Claim of reversed human evolution provokes skepticism, interest
and World Science staff
Scientists’ reactions have ranged from deep skepticism to interest in a report of a mutation that makes people walk on all fours, cited in a Turkish study as a possible instance of “backward evolution.”
The bizarre case, reported last week in World Science, has also attracted the attention of several scientists in Europe, some of whom are working on a BBC documentary about it.
Three researchers with the University of Cambridge, U.K., and the London School of Economics recently wrote that the mutation could represent a “rediscovery” of a walking style much like that of human ancestors. They added that this might help resolve an old debate over how our forebears walked.
“There’s an interesting story here,” wrote one of the three, Nicholas Humphrey of the London School of Economics’ Centre for Philosophy of Natural and Social Sciences, in an email.
Five members of a Turkish family carry the mutation, according to researchers who have studied them, and also are mentally retarded with limited language skills.
Uner Tan, a researcher with Cukurova University Medical School in Adana, Turkey, has gone even further than the British researchers: citing the victims’ ape-like walking and “primitive” language, he has proposed the mutation could represent “backward evolution.”
Reverse evolution is far from a new concept in biology; many biologists accept it as a real phenomenon. Nonetheless, it is controversial and somewhat ill-defined.
Humphrey distanced himself from Tan’s findings. He said he disagrees with Tan’s interpretations of the case, described in a study in the March issue of the International Journal of Neuroscience. Humphrey also has declined to follow Tan’s lead in the naming of the condition—which Tan named after himself, Unertan syndrome.
Humphrey declined to specify why he disagrees with Tan. But other researchers disputed the whole idea that the syndrome might represent a sort of reversion or backward evolution.
The question is “untestable,” wrote Henrique Teotónio, an evolutionary geneticist at the Gulbenkian Science Institute in Oneiras, Portugal, in an email. “It is simply impossible to judge it,” added Teotónio—who has published studies on reverse evolution in flies—because scientists don’t know which genetic changes were responsible for human evolution.
“The chance that this human disorder is related to the evolution of our early ancestors and their mode of walking is remote,” agreed Laurence Mueller, an evolutionary biologist at the University of California at Irvine, in an email.
Both researchers also expressed doubt over a key implication of Tan’s finding: that just one, or a few, genes could underlie such a complex trait as upright walking. They voiced this skepticism even though other new findings have surprised scientists with how much a single gene can do; one gene has been found to orchestrate a suite of complex courtship rituals in fruit flies, for example.
In response to the criticisms, Tan conceded that his idea is debatable. But he also noted in his defense new studies mapping the mutation to a region of the genome known as 17p, which he said is one of the areas of greatest differences between humans and their closest living ancestors, chimpanzees.
German researchers published those findings in the Dec. 21 issue of the Journal of Medical Genetics. They wrote that the affected gene might shed light on “the evolution of this unique hominid trait,” bipedalism.
The British team offered its own views on the syndrome in a paper presented to colleagues on Oct. 3 and published online. “We report here,” they wrote, “the case of five children, of a single large family, who… as adults have continued to walk—highly effectively—on hands and feet.”
Humphrey’s group added that it’s possible, but debatable, that “we are indeed seeing the ‘rediscovery’ of something very like the quadrupedal [four-limbed] gait used by our ancestors.”
The affected people’s gait, the researchers added, “appears to be a development of the ‘bear crawl’ which they adopted as infants.” This crawl “sometimes occurs as a transitional stage on the way to bipedality,” or upright walking, in children.
The walking style seems to have “unprecedented features,” they added. “The hands are placed palm-down, with all the forward weight being taken on the wrists. Such ‘wrist walking,’ as we shall call it, is not seen in normal infants. It is quite different from the knuckle-walking of the great apes.”
Based on brain scans, Humphrey’s group ascribed the condition to a type of disorder called an ataxia. This arises from an underdevelopment in the cerebellum, a brain structure linked to coordination. Yet ataxia alone doesn’t explain the condition, they added: normally, “if the ataxia is so severe as to prevent bipedalism, then it also rules out effective quadrupedalism,” or four-limbed walking.
“The local villagers laugh at and tease” the affected people, the researchers added. “Because of this, the females tend to stay close to the house, but the male sometimes wanders for several kilometres. He helps raise money for his family by collecting cans and bottles, which he carries home in a pouch made from his shirt, held by his teeth.”
“He is remarkably agile,” the paper added.
“We watched him moving easily across rough terrain in search of collectibles… his hands anticipated the contours of the rocks, so that he placed them deftly without looking down.”
Tan has reported that the affected people are retarded and speak a primitive language of a few hundred words. Humphrey’s group portrayed their mental abilities more positively.
“They can all speak and understand Kurdish well enough to communicate within their own family, and three of them also speak some Turkish; but their articulation is poor, and it seems they have a restricted vocabulary and difficulties with syntax. They have reasonably good interpersonal skills. They interacted with us as visitors in a friendly and courteous way.”
Although the affected people can walk on two legs briefly, Humprey noted, they would rather go on all fours, even on stairs. “They move in this way fluently and effectively, and seemingly without discomfort,” they added—unlike normal adults, who “find such a gait – if and when they try it – tiring and uncomfortable even after practice.”
The syndrome could be “the result not simply of a cerebellar problem but of a combination of unusual factors—genetic, physiological, psychological and social,” the researchers added.
The German researchers noted that carriers of the mutation also tend to have abnormally small heads, a condition called microcephaly. None other than Charles Darwin cited microcephaly as a possible type of evolutionary “regression” in his Origin of the Species.
If the affected people do walk similarly to the way human ancestors did, this might throw light on a longstanding debate over how they did so, Humphrey’s group argued. Various theories suggest humans evolved either from knuckle-walkers, who walked on their knuckles; tree climbers; or “wrist-walkers” who moved on the ground with palms down, they added.
A fossil analysis published in the March 23, 2000 issue of the research journal Nature pronounced findings in favor of knuckle-walking. “But the discovery of these human wrist-walkers changes the situation,” Humphrey’s team wrote.
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