Are modern humans simply bad at smoking?

Mixed blessing

The art of making and using fire was one of the greatest discoveries 'ever made by man', wrote Charles Darwin. Besides providing protection against cold temperatures, the use of fire in food preparation and the introduction of energy-rich cooked foods in our prehistoric diet had a major impact in the development of humankind. However, fire use comes at a cost. Exposure to the toxic compounds in smoke carries major risks for developing pneumonia, adverse pregnancy outcomes in women and reduced sperm quality in males, as well as cataracts, tuberculosis, heart disease, and chronic lung disease. In short, the use of fire is a mixed blessing.

Debate

This mixed blessing, however, put researchers at Leiden University and Wageningen University on the trail of finding genetic markers for the use of fire in prehistoric and recent humans. The use of fire is notoriously difficult to 'see' for archaeologists, and this has led to strong disagreement over the history of its usage. A very early start is advocated by Harvard primatologist Richard Wrangham, who argues that our Homo erectus ancestors were already using fire around two million years ago. However, numerous excavations and intensive research carried out by archaeologists in Europe and the Near East suggest that control of fire occurred much later, around 350,000 years ago.

Genetic markers for fire use

In order to bring fresh data into this 'hot' debate, the Leiden/Wageningen team studied the biological adaptations of prehistoric and recent humans to the toxic compounds of smoke: fire usage implies frequent exposure to hazardous compounds from smoke and heated food, which is expected to result in the selection of gene variants conferring an improved defence against these toxic compounds. To study whether such genetic selection indeed occurred, the team investigated the gene variants occurring in Neanderthals, in Denisovans (contemporaries of the Neanderthals, more related to them than to modern humans), and in prehistoric modern humans.

Tobacco

Single nucleotide variants in 19 genes were tested that are known from modern tobacco-smoking studies to increase the risk of fertility and reproduction problems when exposed to smoke and hazardous compounds formed in heated food.

These genes were compared with variants observed in Neanderthals and their Denisovan cousins, and were also studied in chimpanzees and gorillas, two closely related species that are obviously not using fire, and are therefore not exposed to smoke on a regular basis.

Neanderthal more efficient in handling smoke?

In a study now published in PLOS ONE, the team shows that Neanderthals and the Denisovan predominantly possessed gene variants that were more efficient in handling the toxic compounds in smoke than modern humans. Surprisingly, these efficient variants were also observed in chimpanzees and gorillas, and therefore appeared to be evolutionary very old (ancestral) variants.

Plant toxins

The less efficient variants are observable from the first modern human hunter-gatherers for which we have genetic information onward, i.e. from about 40,000 years ago. The efficient defence against toxic compounds in chimpanzees and gorillas may be related to the toxins in their plant food. Smoke defence capacities in humans apparently hitchhike on those adaptations, developed deep in our primate past. Our prehistoric ancestors were probably already good at dealing with the toxic compounds of smoke, long before they started producing it through their campfires. What allowed for the emergence of less efficient hazardous chemical defence genes in modern humans is a question for future research.