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Five siblings in the family, who live in a remote corner of Turkey, walk exclusively on their hands and feet. Since they were discovered in 2005, scientists have debated the nature of their disability, with speculation they represent a backward stage of evolution. Shapiro’s study, published online this month in PLOS One, shows that contrary to previous claims, people with the family members’ condition, called Uner Tan Syndrome (UTS), do not walk in the diagonal pattern characteristic of nonhuman primates such as apes and monkeys.
Barcelona, Spain: What are the genes implicated in upright walking of humans? The discovery of four families in which some members only walk on all fours (quadrupedality) may help us understand how humans, unlike other primates, are able to walk for long periods on only two legs, a scientist will tell the annual conference of the European Society of Human Genetics.
Mutations causing VLDLR deficiency are also found in Hutterites, a group of Anabaptists who live in colonies of North America. There, however, most of the affected individuals cannot walk at all. The neurological characteristics of the affected members of the Turkish families and the Hutterites seem similar, with the most striking difference being that the Turkish individuals are able to walk on all fours, said the scientists. They hypothesize that the Hutterites may be more profoundly affected due to the deficiency in VLDLR and a neighbouring gene, and therefore lack the motor skills even for quadrupedal locomotion.
Although the families lived in isolated villages 200-300 km apart and reported no ancestral relationships, the scientists expected to find a single genetic mutation implicated in the condition. They were surprised to find that this was not the case.
“We carried out genome-wide screening on these families”, said Professor Ozcelik, “and found regions of DNA that were shared by all those family members who walk on all fours. However, we were surprised to find that genes on three different chromosomes are responsible for the condition in four different families.
“In families A and D there were mutations in VLDLR on chromosome 9, and in family B the phenotype maps to chromosome 17 to a region that contains at least 157 genes, and we are still looking for the precise mutation. Neither region appears to be implicated for family C.”
In all cases, the affected individuals were the offspring of consanguineous marriages, which suggests that if they had married outside the family they would not have had the condition. All of them had significant developmental delay in infancy. “Whereas normal infants make the transition to walking on two legs in a relatively short period”, said Professor Ozcelik, “these individuals continued to move on their palms and feet and never walked upright. Although they can stand from a sitting position and maintain this upright position with flexed hips and knees, they virtually never initiate bipedal walking on their own.”
All the families do share genetic factors in common, and Ozcelik’s team says similar genetic factors are found in a few North American families whose members cannot walk at all.
According to a theory developed by Uner Tan of Cukurova University in Turkey, people with UTS are a human model for reverse evolution, or “devolution,” offering new insights into the human transition from four-legged to two-legged walking. Previous research countering this view has proposed that the quadrupedalism associated with UTS is simply an adaptive response to the impaired ability to walk bipedally in individuals with a genetic mutation, but this is the first study that disproves claims that this form of walking resembles that of nonhuman primates.
The study’s co-authors are Jesse Young of Northeast Ohio Medical University; David Raichlen of the University of Arizona; and Whitney Cole, Scott Robinson and Karen Adolph of New York University.
As part of the study, the researchers analyzed 518 quadrupedal walking strides from several videos of people with various forms of UTS, including footage from the BBC2 documentary of the five Turkish siblings, “The Family That Walks on All Fours.” They compared these walking strides to previous studies of the walking patterns of healthy adults who were asked to move around a laboratory on all fours.
According to the findings, nearly all human subjects (in 98 percent of the total strides) walked in lateral sequences, meaning they placed a foot down and then a hand on the same side and then moved in the same sequence on the other side. Apes and other nonhuman primates, however, walk in a diagonal sequence, in which they put down a foot on one side and then a hand on the other side, continuing that pattern as they move along.
“Although it’s unusual that humans with UTS habitually walk on four limbs, this form of quadrupedalism resembles that of healthy adults and is thus not at all unexpected,” Shapiro says. “As we have shown, quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions.”
The study also shows that Tan and his colleagues appeared to have misidentified the walking patterns among people with UTS as primate-like by confusing diagonal sequence with diagonal couplets. Sequence refers to the order in which the limbs touch the ground, while couplets (independent of sequence) indicate the timing of movement between pairs of limbs. People with UTS more frequently use diagonal couplets than lateral couplets, but the sequence associated with the couplets is almost exclusively lateral.
“Each type of couplet has biomechanical advantages, with lateral couplets serving to avoid limb interference, and diagonal couplets providing stability,” Shapiro says. “The use of diagonal couplets in adult humans walking quadrupedally can thus be explained on the basis of biomechanical considerations, not reverse evolution.”
They’re living, breathing men and women, but they walk on all fours, just as we did four million years ago. And until this film was shot, they were hidden away, unseen by the outside world.