In a report published in PLOS One, the researchers said the family moves laterally - unlike primates, who walk in a diagonal sequence, repeatedly putting a hand on one side and a foot on the other.
They claimed the siblings' walk is, in fact, a by-product of a hereditary condition that causes cerebellar hypoplasia, complicating their sense of balance.
To adapt to this condition, the family has developed quadrupedalism, they said. 'I was determined to set the record straight, because these erroneous claims about the nature and cause of the 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,' lead researcher Liza Shapiro, of the University of Texas, told The Washington Post.
'We have shown that the quadrupedalism resembles that of healthy human adults asked to walk quadrupedally in an experimental setting. [They do not] prefer the diagonal sequence/diagonal couplets gait characteristic of nonhuman primates.'
In 2005, British researchers agreed in a separate study that the siblings' walk differs from that of some primates, noting that they put all of their weight on their palms and wrists, not their knuckles.
However, Turkish evolutionary biologist Uner Tan previously claimed that the siblings, who also appear to have brain damage, suffered from a condition called Uner Tan Syndrome.
He declared that people affected by this syndrome walk with quadrupedal locomotion and are afflicted with 'primitive' speech and severe mental retardation, suggesting 'reverse evolution'. 'The idea of reverse evolution was just a flash, an "aha" experience,' Mr. Tan told NeuroQuantology. 'I suddenly realized they were exhibiting the walking style of our ape-like ancestors.
The five siblings, who have 14 other brothers and sisters who are not affected by the condition, spend most of their time sitting outside the family's basic rural home.
However, one brother travels to the local village where he engages in basic interactions with people. The latest research was carried out by Ms. Shapiro, Whitney Cole, Scott Robinson, and Karen Adolph, all of New York University, Jessica Young, of Northeast Ohio Medical University and David Raichlen, of the University of Arizona.
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