Trilobites: Smoke, Fire and Human Evolution

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 arose 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 the Dresden University of Technology in Germany who was not involved with the study, has wondered whether humans have also developed 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.