The Grizon Ulas family is a Kurdish family of 19 from rural southern Turkey, five (except for another, who had died) of whom walk on all fours with their feet and the palms of their hands in what is called a "bear crawl". Their quadrupedal gait has never been reported in anatomically intact adult humans. The gait is different from the knuckle-walking quadrupedal gait of apes. In 2006, the family was the subject of a documentary: The Family That Walks On All Fours.
The affected siblings have a form of non-progressive congenital cerebellar ataxia. The brain impairments include cerebellar hypoplasia, mild cerebral cortex atrophy and a reduced corpus callosum. They are also mildly mentally retarded and have problems in balancing on two legs. However, they do not show the poor coordination of hands, speech, and eye movements often found in cerebellar ataxia. The four sisters can do needlework. They all share a recessive mutation on chromosome 17p.
Uner Tan of Cukurova University Medical School in Adana, Turkey, has claimed that they show characteristics of the primate ancestors of Homo sapiens, before the move to bipedalism. He calls the process "backward evolution" and he named the condition Uner Tan syndrome.
However, Nicholas Humphrey, John Skoyles, and Roger Keynes have argued that their gait is due to two rare phenomena coming together. First, instead of initially crawling as infants on their knees, they started off learning to move around with a “bear crawl” on their feet. Second, due to their congenital brain impairment, they found balancing on two legs difficult. Because of this, their motor development was channeled into turning their bear crawl into a substitute for bipedality.
- Türkmen S, Demirhan O, Hoffmann K, et al. (May 2006). "Cerebellar hypoplasia and quadrupedal locomotion in humans as a recessive trait mapping to chromosome 17p". J. Med. Genet. 43 (5): 461–4. doi:10.1136/jmg.2005.040030. PMC 2564522. PMID 16371500.
- Tan U (March 2006). "A new syndrome with quadrupedal gait, primitive speech, and severe mental retardation as a live model for human evolution". Int. J. Neurosci. 116 (3): 361–9. doi:10.1080/00207450500455330. PMID 16484061.
- Humphrey N., Keynes R., Skoyles J.R. (2005). "Hand-walkers : five siblings who never stood up" (PDF). Discussion Paper. London, UK: Centre for Philosophy of Natural and Social Science.