Bilkent University researchers claim they have discovered the gene variants that directly influence how humans walk—on their two feet or on all fours.
Two of four Turkish families that garnered world attention in 2006, with members who suffer from "quadrupedal locomotion syndrome" walking on their hands and feet, share a variant of the VLDLR gene, according to the researchers.
Publicity about the Turkish families and their shared traits of moving on all fours caught world attention because the shift from quadrupedal to bipedal walking is seen as one of the key developments in human evolution.
The families that live in a rural area of Turkey have several members walking on both hands and feet all through their lives.
The condition of their strange mobility was also linked to speech problems and mental retardation, and was labeled "Uner Tan Syndrome" after the scientist who first wrote about it.
Researches claim they have found that the mutation is in the VLDLR gene, which controls the levels of proteins needed for the growth of the brain's cerebellum.
Prof Tayfun Ozcelik of Bilkent University said, "We think this protein is critical for the proper development of the nervous system and our unique ability to balance and adopt a bipedal gait."
Prof Ozcelik presented his study at the European Society of Human Genetics conference in Barcelona. The findings could alter our understanding of how and why humans started walking upright.
Critics however are not ready to accept the study as evidence of an "all four gene". They are of the opinion that quadrupedal locomotion syndrome is most likely the result of several different genetic factors.
They point out that two other Turkish families with the condition do not share the same gene variant. Also, tests on Iraqi and Brazilian sufferers of a similar condition have identified different causal genes.
Professor Nicholas Humphrey of the London School of Economics, who has also examined the Turkish families, said that the gene variant did not force people to walk on all fours. He observed that other people with the variant had learned to walk upright.
"We know that other people with defects in this gene often have problems in the development of a part of the brain which means that they have trouble with their sense of balance," Prof Humphrey said.
He suggested social factors were involved in the transition from quadrupedal movement to bipedal movement.
According to him, other people with VLDLR defects in the US had been successfully taught to walk upright, and the principal difference was that the Turkish families had been "tolerant" of their children walking on all fours.
"I do not believe for a moment that what we have here is a gene for walking on all four legs," said Prof Humphrey.
"These family members basically started off crawling on all fours, for this reason, and stayed that way," he added.
However, Professor Ozcelik insisted that some of the families had made strenuous efforts to correct their children's gait.