As I was walking to the parking lot after work today, the sun hit my right eye directly through the space between my head and my sunglasses. I marveled about how life evolved a sensory organ that can so easily be burnt out by something that is in the sky all over the planet for 12+ hours for half of the year. At first, this seemed odd to me, and I began wondering whether this level of photosensitivity is a “standard” trait. In other words, if we lived next to a warmer but dimmer sun, would we have evolved eyes like ours that would not be burnt out by looking directly at our sun, or would we have eyes with a proportionately higher photosensitivity to make that hypothetical sun appear just as bright as our brighter sun appears to us here on Earth? Conversely, would we have eyes that could stare at our sun here on Earth if we had evolved on a planet with a cooler but brighter sun?
This led me to marveling about our “comfortable” temperature range. Obviously our comfort level is somewhat limited by the freezing (0 oC) and boiling (100 oC) points of water at terrestrial pressure, which is essential for life as we know it and constitutes 65-90% of our living cells. Our body temperatures are relegated to a temperature that falls right at about 1/3 of that temperature range, 37 oC. I started to wonder why it is so low in comparison to the full range of temperature available to us within the regime of liquid water. According to Vostok ice cores, the average global temperatures when modern humans evolved 200,000 years ago and even earlier ranged from 8 oC cooler to 3 oC warmer than they are today. The average temperature in central Africa (we evolved in Africa, but where and what climate are still somewhat debated, so this is just a starting point for thinking about the problem) is about 27 oC, 10 oC cooler than our internal body temperature. However, we are actually made up of millions of metabolic furnaces (our cells), and we need a heat sink for all the energy released just from the chemical processes that keep us alive, so it just makes sense that we’d evolve to have an average internal temperature relegated to above the average annual temperatures where we evolved. It also makes sense that our average internal temperatures would be higher than the typical expected summer high temperatures, but not much higher than that so that we wouldn’t have to waste energy heating ourselves year-round. I think I can say that our internal body temperature therefore makes sense to me, after really thinking about it.
However, our experience of discomfort when touching cold or hot things and where those temperatures lay within the regime of liquid water still seemed odd to me. In my own experience, it’s not that uncomfortable to touch extremely cold things (rather briefly, mind you), even down to -80 oC if the substance isn’t that conductive (like dry ice). For conductive things, I still only begin to experience discomfort below the freezing point of water. However, I begin to experience discomfort touching hot things well below the boiling point of water (like sink full of really hot dishwashing water). This seemed odd to me. Why would we be okay touching things that are so very cold compared to the temperatures that would freeze our cells to death and not be okay touching things that aren’t really all that hot compared to the temperature that would boil our cells to death?
I started to think about this, too. When we evolved, we were in a relatively warm climate. We didn’t have much exposure to the ultra-cold temperatures that would freeze us to death, so there wasn’t much stimulus to develop nerve cells that would give an immediate “remove your hand from that, right now!” neural response to touching something extremely cold. However, there was definitely fire, even if we couldn’t control it when we evolved. Fire is a natural phenomenon on a planet covered in reduced carbon (living and dead life forms) under an oxygen-rich atmosphere.
Begin digression. We’re always in a thermodynamic dis-equilibrium that would result in a big boom of fire if it weren’t for a nice activation energy barrier that must be overcome. The best way to get over this activation energy barrier is called a “spark” when there isn’t enough water to absorb and dissipate the energy away from a potential flame, which is what we call the rapid combination of oxygen and reduced carbon that actually should be happening all the time. If there weren’t the nice activation energy barrier and lots of water around keeping us all safe. That also freaks me out, but may be another story for another time. End digression.
Anyway, our ancestor hominids definitely evolved in a landscape that included fire, and probably not much ice. They definitely were not exposed frequently to items with temperatures below -20 oC, but they may have frequently (on evolutionary time-scales) been exposed to temperatures ranging up to 800-1000 oC, or the temperature of a grass fire. If a grass fire is headed your way, and they can move at speeds of more than 80 mph, then your nerve cells had better warn you early to get the heck out of dodge. Near a stationary fire that isn’t roaring to get you at 80 mph, the distance between “I’m SO COLD” and “I’m BURNING HOT” isn’t very far, as anyone knows who has stood facing a campfire in loose-fitting jeans and upon sitting down experienced the joint discomfort of really cold pants hitting the back of the leg and really hot pants hitting the front of the leg. Therefore, I suppose it makes sense that we evolved nerve cells that warn us “early,” well below the boiling point of water and long before we can actually get our hand to a hot flame, that we are about to stick our hand in a fire.
Now back to my original question: would our eyes burn out looking at a dimmer sun if we had evolved on a planet with a dimmer sun? I don’t have the answer, but I bet it could be found by studying species that evolved to take advantage of the low light environments of caves. Maybe the answer is already out there in the scientific literature or even on the information highway we call the internet. If you know, please tell me!
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