A case study of multiple individuals with a supernumerary vertebra and sacralization in the prehistoric Rio Muerto Tiwanaku cemetery (AD 700-900), Moquegua, Peru

•Rare case of 16 Tiwanaku individuals from Peru with a supernumerary vertebra.•Individuals had lumbosacral transitional vertebrae and sacralization.•Supernumerary vertebra is a rare trait that can be linked to Hox genes.•The number of vertebrae can be inherited, suggesting biological relatedness.•Rio Muerto people may have practiced exogamy with Omo-style subgroups in this area. This paper presents a rare cluster of 16 Tiwanaku colonists (AD 700-1000) buried in the Rio Muerto cemeteries M70B and M43A (Moquegua Valley, Peru) who have a supernumerary sixth lumbar (L6) vertebra in their complete spine. While some nonhuman primates have an extra lumbar vertebra for quadrupedal flexibility, an additional lumbar vertebra in humans is considered a rare trait and associated with a Hox gene mutation. More than half of adolescent and older individuals in the study population were observed to have this trait (M70B: 14 of 26; M43A: 2 of 3). Almost all these people also have full or partial L6 sacralization. At least three other Tiwanaku colonists within the Moquegua Valley have been noted with this trait, as well as one case from the Tiwanaku heartland site of Lukurmata, Bolivia. Finding this many people with this rare condition concentrated in one archaeological site suggests a shared connection, such as a congenital condition. We discuss the distribution of this trait at the Rio Muerto site, and how this may represent a subgroup of a migrant community within the larger Tiwanaku population. We also consider mortuary evidence and previously published strontium indications of migration, potential shared descent, community connections, and studies of intermarriage within this region.