Pennsylvania State University researchers claim they've uncovered a gene that helps humans cope with the toxicity of smoke. This gene wasn't found in specimens belonging to Neanderthals or Denisovans, two other hominin species which were contemporary with homo sapiens for thousands of years before becoming extinct.
A smokey edge
The researchers posit that this mutation must have given humans an evolutionary edge against the two other species. Some 50,000 years ago, humans, Neanderthals and Denisovans lived mostly in caves where fires must have clouded everything in smoke.
When we eat grilled meat or inhale smoke, toxic byproducts like polycyclic aromatic hydrocarbons (PAHs) are absorbed by the body which cause DNA mutations, cancer or sudden death. Luckily, when PAHs enter the body enzymes are produced to break down the chemicals and flush them out. If there's too much smoke, however, the body goes into enzyme production overdrive, which triggers the formation of a number of even more toxic byproducts -- but not if you have a gene mutation in the aryl hydrocarbon receptor, found in the in the middle of the ligand-binding domain.
All modern humans have this mutation, but when the researchers led by Gary Perdew, the John T. and Paige S. Smith Professor in Agricultural Sciences, Penn State, sequenced the DNA of three Neanderthals and one Denisovian they couldn't find any. Not having this mutation could have made all the difference -- from a hundred to a thousand-fold more aryl hydrocarbon receptor ligand sensitivity.
"For Neandertals, 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 in a statement. "The evolutionary hypothesis is, if Neandertals 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."
The same tolerance also helped humans pick up some bad habits, like smoking tobacco, the researchers wrote in their paper published in Molecular Biology and Evolution.
All hominids included in this study knew how to make fire. In fact, a previous study found Neanderthals were clever enough to use manganese dioxide -- a substance commonly used in batteries today -- to light their camps. The first hominid to use fire was likely Homo erectus, a species which regularly made use of the blaze to cook, fend off predators, provide warmth and possibly ritualistic purposes 1.9 million years ago. Any species which followed H. erectus must have learned to light and maintain fires as well.
"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. "Cooking could also help increase the digestibility of other foods, both in chewing time and reduced energetic investment in digestion."
"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," he added.
I would caution, however, readers that this is still a tentative hypothesis. After all, the researchers could only look at three Neanderthals and a Denisovan. A lot more DNA from more specimens should be sequenced before any definite conclusion can be made. Already, some critics aren't convinced.
“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,” David Wright, an archaeologist at Seoul National University and the University of York, told The Guardian.
“That somehow Homo erectus and Neanderthals and dozens of other hominid species couldn’t handle sitting around a fire, it doesn’t make any sense to me,” he added. “The problem is it’s really difficult to test, because we can’t take a Neanderthal and sit them next to a fire to see how they react.”