Fires have been important to our family for at least a million years. Maybe even longer. However, not everything about this key invention is good for us. Fire produces toxicants that can cause cancer and reduce fertility. They can also wreak havoc with our lungs. This damage is thought to have been what allowed tuberculosis to become such a problem in our species.
Give we've been dealing with fire so long, some researchers speculated that we may have evolved to deal with these problems. That we might have adapted to become better at detoxification. Examination of Neanderthal genes reveals they were certainly good at dealing with toxicants from a fire. Even the Denisovans also had this additional capacity.
However, contrary to expectations we don't. The genes which allowed our relatives to deal with fire have gained harmful mutations in us. As a result of this, we're a lot worse at dealing with the side-effects of fire. This flies in the face of how you'd think natural selection should work.
So what happened?
Ancient fire
Around 1 million years ago a group of hominins lived in a South African cave. We now call it Wonderwerk, which means "miracle" in the local tongue. But we have no way of knowing what these hominins called it. But we do know that they started a fire there.
In fact, they must have started several. Burnt bones, plants, and flecks of clay are found across multiple rock layers at the site. This repeated burning over a long period of time at a single location indicates these individuals had control over fire. If that wasn't enough to convince you, there's hotter evidence. Literally. Discolouration, damage, and other indicators of how hot these objects burnt place them at ~500 degrees. Hotter than most natural fire.
This started a long, proud tradition of burning stuff. Hominins continue to set stuff on fire at increasing frequency, all the way up until the present day (although that increased frequency could be preservation bias). Sites like Gesher Benot Ya'aqov preserve evidence of several hearths. People congregated around them, making tools, cooking, and eating.
However, this tradition of being a firestarter might also extend further back in time. Several African sites also show evidence of burnt materials as early as 1.7 million years ago. Nearly double the age of Wonderwerk. But the authenticity of these finds is debated. Some could simply be tree trunks that burned in bush fires.
Ancient genes
The exact age of the first hominin-made fire is debated. Regardless, it's clear that it's ruddy old. Even assuming Wonderwerk is the oldest, that would make fire five times as old our species.
It has long been thought that this ancient interaction with fire influenced our evolution. Fire can keep us warm, allowing us to expand out of Africa. It can cook food, fueling our large brains. Cooking food would also kill off germs, helping our ancestors flourish.
All of this ignores the fact that fire isn't always a good thing. Even excluding when some of it gets on you. Fire produces toxicants (the non-animal made version of toxins). These can get in your lungs and cause damage, which is part of the reason smoking is bad. This damage can also increase your risk for other conditions, like TB. Even cooking food isn't all good. Charred stuff contains a high concentration of toxicants that pose a cancer risk.
As such, some researchers speculated that these negative side-effects might also have influenced human evolution. That as we became more reliant of fire we adapted to detoxify it. Unfortunately, fossils don't provide much information on how much fire made them cough.
So these researchers looked at genes instead. Several have been found that correlate with our ability to detoxify fire. They predicted that the genes of our more ancient and distant relatives - like Neanderthals and chimps - should lack our "evolved" detoxification alleles. But it turned out the opposite was true. Modern humans had many high-risk alleles, whilst apes and other hominins had lower risk variants.
Modern vulnerability to toxicants
This research revealed two important things. First, that resistance to toxicants is ancient. Neanderthals, chimps, and Denisovans have it. So it likely came from our common ancestor, which lived 7-14 million years ago. In other words, the ability to deal with fire preceded our use of it. This raises the question of how common are these adaptations? It might be the case that almost all mammals have the ability to deal with fires, since they can be encountered in wild.
The other key discovery is that this an ability we lost. Or rather, that has been diminished in modern humans. Since both Neanderthals and Denisovans haven't undergone this change, it likely occurred after we all split. This was around half a million years ago. A review of an ancient human genome, dating to 45,000 years ago, found they also had many high-risk alleles. So at some point between those data points, we sacrificed some resistance to toxicants.
This raises the obvious question of "why"? The answer might simply be "chance". The 45,000-year-old date places this genetic shift in the out of Africa time frame. When modern humans began venturing out of their homeland. Obviously, the groups which made this journey weren't a perfect cross-section of all Africans. This would create a genetic bottleneck, where some variation was lost. Perhaps this lost variation includes the low-risk alleles.
There is more support for this than "the timing lines up". Sub-Saharan Africans have a higher proportion of low-risk alleles than the groups which spread from Africa. This would suggest that the division between the two did lead to some helpful alleles being lost.
References
Aarts, J.M., Alink, G.M., Scherjon, F., MacDonald, K., Smith, A.C., Nijveen, H. and Roebroeks, W., 2016. Fire Usage and Ancient Hominin Detoxification Genes: Protective Ancestral Variants Dominate While Additional Derived Risk Variants Appear in Modern Humans. PloS one, 11(9), p.e0161102.
Berna, F., Goldberg, P., Horwitz, L.K., Brink, J., Holt, S., Bamford, M. and Chazan, M., 2012. Microstratigraphic evidence of in situ fire in the Acheulean strata of Wonderwerk Cave, Northern Cape province, South Africa. Proceedings of the National Academy of Sciences, 109(20), pp.E1215-E1220.
Chisholm, R.H., Trauer, J.M., Curnoe, D. and Tanaka, M.M., 2016. Controlled fire use in early humans might have triggered the evolutionary emergence of tuberculosis. Proceedings of the National Academy of Sciences, p.201603224.
Gowlett, J.A.J., 2016. The discovery of fire by humans: a long and convoluted process. Phil. Trans. R. Soc. B, 371(1696), p.20150164.
