Neanderthals is especially striking and clear, providing strong evidence of a bottleneck event," said Conde-Valverde, a paleontologist from Universidad de Alcalá and co-author of the study. The findings reveal that the morphological diversity of the semicircular canals of classic Neanderthals is lower than that of pre-Neanderthals and early Neanderthals. This aligns with previous paleogenetic results, suggesting that Neanderthals underwent a loss of genetic diversity during their evolution.
Late Neanderthals exhibited less variation than modern humans, both genetically and morphologically. Studies have shown that late Neanderthals were a fairly uniform group by the beginning of the Late Pleistocene. This reduced variation might be the result of a bottleneck event—a decline in population size that leads to a loss of genetic diversity.
"This challenges the common assumption of a bottleneck event at the origin of the Neanderthal lineage," stated Urciuoli, according to Phys.org. The research suggests that the bottleneck occurred later in Neanderthal evolution than previously believed, possibly leading to the derived morphology of Late Pleistocene Neanderthals. Drastic climatic changes likely had profound impacts on the genetic and morphological variability of the Neanderthal lineage. Several studies have shown that Neanderthals had a small effective population size, suggesting a highly fragmented metapopulation.
In the absence of ancient DNA evidence for this period, the morphology of phylogenetically informative anatomical structures, such as the bony labyrinth, is the best source of data to investigate the evolutionary history of Pleistocene humans. The bony labyrinth is housed in the petrosal portion of the temporal bone and comprises the semicircular canals, vestibule, and cochlea. Petrosals are abundant in the fossil record and are often well-preserved, even in fragmentary skeletal remains.
"The development of the inner ear structures is known to be under very tight genetic control, since they are fully formed at the time of birth. 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. The present study represents a novel approach to estimating genetic diversity within Neandertal populations," explained Rolf Quam, a Binghamton University anthropologist who participated in the study, according to Gizmodo.
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