Siblings in Kars, Turkey, with Uner Tan syndrome (quadrupedal locomotion, severe mental retardation, and no speech): a novel theory for the evolution of human bipedalism.

Objective: To investigate siblings from Kars (n  =  2), Turkey, with diagonal-sequence quadrupedal locomotion (QL), severe mental retardation, and no speech (Uner Tan syndrome, UTS), in relation to the evolutionary emergence of human bipedal locomotion (BL). Methods: Video recordings were made to assess gaits. Brain MRI scanning was performed to visualize the cerebro-cerebellar malformations. Genome-wide association analyses were performed in venous blood samples. Results: One of the two men with UTS showed early-onset QL and late-onset BL without infantile hypotonia, the other consistent QL with infantile hypotonia. No homozygosity was found in the genetic analysis. The family lived under extremely poor socioeconomic conditions. Conclusions: Low socioeconomic status may be a triggering factor for the epigenetic emergence of UTS. The neural networks responsible for the ancestral diagonal-sequence QL, evolutionarily preserved since about 400 MYA, may be selected during locomotor development, under the infl...