A much-derided theory that five people who walk on all fours are products of “backward evolution” is plausible, and testable, said a U.S. biologist who weighed in on the controversy last week.

The debate erupted last month after a Turkish scientist proposed that the five siblings in Turkey, who also speak what he called a primitive language, had undergone backward evolution. The claim met with skepticism, even jeers, from some fellow scientists.

But Keith Crandall of Brigham Young University in Provo, Utah, said the idea is nothing extraordinary, calling it a “nice and testable hypothesis.”

Reverse evolution occurs when an organism returns to the genetic state of its ancestors, said Crandall, who wrote a paper on the topic in the Oct. 2003 issue of the research journal Trends in Ecology and Evolution. In that work, he wrote that reverse evolution is documented in various organisms, such as fish that lose their eyes after living in dark caves for generations.

The Turkish researcher, Uner Tan of Cukurova University Medical School in Adana, Turkey, argued that something similar may have afflicted the family in Turkey: a mutation stripped them of the genes or genes that let humans walk upright, returning them to the pre-human state of quadrupedalism, or four-limbed walking.

The stakes are high. If Tan is right, it would suggest the quadrupedal siblings accurately reflect what our ape-like ancestors were like in some ways. They thus could give biologists unprecedented insights into human evolution.

Laurence Mueller, a biologist at the University of California, Irvine, was skeptical.

“My opinion is that the chance that this human disorder is related to the evolution of our early ancestors and their mode of walking is remote,” he wrote in an email.

It’s doubtful that just one or a few genetic changes in evolution could have caused upright walking, he said, but that’s what Tan’s theory implies: if one mutation caused the four-limbed walking in the siblings, and this is reverse evolution, then one mutation would distinguish upright walkers from quadrupeds.

The key difficulty in proving a case of reverse evolution, Crandall said, is that what looks like a real case of it, could really be an evolution of new genes that mimic the effects of the old.

“Genetics research is rife with examples” of similar events, Mueller wrote.

Henrique Teotónio, of the Gulbenkian Science Institute in Oneiras, Portugal, sided with Mueller. He called the idea that one mutation caused upright walking “untestable,” arguing that to test it, one would need to find another lineage of upright-walking primates besides humans. Then one would have to compare the species. The fossil evidence so far, he argued, points the opposite way, hinting that not one but “several gene changes were involved.”

Crandall argued that modern genetic techniques can test the contrasting claims, because biologists can use these methods to reconstruct an organism’s genetic past with some confidence.

To demonstrate reverse evolution here, he wrote, one would have to identify the genetic changes associated with four-limbed walking, then show that the same changes appear in a reconstruction of ancestral human genes.

Tan said he has begun such a testing process, by mapping the defect causing the quadrupedalism to one of the areas of the genome that is most different between humans and other primates. That hints that the gene at issue was important in human evolution, consistent with his hypothesis, he suggested.

Crandall said more work is needed to show this, in particular to prove that evolution promoted the gene’s spread as a beneficial mutation. Evolution is the process in which occasional helpful mutations spread throughout a population, as the organisms that have them reproduce more. This leads to gradual changes in whole species.

Reverse evolution would occur when genes recently acquired through evolution are lost again, or when genes become reactivated after falling into disuse.

Tan has gone as far as to propose that the “reverse evolution” of the family in Turkey affects the mind as well as the body, noting that victims of the syndrome are retarded. Crandall said he doesn’t buy many of Tan’s ideas, but that at least with regard to walking, Tan may have suggested a concept worth testing.

Scientific doubts on reverse evolution, Crandall wrote, have nothing to do with a popular misconception that evolution “has no direction.” It does to some extent, he argued—species tend to become better suited to their environment—and that may be irrelevant anyway, since a return to an ancestral state can occur whether or not one thinks of evolution as directed.

“I don’t know of any evolutionary biologists who would subscribe to the notion that reverse evolution is impossible,” Crandall wrote. “It doesn’t take long in evolutionary biology to figure out that nothing is impossible!”

ABOUT THE AUTHOR

Science writer Jack Lucentini is founder and editor of the World Science science news webzine. He has worked as a staff writer at three daily newspapers, and as a freelance science writer for a range of publications including The Washington Post, Discover magazine and The Scientist magazine. He earned his bachelor's degree at Oberlin College in Oberlin, Ohio in 1993.