A genetic mutation may have helped modern humans adapt to smoke exposure from fires and perhaps even sparked an advantage over Neanderthals and other competitors.
Modern humans are the only primates that carry the mutation that may have increased tolerance to toxic materials produced by fires for cooking, protection, and heating, says Gary Perdew, professor in agricultural sciences at Penn State.
At high concentrations, smoke-derived toxins can increase the risk of respiratory infections. For expectant mothers, exposure 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—compared to other hominins.
“If you’re breathing in smoke, you want to metabolize these hydrophobic compounds and get rid of them, however, you don’t want to metabolize them so rapidly that it overloads your system and causes overt cellular toxicity,” Perdew says.
Researchers suggest 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 desensitized humans to certain smoke toxins. The mutation in the receptor is located in the middle of the ligand-binding domain and is found in all present-day humans.
Ligands are small molecules that attach to receptor proteins in certain areas in much the same way that keys fit into locks.
“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,” Perdew says.
“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 metabolizing these compounds.”
“Our tolerance has allowed us to pick up bad habits.”
There is evidence that both humans and Neanderthals used fire, says George Perry, assistant professor of anthropology and biology. “Our hominin ancestors—they would technically not be called humans at that time—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.
“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 says. “Cooking could also help increase the digestibility of other foods, both in chewing time and reduced energetic investment in digestion.”
Fire also provided warmth, particularly in the higher latitudes. “Besides heating and cooking, humans used—and still use—fire for landscape burning and as part of hunting and gathering, and now as part of agriculture.”
The study, published in the journal Molecular Biology and Evolution, may also lend support to the theory that the invention of cooking may have helped humans thrive, Perdew says. But the receptor might have also given humans a better tolerance for cigarette smoke, putting them at risk of cancer and other chronic diseases. “Our tolerance has allowed us to pick up bad habits.”
The researchers used computational and molecular techniques to examine the difference in the genetics of polycyclic aromatic hydrocarbon tolerance between humans and Neanderthals. They examined a genomic database of humans, Neanderthals and a Denisovan, a hominin more closely related to Neanderthals than to humans.
“We thought the differences in aryl hydrocarbon receptor ligand sensitivity would be about ten-fold, but when we looked at it closely, the differences turned out to be huge,” Perdew says. “Having this mutation made a dramatic difference. It was a hundred-fold to as much of a thousand-fold difference.”
Other researchers from Penn State, Evan Pugh University, and Oregon State University are coauthors of the study, which the National Institutes of Health supported.
Source: Penn State