A genetic mutation may have helped modern humans adapt to smoke exposure from fires and perhaps sparked an evolutionary advantage over Neanderthals and other non-human primates, a new study has found. Modern humans are the only primates that carry this genetic mutation that potentially increased tolerance to toxic materials produced by fires for cooking, protection and heating, said Gary Perdew, professor at Pennsylvania State University. At high concentrations, smoke-derived toxins can increase the risk of respiratory infections. For expectant mothers, exposure to these toxins can increase the chance of low birth weight and infant mortality. The mutation may have offered ancient humans a sweet spot in effectively processing some of these toxins - such as dioxins and polycyclic aromatic hydrocarbons. "If you're breathing in smoke, you want to metabolise these hydrophobic compounds and get rid of them, however, you don't want to metabolise them so rapidly that it overloads your system and causes overt cellular toxicity," said Perdew. The researchers suggested that a difference in the aryl hydrocarbon receptor - which regulates the body's response to polycyclic aromatic hydrocarbons - between humans, Neanderthals and other non-human primates may have made humans more desensitised to certain smoke toxins. "For Neanderthals, inhaling smoke and eating charcoal-broiled meat, they would be exposed to multiple sources of polycyclic aromatic hydrocarbons, which are known to be carcinogens and lead to cell death at high concentrations," said Perdew. "The evolutionary hypothesis is, if Neanderthals were exposed to large amounts of these smoke-derived toxins, it could lead to respiratory problems, decreased reproductive capacity for women and increased susceptibility to respiratory viruses among preadolescents, while humans would exhibit decreased toxicity because they are more slowly metabolising these compounds," he said. There is evidence that both humans and Neanderthals used fire, said George Perry, assistant professor at Penn State. "Our hominin ancestors were likely using fire at least a million years ago, and some infer an earlier control and use of fire approximately 2 million years ago," said Perry. Fire would have played an important role for both humans and Neanderthals. "Cooking with fire could have allowed our ancestors to incorporate a broader range of foods in our diets, for example, by softening roots and tubers that might otherwise have been hard to chew," Perry said. The study may also lend support to a recent theory that the invention of cooking may have helped humans thrive, according to Perdew. The findings appear in the journal Molecular Biology and Evolution.