A genetic mutation may have helped early humans fend off toxic fumes that wafted into the air from ancient cave fires, a team of researchers claim. The scientists say the altered DNA was found in humans but not Neanderthals, and may have given our early human ancestors an edge over our extinct cousins, who had also learned to control fire. Researchers at Pennsylvania State University looked at what is called the aryl hydrocarbon receptor gene which plays a role in the breakdown of certain noxious substances. When organic matter such as meat or wood is burned it releases polycyclic aromatic hydrocarbons (PAHs), which can mutate DNA and cause cancer, as the World Health Organization cautioned against in 2015. The PAHs are absorbed when we eat grilled meats or breathe in smoke. Once inside the body, their presence triggers aryl hydrocarbon receptors into producing enzymes that break them down so they can be flushed out. But according to the researchs author Gary Perdew, when we breathe in too many toxic substances, just as we would in smoky caves, enzyme production goes into overdrive. The extra enzymes create a slew of toxic by-products that Perdew describes as uber-toxicity. The mutation Perdew and his team identified appears to slow down enzyme production to a rate that limits the risk of this toxicity. They theorise this may have made humans less sensitive to smokes harmful effects, allowing them to be around fires more often and offered an evolutionary advantage. Neanderthals, who appear to lack the mutation, would have struggled with more smoke-related respiratory infections, fertility problems, and mortality. We prospered because of this mutation, Perdew said. I wouldnt say Neanderthals died out because of it, but it could have been a contributing factor. The forerunner of modern humans and Neanderthals, Homo erectus, learned to control fire for warmth and cooking at least 1.9m years ago in Africa. Both picked up the practice from this common ancestor, but with fire came greater exposure to smoke in enclosed caves. They werent the greatest cooks, said Perdew. And it wasnt easy to start a fire, so they might have kept a little fire burning all the time. They were probably burning grasses, whatever they could get their hands on. The scientists studied the gene in nine modern humans, including one from 45,000 years ago, three Neanderthals, and one member of the related but mysterious Denisovans in Siberia. The mutation was not present in the Neanderthals or Denisovan, but it was found in all of the modern humans. If the claims hold up, the study published in Molecular Biology and Evolution would be one of the first examples of human evolution in response to environmental pollution. However, given the small sample size of Neanderthals and the threadbare fragments of their DNA, the hypothesis is still tentative. David Wright, an archaeologist at Seoul National University and the University of York, was cautious about the interpretation, and said that historical evidence does not seem to line up with the theory. Neanderthals were the ultimate cave-dwelling fire users. If there was some selective disadvantage against this, then they would have died out a long time before they did. But they were actually one of the more successful stories in human evolution and lasted a really long time compared to other hominids, he said. That somehow Homo erectus and Neanderthals and dozens of other hominid species couldnt handle sitting around a fire, it doesnt make any sense to me, he added. The problem is its really difficult to test, because we cant take a Neanderthal and sit them next to a fire to see how they react. Emily Monosson, an environmental toxicologist at the University of Massachusetts in Amherst said: It wouldnt be unexpected that being exposed to a contaminant that makes you sick would be a selective pressure on humans. However, she agreed with being cautious: Youre talking about one helpful adaptation against one chemical, but theres a lot of other harmful particulates in smoke. The team have plans to keep studying the gene to figure out where it came from and how it evolved. Perdew said: Something happened, where we picked up this mutation and we all have it, thats undeniable. The question is why?
The Guardian, 3 August 2016 ;http://www.guardian.com ;