When early humans discovered how to build fires, life became much easier in many regards. They huddled around fire for warmth, light and protection. They used it to cook, which afforded them more calories than eating raw foods that were hard to chew and digest. They could socialize into the night, which possibly gave rise to storytelling and other cultural traditions.  But there were downsides, too. Occasionally, the smoke burned their eyes and seared their lungs. Their food was likely coated with char, which might have increased their risk for certain cancers. With everyone congregated in one place, diseases could have been transmitted more easily.  Much research has focused on how fire gave an evolutionary advantage to early humans. Less examined are the negative byproducts that came with fire, and the ways in which humans may or may not have adapted to them. In other words, how did the harmful effects of fire shape our evolution?  It’s a question that’s just starting to attract more attention. “I would say it’s mostly barroom talk at the moment,” said Richard Wrangham, a professor of biological anthropology at Harvard University and the author of “Catching Fire: How Cooking Made Us Human.” His work suggested that cooking led to advantageous changes in human biology, such as larger brains.  Now, two new studies have proposed theories on how negative consequences of fire might have shaped human evolution and development.  In the first, published Tuesday, scientists identified a genetic mutation in modern humans that allows certain toxins, including those found in smoke, to be metabolized at a safe rate. The same genetic sequence was not found in other primates, including ancient hominins such as Neanderthals and Denisovans.  The researchers believe the mutation was selected for in response to breathing in smoke toxins, which can increase the risk of respiratory infections, suppress the immune system and disrupt the reproductive system.  It’s possible that having this mutation gave modern humans an evolutionary edge over Neanderthals, though it’s speculation at this point, said Gary Perdew, a professor of toxicology at Pennsylvania State University and an author of the paper. But if the speculation is correct, the mutation may have been one way that modern humans were inured against some adverse effects from fire, while other species were not.  Thomas Henle, a chemistry professor at Dresden University of Technology in Germany who was not involved with the study, has wondered whether humans also have unique genetic mutations to better handle, or even take advantage of, byproducts of fire in food. In 2011, his research group showed that the brown molecules that come from roasting coffee can inhibit enzymes produced by tumor cells, which might explain why coffee drinkers may be at lower risk for certain cancers.  Other research has suggested that these roasting byproducts may stimulate the growth of helpful microbes in the gut.  A genetic mutation that may help humans tolerate smoke toxins could be just one of many adaptations, Dr. Henle said. “I am sure that there are further human-specific mechanisms, or mutations, which are due to an evolutionary adaptation to eating heat-treated foods.”  Understanding how humans might have uniquely adapted to the risks from exposure to fire may have implications for how scientists think about medical research, Dr. Wrangham said. Other animals that didn’t evolve around fire, for instance, may not be the best models for studying how we process food or detoxify substances.  One example, he suggests, is the study of acrylamide, a compound that forms in foods during frying, baking or other high-temperature cooking. When given to lab animals in high doses, acrylamide has been shown to cause cancer. But so far, most human studies have failed to link dietary acrylamide to cancer.  “People keep ‘wanting’ to find a problem for humans,” Dr. Wrangham said, but there’s “nothing obvious at all.”  Humans may not have been able to adjust to all of the dangers of fire. The second study, published last week in Proceedings of the National Academy of Sciences, suggests that with fire’s advantageous effects for human societies also came profound new damage. It offers conjecture that the early use of fire might have helped spread tuberculosis by bringing people into close contact, damaging their lungs and causing them to cough.  With mathematical modeling, Rebecca Chisholm and Mark Tanaka, biologists at the University of New South Wales in Australia, simulated how ancient soil bacteria might have evolved to become infectious tuberculosis agents. Without fire, the probability was low. But when the researchers added fire to their model, the likelihood that tuberculosis would emerge jumped by several degrees of magnitude.  It is thought that tuberculosis has killed more than a billion people, possibly accounting for more deaths than wars and famines combined. Today it remains one of the deadliest infectious diseases, claiming an estimated 1.5 million lives each year.  Many experts believe tuberculosis arose at least 70,000 years ago. By then, humans were most certainly controlling fire. (Estimates of when human ancestors started regularly using fire vary greatly, but the consensus is that it was at least 400,000 years ago.)  “We realized that the discovery of controlled fire must have caused a significant shift in the way humans were interacting with each other and with the environment,” factors known to drive the emergence of infectious diseases, Dr. Chisholm said.  She and Dr. Tanaka believe that fire might have helped spread other airborne diseases, not just tuberculosis. “Fire, as a technological advantage, has been a double-edged sword,” Dr. Tanaka said.  Negative cultural consequences came with fire, too — and continue to leave an imprint. Anthropologists have speculated that inhaling smoke led to the discovery of smoking. Humans have long used fire to modify their environment and burn carbon, practices that now have us in the throes of climate change. Fire is even tied to the rise of patriarchy — by allowing men to go out hunting while women stayed behind to cook by the fire, it spawned gender norms that still exist today.  Investigating how fire’s harmful effects have shaped human history and evolution can provide a rich look into the relationship between culture and biology. Did we evolve biologically to guard against the health risks of inhaling smoke? Did that help us pick up the cultural practice of smoking? There are many other possibilities.  “It’s a fascinating feedback loop,” said Caitlin Pepperell, a professor at the University of Wisconsin-Madison who studies the evolution of human diseases. “I hope these studies will spur us to think more about fire, and take it in all the different directions it can go.”  A version of this article appears in print on August 9, 2016, on page D5 of the New York edition with the headline: The Lingering Embers of Discovering Fire. Order Reprints| Today's Paper|Subscribe
Figuring out how to make fire was no doubt an evolutionary boon to our ancestors. But it may have led to our smoking habit and the emergence of tuberculosis.