Family That Walks On All Fours Not A Product Of 'Reverse Evolution'

In Turkey, there's a family with an apparent genetic disorder that causes them to walk on all fours. Scientists have speculated that they're an example of "devolution" — a backwards step towards our quadrupedal past. A new paper challenges this assumption, offering a far more reasonable explanation.

This condition came to light in a 2005 BBC documentary titled "The Family That Walks On All Fours." The film explored the story of five individuals, four females and one male, of the Ulas family who were exhibiting a previously unreported quadruped walking style. The condition has since been documented in other Turkish families, leading to a proposed genetic condition known as Üner Tan Syndrome (named after its discoverer, a Turkish evolutionary biologist).

In addition to the curious walking style, the disorder is characterized by a number of other conditions, including cerebellar hypoplasia — resulting in a loss of balance and coordination — and impaired cognitive abilities.

Some scientists have interpreted the habitual use of the quadrupedal gait through an evolutionary lens, suggesting that it's an ancient expression of our quadrupedal primate past, or "devolution." The condition, argues Tan, is a repressed vestigial trait that's lurking beneath the genetic surface — not unlike a tail that appears in some babies.

Not A Primate-Like Walk

But a new study by Liza Shapiro and her colleagues claims this is nonsense, and that in reality it's merely an adaptation to the physical limitations wrought by the genetic disorder.

To prove it, Shapiro compared the gaits of people afflicted with Uner Tan Syndrome to the walking styles of nonhuman quadrupeds, including baboons and cats.

Family That Walks On All Fours Not A Product Of 'Reverse Evolution'

Credit: Shapiro et. al./PLOS.

After observing over 500 quadrupedal strides from video sequences of individuals with UTS, the researchers found that these humans used lateral sequence — and not diagonal sequence — quadrupedal gaits. Primates walk in a diagonal sequence in which they put a hand on one side and a foot on the other, repeating this pattern as they move forward. A lateral sequence, on the other hand, is not primate-like at all.

The researchers conclude in their study:

In fact, the quadrupedalism exhibited by individuals with UTS resembles that of healthy adult humans asked to walk quadrupedally in an experimental setting. We conclude that quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions.

Thank you, science, for once again showing that the simplest explanation is often the best one.

Read the entire study at PLoS One: "Human Quadrupeds, Primate Quadrupedalism, and Uner Tan Syndrome".

Image: BBC.