Five siblings in Turkey are forced to walk on all fours due to a rare physical disorder.

The siblings, first discovered in 2005, also have a vocabulary of just 100 words and communicate with each other in a language that they developed, according to the Documentary, The Family that walks on all fours.

The siblings can travel for miles on all fours, with the skin on the heels of their hands as thick as it is on the soles of their feet.

American researchers have confirmed that the condition, previously thought to be a rare example of 'reverse evolution', it is in fact caused by a weak sense of balance, and has no relation to any primitive genetics.

"I was determined to set the record straight because these erroneous claims about the nature and cause of quadrupedalism have been published over and over again, without any actual analysis of the biomechanics of their gait, and by researchers who are not experts in primate locomotion," said lead researcher Liza Shapiro from the University of Texas in the documentary.

Turkish psychologist Stefany Arabella says the siblings' father worries about their wellbeing after he passes away.

"They are occasionally beaten up by the village children and [are not] socially accepted," said Arabella.