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Uner Tan syndrome, Unertan syndrome or UTS is a syndrome proposed by the Turkish evolutionary biologist Üner Tan. According to Tan, persons affected by this syndrome walk with a quadrupedal locomotion and are afflicted with "primitive" speech and severe mental retardation. Tan postulated that this is an example of "reverse evolution".[1] The proposed syndrome was featured in the 2006 BBC2 documentary The Family That Walks On All Fours.



The Ulaş family of nineteen from rural southern Turkey has been the primary example of the proposed syndrome. Tan described five members as walking with a quadrupedal gait using their feet and the palms of their hands. In infants, where this is a rare but a normal stage prior and sometimes following bipedal walking, such a gait is called "bear crawl". The affected family members are also severely mentally retarded and their speech is affected. Tan proposed that these are symptoms of Uner Tan syndrome.[1]

In January 2008, Tan reported on another family (four males and two females) located in southern Turkey.[2]

Four other unrelated cases in families are described as having various degrees of UTS.[2] Males may be affected more than females. It is also claimed that some individuals are unaware of time, lack language, have severe mental retardation with no conscious experience, and communicate by using sounds. Two males are unable to stand up, while in other cases, can stand up but cannot make a step when standing. Less severe cases use toe walking, which is a normal phase in child gait development.


Neuroscientist and evolutionary psychologist Roger Keynes, psychologist Nicholas Humphrey and medical scientist John Skoyles have argued that the gait of these individuals is due to two rare phenomena coming together, not atavism.[3] First, instead of initially crawling as infants on their knees, they started off learning to move around with a "bear crawl" on their feet.[3] Second, due to their congenital brain impairment, they found balancing on two legs difficult.[3] Because of this, their motor development was channeled into turning their bear crawl into a substitute for bipedalism.[3]

Uner Tan and colleagues claim that UTS differs from disequilibrium syndrome [2]

It's terribly easy to be led away by some notion of living fossils...I'm not going to make any bones about this. I think that Professor Tan's description of this family as a "devolution," as an evolutionary throwback, is not only scientifically irresponsible, but is deeply insulting to this family.

See alsoEdit


  1. ^ a b Tan, Üner (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. 
  2. ^ a b c Tan U, Karaca S, Tan M, et al. (January 2008). "Unertan syndrome: a case series demonstrating human devolution". Int. J. Neurosci. 118 (1): 1–25. doi:10.1080/00207450701667857. PMID 18041603. 
  3. ^ a b c d 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. 

Further readingEdit

  • Tan, U. (2010). Uner Tan Syndrome: History, Clinical Evaluations, Genetics, and the Dynamics of Human Quadrupedalism. The Open Neurology Journal, 4, 78-89 doi:10.2174/1874205X01004010078.
  • Tan, U. (2012). Development of bipedal and quadrupedal locomotion in humans from a dynamical systems perspective. In: Human Development; Different Perspectives, Ed. by Maria Lucia Seidl-De-Moura. InTech Publications, Croatia, pp. 43-62.
  • Tan, U., Tamam, Y., Karaca, S., Tan, M. (2012). Uner Tan syndrome: review and emergence of human quadrupedalism in self-organization, attractors and evolutionary perspectives. In: Latest Findings in Intellectual and Developmental Disabilities Research. Uner Tan (Ed.). InTech Publications, Croatia, pp. 1-44.