An investigation based on the analysis of bone remains found in the El Sidrón cave (Asturias), directed by the Higher Council for Scientific Research (CSIC) and published in the journal PLOS ONE, highlights that the Homo neanderthalensis of the Cantabrian coast lived in small groups, relatively isolated and with little genetic diversity.
According to Antonio Rosas, director of the study and researcher at the CSIC at the National Museum of Natural Sciences (MNCN), the high incidence of congenital anomalies in the first cervical vertebra, called atlas, together with the retention of milk canines, may be the result of inbreeding in the Neanderthal community of 13 individuals that lived in that cave. Among the most frequent anomalies of the atlas, which forms a ring of bones on which the skull rests, is the so-called dehiscence or lack of closure of the posterior arch in the mid-sagittal line.
In contrast, among current human populations, this type of incidence barely appears in 1% of individuals . Luis Ríos, another MNCN scientist who participated in the study, states that in clinical practice this defect in the atlas is only discovered by chance in most cases thanks to diagnostic imaging tests (x-rays, axial tomography) that are performed when there is trauma to the cervical region.
On the other hand, one of the atlases from El Sidrón presents this defect and another second atlas is affected by a dehiscence of the anterior arch also in the mid-sagittal line. Since it is a defect that in current human populations does not usually exceed 0.1%, how do you explain that it occurs twice congenitally in such a small group of Neanderthals? For Antonio Rosas, there are three possible explanations. The first is that this congenital anomaly has a genetic basis and that since the frequency is so high, these two Neanderthals were related, which coincides with the genetic studies carried out in El Sidrón, which point to consanguinity relationships.
The second explanation, consequent to the first, points to inbreeding: the practice of marrying or mating people of common descent or natives of a small town or region. The demographic scenario of the Pleistocene, based on small and dispersed groups, would obviously favor pairing within the same group and in the long term, within the same family.
Over time, inbreeding can generate an increase in the frequency of variants and congenital anomalies of the skeleton (tooth transposition, vertebral defects such as atlas dehiscences, cervical ribs). In human populations on the island or located in isolated areas, a higher frequency of these anomalies has been observed. On the other hand, the most recent genetic studies carried out in Neanderthals both in El Sidrón, as in Vindija (Croatia) and Siberia (Russia), indicate a tendency to a frequent pairing with closely related individuals, and congenital anomalies of the atlas in El Sidrón could be interpreted as another sign, in this case skeletal, of inbreeding in the Neanderthals of this geographical area.
In the long run, the demographic panorama of Neanderthal groups clarified in recent years allows us to explain the numerical decline of their populations, which could finally lead to their definitive extinction, based on biology.
