Comparison of footfall sequence in primate (baboon, above) and nonprimate (cat, below). Footfall sequence is depicted numerically, beginning with the right hind limb in each animal. The primate is walking in diagonal sequence (RH-LF-LH-RF), and the nonprimate is walking in lateral sequence (RH-RF-LH-LF). Image from Muybridge E (1887) . Animal Locomotion: An Electro-photographic Investigation of Consecutive Phases of Animal Movements, 1872-1885: 112 Plates. Published under the auspices of the University of Pennsylvania.

The family that walks on all-fours does not constitute reverse evolution

bbc_walk_on_all_four

In 2006 , the BBC aired a fascinating documentary that featured that featured a family of five siblings from a remote corner of Turkey that remarkably solely moved about by walking on all fours. Many anthropologists of the time saw this behavior as evidence of reverse evolution and sought to extensively study the phenomenon in order to gain insights on how human bipedal locomotion can to be. Researchers from the US, however, claim they have proof that the family that walks on all-fours, as they’re commonly referred to in literature, simply adapted to an unfortunate neurological syndrome and their behavior does not constitute backward evolution.

Here’s the documentary:

At the time of their discovery, Uner Tan of Cukurova University asserted that the family members’ condition,  called Uner Tan Syndrome (UTS), acts like a model for reverse evolution, mimicking the way non-human primates, like monkeys or apes, move about. This view, while it gained considerable traction, has been repeatedly countered by studies which found people with UTS simply adapted to the impaired ability to walk bipedally.

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The team, comprised of researchers at Northeast Ohio Medical University, University of Arizona, New York University, carefully analyzed footage of 518 quadrupedal walking strides and compared them to walking patterns of healthy adults who were asked to move around a laboratory on all fours. More than 98% of participants, whether they were asked to walk on all fours or were forced to because of UTS, walked in lateral sequences – they placed a foot down, then the corresponding hand from the same side, before performing the same sequence on the other side. Non-human primates, however, use what’s called a diagonal quadruped 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,” says  Liza Shapiro, an anthropologist at The University of Texas at Austin. “As we have shown, quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions.”

Comparison of footfall sequence in primate (baboon, above) and nonprimate (cat, below). Footfall sequence is depicted numerically, beginning with the right hind limb in each animal. The primate is walking in diagonal sequence (RH-LF-LH-RF), and the nonprimate is walking in lateral sequence (RH-RF-LH-LF). Image from Muybridge E (1887) . Animal Locomotion: An Electro-photographic Investigation of Consecutive Phases of Animal Movements, 1872-1885: 112 Plates. Published under the auspices of the University of Pennsylvania.

Comparison of footfall sequence in primate (baboon, above) and nonprimate (cat, below). Footfall sequence is depicted numerically, beginning with the right hind limb in each animal. The primate is walking in diagonal sequence (RH-LF-LH-RF), and the nonprimate is walking in lateral sequence (RH-RF-LH-LF). Image from Muybridge E (1887) . Animal Locomotion: An Electro-photographic Investigation of Consecutive Phases of Animal Movements, 1872-1885: 112 Plates. Published under the auspices of the University of Pennsylvania.

The researchers believe initial research in favor of reverse evolution was misguided 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.”

Findings were reported in the journal PLOS ONE.

One thought on “The family that walks on all-fours does not constitute reverse evolution

  1. Üner Tan

    Please see my recent article on this topic:

    Tan U. Quadrupedal Locomotor Characteristics of Uner Tan Syndrome Cases, Healthy Humans, and Nonhuman Primates in Evolutionary Perspectives. WebmedCentral NEUROSCIENCES 2015;6(12):WMC005032

    Abstract

    Introduction: Uner Tan syndrome (UTS) consists of quadrupedal locomotion (QL), impaired intelligence, and dysarthric or no speech. Previously, I described the walk of cases with UTS as diagonal sequence (DS) because of ipsilateral limb interference, which is mostly observed in nonhuman primates with DS QL. The only gait analysis previously performed for UTS was of a few cases from only one family. They exhibited lateral sequence (LS) QL. The current work presents a gait analysis of UTS in more cases from more families, to obtain a representative sample.

    Methods: Hip and knee angles during quadrupedal standing were measured in UTS cases, healthy controls with requested QL, and nonhuman primates. Limb phases were assessed from video footages, as the percent of the hind limb’s stride durations.

    Results: UTS cases and nonhuman primates exhibited quadrupedal standing with straight legs nearly perpendicular to the ground. Healthy individuals could not walk quadrupedally like the UTS cases, but could perform QL only with flexed legs. UTS cases and healthy individuals with free (flexed-leg) QL used predominantly lateral sequence-diagonal couplet (LSDC) walks. Terrestrial primates preferred DS gaits. The healthy individuals with free QL were similar to arboreal primates in quadrupedal posture.

    Conclusions: Healthy individuals could not imitate the QL of the UTS cases, so a comparison of the UTS cases with healthy individuals is not justified. Although these results do not seem to support the thesis of locomotor evolution in reverse, nobody knows with certainty who our ancestors were or how they walked, and so the possibility of UTS as an example for the ancestral reappearance of QL in human beings cannot be positively excluded. This locomotor evolution in reverse was supported by experimental evidence that proved reverse evolution occurs as a scientific fact.

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