Extending the day into night by controlling light may have helped ignite human evolution but we are only beginning to understand how our use of light is impacting life on this planet.

The history of humankind is also the history of controlling fire. Myths and folklore from all over the world attest to the central role that fire has played in our history. Matarisvan in the Hindu Rig Veda and Coyote in Native American legends are credited with gifting fire to humans. The Greek legend of Prometheus recounts how Prometheus – always looking out for human welfare and development – angered Zeus by sharing the secret of fire with humans. As a punishment for his humanist deed, Prometheus was chained to a rock. He endured the pain of an eagle preying on his liver, which would regenerate each day and thus perpetuate the daily trauma of having part of his liver ripped out by an eagle. The magnitude of this Promethean torment only underscores the significance that the Ancient Greeks attributed to the usage of fire by humans.

We do not need to rely on ancient myths to understand the importance of fire in our history. The discovery of burned bones and plant remains in a cave in South Africa suggests that our hominid ancestors used controlled fire at least one million years ago, and there is ample evidence for regular fire usage approximately 350,000 years ago. Was controlled fire just one of the many tools that hominins harnessed and used or was fire essential for the evolution of our species? There are obvious benefits to using fire such as staying warm when it is freezing cold outside or fending off potential predators. The primatologist Richard Wrangham famously formulated the “cooking hypothesis” which stated that central benefit of fire usage was that it enabled hominins to cook their food. Cooked food allowed our ancestors to increase the amount of energy their bodies could extract from food. Instead of wasting energy trying to digest raw food, fire-controlling hominins could utilize this additional energy to grow bigger brains and evolve cognitively.

An even more intriguing perspective for the indispensable role of fire has been put forward by the primatologist Frances Burton from the University of Toronto in her book, aptly titled, “Fire: The Spark That Ignited Human Evolution”. Burton does not discount that fire provided warmth and a means for cooking but she proposes that the key benefit our ancestors derived from controlling fire was that it allowed them to generate light. Instead of having to rely on natural sunlight, hominins used fire as a light source at night to artificially extend the day. Burton suggests that the light emitted from campfires changed the hormonal patterns of our ancestors. It suppressed the hormone melatonin which is released by the pineal gland in our brain at night and functions as an important regulator of our internal biological clock. According to Burton, the changes in hormonal patterns by campfires liberated our ancestors from the natural light-dark cycles and seasonality and increased their ability to reproduce.

As intriguing as Burton’s idea is, there is little scientific evidence to back up the notion that light emitted by the campfires of our ancestors was sufficient to propel human evolution forward by affecting internal biological clocks. Humans, like other animals and plants, have an internal biological timekeeping system called a circadian clock. The name “circadian” is derived from “circa” = ‘approximately” and “dian” = “one day” and refers to the fact that our internal clock runs at a period of approximately 24 hours, mirroring the 24 hour day/night cycle of the earth. The internal circadian clock allows our body to anticipate the external time of the day. Our body temperature, hormones and many other important processes in the body would keep on cycling in a 24 hour rhythm even if we were to spend a few days in continuous light or in continuous darkness.

Our internal clock is located in a part of the brain called the suprachiasmatic nucleus (SCN). It does not just function as a passive timekeeper but is smart enough to adapt to our environment. If the internal clock is currently anticipating nighttime and unexpectedly encounters bright light, it realizes that it needs to be reset. It sees the bright light as a cue for daytime sunlight and shifts to ensure that there is harmony between the internal time and the external time. Jet lag experienced by people who fly across time zones is a classic example of the gradual resetting process. Individual circadian clocks differ in their ability to adapt. Annoying globe trotters boast that they experience hardly any jet lag, whereas others may need a week to adjust to the new day/night cycle when they fly from Beijng to Munich. A good rule of thumb is that exposing internal clocks to bright sunlight facilitates the adjustment to the new time zone because circadian clocks are pretty hardy and do not shift easily with the low light intensities. This makes sense because overly sensitive internal clocks would be rather useless for our bodies. They would respond to minor changes in light intensities that occur when we go about our lives, moving in and out of caves or buildings.

Burton measured the light intensity emitted from campfires and found that it was only 3 lux at 2 meters, a typical distance at which our ancestors might have huddled around a campfire. Just to put this in perspective, here are some typical light intensities that we encounter (measured in lux units and adapted from an article by the researcher Kevin Gaston and his colleagues at the University of Exeter):

Bright sunlight is the most effective timing cue, but researchers have found that even light intensities as low as 10-100 lux can suppress our clock hormone melatonin and reset our clocks. The circadian biologist Anne-Marie Chang, who is now at Pennsylvania State University, recently discovered that human subjects who used digital tablets or E-readers late at night for four hours showed signs of melatonin suppression, disruption of the onset of sleep phases and reduced alertness on the next morning when compared to fellow subjects who spent the same time reading a paper book. It is important to note that the digital tablets were set to emit roughly 30 lux of light intensity which is still substantially higher than the dim light that would reach hominins sitting around a campfire.

The findings of Chang and her research team also point to another reason why it is unlikely that suppressing the clock hormone melatonin was instrumental in accelerating human evolution. Artificial light levels at night which suppress melatonin are detrimental for our health. Chang’s study subjects merely showed reduced alertness in the mornings but the effects of light at night and melatonin suppression can be far more deleterious than has been previously appreciated.

The problem with attributing too many benefits to anthropogenic light is that scientists are increasingly discovering the dark side of light. Yes, light allows us to work, read and socialize at night but this control over light comes at a cost. By exposing our internal clocks to light at night, we are throwing them into a state of temporal disarray. Sunlight during daytime hours resets them to the solar day/night schedule but whenever we turn on the lights in our homes and offices at night, our clocks are again reset and deluded into thinking that it is once again daytime outside. Night-time alertness levels increase and the internal drive to sleep is suppressed or delayed but night-time light exposure has also been linked to diseases such as increased levels of obesity and diabetes. The precise biological mechanisms are still being investigated. Is it because light exposure directly interferes with our metabolism? Or is it because nighttime light makes us stay up late, develop poor dietary habits and eat food at a time when our metabolism is not ready for large amounts of caloric intake? Studies with mice suggest that even dim light exposure at night can make animals more obese than their counterparts who live in darkness at night, even if the total amount of food intake during a single 24-hour period is the same.

Satellite images of our planet show a steady increase in night-time light. Our quest for omnilumination at night not only harms fellow humans by disrupting hormonal rhythms, sleep schedules and metabolism, it can also have devastating effects on other animals and plants. Many animals are far more sensitive to light than we are and light intensities that are typically found at night-time in urban settings can suppress the immune system of hamsters or disrupt flight patterns of bats. Street-lighting at night is thought to increase safety for drivers and pedestrians but we often forget that it can act as an environmental pollutant which disrupts eco-systems. This is not only true for experiments in laboratory settings where animals are exposed to artificial light at night. Numerous recent studies have conducted field research in real-life urban settings. A study by Gaston and his colleagues compared the animal species living on the grass directly underneath street lights (approximately 19 lux at night) and those in between street lights (3 lux at night). The composition of the species differed markedly with the street lights tipping the balance in favor of predators and scavengers and reducing the numbers of grazers. The numbers of ground beetles directly underneath the street lights quadrupled at night whereas the numbers of snails and slugs were cut down by half!

Having a few less snails and more beetles alongside a street may not seem like a major threat to our planet but these studies are just providing snapshots of the disruption in the ecological balance. The Philosophical Transactions of the Royal Society, the oldest English-language science journal in the world, published a whole issue devoted to light pollution in 2015, its 350th anniversary year. The issue “The biological impacts of artificial light at night: from molecules to communities” showcases research on how artificial light can have adverse effects on the migration of bat species, the biological rhythms and reproductive maturity of city blackbirds and the immune system of crickets. The pervasiveness of the impact of artificial light should give us pause because it means that the long-term consequences of artificial light may be both severe and unpredictable.

Franz Hölker and his colleagues at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin have characterized light pollution and the resultant loss of true night as a “biodiversity threat”. By disrupting the feeding and reproduction cycles of the species on our planet, light pollution is changing the relative composition of populations. Daytime predators can extend their day in the setting of artificial light at night and prey on vulnerable nocturnal animals, who are disoriented by the loss of night. Loss of nocturnal species such as bats, which are critical for dispersing seeds, controlling insect populations, and pollinating flowers, can snowball into devastating effects for entire eco-systems.

It has taken humankind quite a bit of time to realize the ecological impact of climate change or anthropogenic global warming (AGW) but the conversation about anthropogenic global lighting (AGL) is only starting. Understanding the biological and ecological effects of light are just the first step, but we will then need to move on to take appropriate measures which contain the ecological impact of night-time light. What are these measures? Simple energy conservation in which we limit light usage is an obvious common-sense solution but more intricate measures that are based on the biology of light perception and circadian clocks will likely play a bigger role in the future. Not all wavelengths of light are equally potent in terms of affecting internal clocks and hormone levels, so using light sources with defined wave-lengths instead of white light which contains all wavelengths might be another option. Creating dark refuges for animals at night in the urban setting, minimizing night-time light exposure in our homes, and reducing shift-work in offices and factories may also contribute to restoring health and the ecological balance.

We started out to examine how the light emitted by the campfires of our hominid ancestors helped us become human and found there is not much scientific data support for a beneficial role of campfires as regulators of our internal clocks. Campfire lights are probably too dim to have had any effect on the clocks of hominins and even if they did, these effects would have been more deleterious than beneficial. Does this mean that the “cooking hypothesis” is the best explanation for the role of fire as a driver of human evolution? What fire did or did not do for hominins 350,000 years ago belongs in the realm of speculation, but I think that Frances Burton’s idea about the significance of campfires as a light source may be true, even if this effect is not necessarily due to the suppression of hormone levels and resetting of circadian clocks.

Fascinating data collected by the anthropologist Polly Wiessner at the University of Utah may shed some new light on the role of fire in human evolution. Wiessner has spent many years studying the Ju/’hoan (!Kung) Bushmen of southern Africa, a contemporary hunter/gatherer society. A modern day hunter/gatherer society does not necessarily function in the same manner as our hunter/gatherer ancestors did several hundred thousand years ago, but as Wiessner points out, there are many characteristics that the Bushmen share with our ancestors, such as the hunter/gatherer diet, living in small groups of 15-40 individuals, residential groups consisting of kin and non-kin as well as a fire-centered nightlife. These similarities allow us to make some educated inferences from contemporary Bushmen practices regarding our evolutionary past.

Wiessner collected and analyzed the daytime and night-time conversations of the Bushmen. Daytime conversations mostly consisted of economic topics and complaints – work, money and grumbling. But night-time conversations took place around a campfire and had a very different focus. Here is an excerpt from Wiessner’s fascinating study:

After dinner and dark, the harsher mood of the day mellowed and people who were in the mood gathered around single fires to talk, make music, or dance. Some nights large groups convened and other nights smaller groups. The focus of conversation changed radically as economic concerns and social gripes were put aside. At this time 81% of lengthy conversations involving many people were devoted to stories; these stories were largely about known people and amusing, exciting, or endearing escapades. Storytellers did not praise heroes or moralize; advancing oneself in the moral hierarchy or demoting others was avoided, as was any form of self-promotion. No doubt, listeners gleaned unspoken lessons from stories. When a story was over, others rehashed details, embellished, and discussed. The language of stories tended to be rhythmic, complex, and symbolic, with individuals repeating the last words of phrases or adding an affirmative “Eh he.” Frequently listeners were stunned with suspense, nearly in tears, or rolling with laughter; they arrived on a similar emotional wavelength as moods were altered.

Night-time is story-telling time. The Bushmen take turns in telling enthralling stories which are full of suspense, humor or even tragedy. These are not moralizing lectures about work and duties but invite listeners to participate, embellish and share their own stories. The light of the fire may not be enough to shift circadian clocks but it allows the Bushmen to see each other’s smiles and tears, providing feedback and communication that cannot occur in complete darkness. The Bushmen do not use firelight to extend the work day into the night, they use it to create a forum for creative story-telling and social exchange.

If our hominid ancestors engaged in similar conversations, then it may not be far-fetched to propose a new hypothesis for the role of fire in human evolution. In addition to the “cooking hypothesis”, we should consider the “creativity hypothesis” which states that controlled fire allowed our ancestors to dedicate leisure time at night for creative story-telling. Let us take a moment to imagine a scene in which hominins sat around a campfire. They had finished cooking and eating their final meal of the day. It was too dark to go hunting or sharpen tools, so they just sat there, cherishing the warm glow emanating from the fire. At some point, a member of the group began using gestures or verbal language to tell a story. It was not a technical communication about where to hunt or about assigning new duties, it was ‘just” a sharing of one’s experiences. Communication for the sake of communication. Imagine the delight that the other members experienced upon hearing these first semblances of a story and the joy of the narrator when she saw the reactions of her enamored audience. Perhaps the idea of story-telling caught on and soon more members of the clan or tribe wanted to share their stories, forcing the group to improve, develop or expand its vocabulary. This development of language skills, the unfettered imagination to create or embellish stories and the social interactions during campfire story-telling may have been some of the sparks which ignited the cognitive evolution of humans. It is this interpretation of Wiessner’s data which lead the anthropologist and evolutionary psychologist Robin Dunbar to conclude that “fire and language may be more closely related than conventional views assume”.

We do not need to only rely on recorded conversations of the Bushmen in order to relate to the “creativity hypothesis” and the role of campfires in the development of language skills in our evolutionary past. Since my youth, I have always felt the strong urge to hear or share stories while sitting next to a campfire with friends. In my own experience, night-time is also when it is easiest to write or read. Free from the mundane demands and drudgery of work, night-time allows one’s imagination to roam free. And just perhaps, when we sit down as groups to watch actors perform on stages or movie screens in the evenings, we are in some ways reconnecting to the creative story-tellers among our ancestors.

It is likely that creative story-telling around the camp-fire was important for the evolution of the human mind. The pre-historic usage of the campfires as an artificial light source may have struck just the right balance: the light was bright enough to allow for social interactions and story-telling but not so bright that it would disrupt our internal clocks or those of the species with whom we share this earth. The current trends in contemporary usage of artificial light are more concerning. Bright lights at night are adversely affecting the biology and physiology of humans and many other animal species. Instead of using artificial light as a means to engage in creative story-telling at night, artificial light is now extending our workday into the night. Leisure time is losing out to economically desirable productivity time. We need to learn how to use artificial light responsibly, in a manner that which allows us to engage in creative activities while preventing the adverse effects of light pollution and unhealthy work schedules at night. The German colloquial word for the end of a work day is “Feierabend”, which literally means “celebratory evening”. We could try to connect to our evolutionary past and celebrate the end of our workday by exercising our playful imagination in the service of story-telling.

Jalees Rehman, MD is a German scientist and physician, and he is currently an Associate Professor of Medicine and Pharmacology at the University of Illinois at Chicago. He initially trained as a circadian biologist but his laboratory now studies the biology of regeneration and cellular metabolism. In addition to working as a scientist, he is also a science writer and essayist. He blogs Fragments of Truth.

He can be followed on Twitter: @jalees_rehman.