Rio Muerto (Pérou) : Rare Spinal Condition And Possible Inbreeding

Kristina Killgrove

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An extremely rare condition of the spine, in which an extra lumbar vertebra is present, was recently discovered in nearly one-fifth of burials in an ancient Peruvian cemetery. Could this inherited genetic condition be the result of inbreeding?

Research at the Moquegua Valley site of Rio Muerto, a cemetery associated with Tiwanaku and dating to 500-1100 AD, was presented at last week’s American Association of Physical Anthropologists conference by Sara K. Becker of the University of California, Riverside and her colleagues. They found 14 cases of sixth lumbar sacralization – both the presence of an extra bone in the lower back and the fusion of that bone with the sacrum at the back of the pelvis.

While normal human anatomy includes 7 cervical, 12 thoracic, and 5 lumbar vertebrae, it is not uncommon to have an extra bone, to lack a bone somewhere in the spinal column, or to see a slight difference in arrangement of the vertebrae. What usually happens is a vertebral “shift” – such as when the last lower back vertebra becomes “sacralized” and fuses at least partially with the sacrum. Irregular fusions such as these can cause pain once the person reaches adulthood.

Becker 1 1200x725Sixth lumbar sacralization in a middle-aged female from Rio Muerte, Tiwanaku, Peru.

Becker and colleagues note that the presence of a sixth lumbar vertebra is uncommon – under 10% in contemporary humans, and associated with differences in the Hox genes, which control organization along the head-to-tail axis of an organism. To have both the presence of a sixth lumbar vertebra and the sacralization of that vertebra, they write, is so rare as to be nonexistent in the clinical literature except anecdotally.

At the Rio Muerto cemetery, the researchers found 14 individuals with this rare condition. Of those 14, eight were probably male and five were female. One was a subadult whose sex could not be determined. Age-at-death ranged from 12 to 60 years.

Interestingly, Becker and colleagues found that all of these individuals were buried in a similar manner as the rest of the population, “which may mean that people did not realize they had this condition.” On the other hand, if they did know about the condition, “they may have considered this normal.” Since most of the people died in early adulthood, most probably did not suffer from pain associated with sacralization of the vertebra.

Becker 2Sixth lumbar sacralization in a teenaged male from the site of Rio Muerto, Tiwanaku, Peru.
Because of the rarity of this condition, Becker and colleagues conclude that “these high rates of L6 with sacralization indicate a degree of inherited genetic relatedness between these 14 burials.” One of the ways that rare conditions become prominent in a group is, of course, though inbreeding. While Becker and colleagues do not yet know whether inbreeding occurred, they also can’t dismiss that hypothesis.

Perhaps more importantly, this trait may make it easier for archaeologists to investigate patterns of migration in the area. A nearby site has a frequency of two individuals out of 31 with sixth lumbar sacralization, which Becker and colleagues write “could indicate some kind of interrelatedness or intermarriage in the Rio Muerto area, as well as within the wider colonial region.”

The researchers plan to investigate this pattern of genetic relatedness through DNA analysis in the future.