Human Quadrupeds Are Not Examples of Devolution

Photographer Eadweard Muybridge’s pictures of boy walking on all fours / U.S. National Library of Medicine CC BY-NC 3.0
Janet Fang 23 Jul 2014, 22:04

In its most extreme form, Uner Tan Syndrome is characterized by loss of balance and coordination, impaired cognitive abilities, and a habitual quadrupedal gait on all fours. Since its discovery, some have tried to interpret this quadrupedalism from an evolutionary perspective, suggesting that it’s an expression of our primate ancestry -- an instance of evolutionary reversal to a primitive state, or “devolution.”

Debunking that notion is a new analysis published in PLoS One last week. Quadrupedalism in healthy adults or those with a physical disability, they found, can be explained using biomechanical principles rather than evolutionary assumptions. 

In 2005, evolutionary biologist Uner Tan described a Turkish family with 19 siblings, five of whom exhibited impaired cognitive abilities, dysarthric speech, cerebellar hypoplasia, and habitual quadrupedal gait on hands and feet. Tan interpreted these symptoms as examples of human devolution, stating how they “may provide us with some important clues about the transition from quadrupedality to bipedality.” A mutation in a receptor gene is said to be the cause of some of the symptoms, though these reported genetic associations appear to only explain part of the story. 

To support their devolution claim, Tan and colleagues said that people with UTS use “primate-like” diagonal sequence quadrupedalism, a type of gait that distinguishes primates from most other mammals. The next time you walk, notice how your right arm swings forward when you take a step with your left foot. Then look at your cat or dog: Their right forelimb and leg move forward together, a gait known as lateral sequence. Here’s a comparison of the footfall sequence of a baboon and a cat:

Recent research has shown how those with UTS walk differently from non-human primates: They put their weight on their wrists, not their knuckles. Now, a team led by Liza Shapiro from the University of Texas at Austin analyzed 518 quadrupedal strides from footage of the Turkish family members with UTS (available through the documentary “The Family that Walks on All Fours”) as well as video sequences from other families. 

They found that individuals with UTS almost exclusively used lateral sequence -- not diagonal sequence -- quadrupedal gaits. Diagonal sequence was only very occasionally used, and it wasn’t the preferred gait. 

Additionally, the team found that the quadrupedalism exhibited by people with UTS resembles that of other adults asked to walk quadrupedally in an experimental setting. However, unlike the volunteers in these experiments, those with UTS can move quite effectively on all fours. 

Their walk is a byproduct of a hereditary and neurological condition that complicates their sense of balance. And to adapt, they have developed quadrupedalism. “I was determined to publish this and set the record straight, because these erroneous claims about the nature and cause of the quadrupedalism in these individuals have been published over and over again,” Shapiro tells The Washington Post, “without any actual analysis of the biomechanics of their gait, and by researchers who are not experts in primate locomotion. 

Images: U.S. National Library of Medicine CC BY-NC 3.0 (top) & 2014 Shapiro et al CC BY 4.0 (middle)

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