¿Es posible la evolusión reversible?

Is it possible reverse evolution? 

Backward evolution” spawns ape-like people

Special to World Science
An editor of a noted scientific journal says he has discovered a genetic defect that seems to set back the clock on human evolution by more than a million years.

imagesIts victims walk on all fours and mouth a primitive language, the scientist reported. He added that the syndrome may literally undo eons of evolution, and thus reflect with some accuracy what our ape-like ancestors were like.

The researcher, Uner Tan of Cukurova University Medical School in Adana, Turkey, has posted an online video clip of an affected woman walking on all fours, her face blurred.

The idea that evolution can run backward isn’t new; some scientists say there have been confirmed cases of it in animals. But it’s also a controversial subject, and considered hard to prove in any given case.

Tan, at any rate, argued that this could be a case of it, so the mutation—known to run in one Turkish family—might offer scientists an unprecedented glimpse into human origins.

“This syndrome interestingly exhibits prehuman features” and represents “possible backward evolution,” he wrote in a paper describing the condition. As such, it “can be considered a live model for human evolution.” 

The paper appears in the March issue of the International Journal of Neuroscience, where Tan sits on the editorial board. He also named the condition after himself: Unertan syndrome. 

The mutation could shed light on the “transition from quadrupedality to bipedality”—from four-legged to two-legged walking, he wrote. Possibly more important, he added, it may illuminate the evolution of the mind.

“The children exhibiting this syndrome originated from a family having 19 children,” he wrote in another recent paper, in the journal Neuroquantology. Five of these, aged 14 to 32 years, “walked on two palms and two feet, with extended legs… They could stand up, but only for a short time, with flexed knees and heads.”

“The patients had a rather primitive language… they spoke to each other using their own language, using only a few hundred words” which the parents could partly understand, Tan wrote.

“They were mentally retarded; they could not count from one to ten. They were not aware of time and space. For instance, they did not know where they live (which country, which village, which city). They were unaware of year, season, day, and time. Otherwise, they had quite strong legs and arms.”

“The sitting posture was rather similar to an ape,” Tan added. “They could not hold their heads upright; the heads were flexed forward with their skulls. They could not raise their heads to look forward. This head posture with flexed skull was rather similar to the head posture of our closest relatives, like chimpanzees.”

Like most primates, Tan observed, victims of the syndrome walk with a characteristic sequence of movements: after a foot touches the ground, the hand on the other side does. “They could walk fairly fast using their strong legs, without any imbalances.”

Tan said in an email that with colleagues, he has mapped the defect to a region of the genome called chromosome 17p, a site of some of the biggest genetic differences between humans and chimps. Other researchers have also recently linked bipedalism to 17p.

Tan’s report is reminiscent of a 2002 discovery that a different mutation, affecting a gene called FoxP2, created severe speech and grammar problems.

That finding has sparked intense research into what scientists think could be the first known “language gene.” FoxP2-mutated patients also have some coordination difficulties, prompting much discussion among scientists of possible links between language and coordination.

Those patients aren’t reported to have problems standing up, though, unlike those Tan studied.

Scientists generally consider the transition to upright walking as the most important event in human evolution, according to Tan. This freed the hands for skilled movements such as throwing and toolmaking, he added, and may have even made consciousness possible—though a growing number of scientists say consciousness might not be unique to humans.

Upright walking became habitual by the age of Homo erectus, an extinct human ancestor believed to have evolved in Africa 1.6 million years ago, Tan wrote. Language mostly evolved later, about 40,000 years ago, he added, though Homo erectus likely had a rudimentary form of language.

Evolution is generally believed to occur when organisms with the worst genes for their environment die off, leaving their fitter peers to survive and spread their genes. This process, called natural selection, can over many generations alter a whole species’ gene pool, eventually producing major changes in the species.

One theory, known as punctuated evolution, holds that evolution takes place more in sudden leaps than in gradual, constant changes. Unertan syndrome suggests this view is correct, Tan argued, because it shows complex traits can both appear and vanish suddenly, probably because they involve a single gene or group of related genes.

But the notion that evolution can run backward is controversial.

Reverse evolution would happen when organisms lose genes they had gained earlier in evolution. Alternatively, it could occur when they regain a previously lost ability or structure, possibly because genes that had fallen into disuse, but weren’t completely lost, become active again.

evolutionIt has been shown to happen in some cases, wrote Megan Porter and Keith Crandall of Brigham Young University in Provo, Utah, in the Oct. 2003 issue of the research journal Trends in Ecology and Evolution. For instance, fish that take up living in dark caves can mostly lose their eyes, since they don’t need them; but they can re-evolve them upon resuming life at the surface.

Yet it’s hard, they wrote, to prove that a given case represents true reverse evolution—the return to an ancestral genetic state. That’s because what seems like backward evolution could just be an evolution of new genes that produce similar effects to the old. 

However, detailed studies can sometimes bolster a case for reverse evolution, they added; for instance, by showing that muscle and nerve patterns in both an old and new structure correspond precisely.

Occasionally, reverse evolution may require a little human help.

In a new study published in the Feb. 21 issue of the journal Current Biology, researchers reported producing chickens with teeth. Birds don’t normally have teeth, but their dinosaur ancestors did. Matthew P. Harris of the University of Wisconsin in Madison and colleagues said they made the toothy chickens by providing carefully chosen molecular signals to the beak region of developing embryos.

This suggests chickens retain some tooth-forming ability, they wrote, noting that the teeth in this case somewhat resembled those of birds’ closest living ancestors—alligators.

¿Es posible la evolusión reversible?

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