4 ms·

Agree, but only at the margins. That one thing, in and of itself isn't a reproductive failure. But over a population with varying levels of, say, hunting profic

Agree, but only at the margins. That one thing, in and of itself isn't a reproductive failure. But over a population with varying levels of, say, hunting proficiency, there will be enough people who are already not very good at hunting and a gene that makes you 3% worse will result in them having less or no children.

lurquer 9y ago
Is there a correlation between reproduction and eyebrows? Seems like a simple thing to study. I doubt anyone has. (And, I doubt anyone will.) It's easier to simply conjure up stories about how eyebrows must have had some beneficial effect and then hand-wave while muttering 'millions and millions.' Given the assumed population levels, the timescales involved, and the minuscule statistical effect of something like eyebrow-ness on survival, I expect 'survival of the fittest' had nothing to do with eyebrows. Rather, it's just a random bit of 'noise' in our DNA.
- toasterlovin 9y ago
  A couple of things: 1. Eyebrows are a pretty elaborate structure to have evolved purely by chance. You don't get from not having eyebrows to having eyebrows via a single mutation. It seems reasonably likely that the specific shape of eyebrows (as in, one person has bushy eyebrows and another has thinner eyebrows) might be tied to sexual selection (as in, the variations don't serve a functional purpose other than being attractive to the opposite sex). But that eyebrows exist at all is probably tied to some survival/reproductive benefit. 2. I think you underestimate the impact that something as a small as, say, a 3% improvement in an outcome can have on the evolution of a specific trait. You can use a spreadsheet to model this yourself. Start with a population where 1% of people possess some trait that allows that subpopulation to have 3% more children per generation that the other 99% of the population. I'm showing that it takes a little over 150 generations for the descendants of that first group to comprise 50% of the population. As a point of comparison, there have been roughly 7,000 generations of humans. So, you can see that natural selection will act even on minute details of anatomy because when you play them out for many generations they matter.
  - Daniel_sk 9y ago
    I guess no one has mentioned this - but they definitely have a function. Their main function is to prevent sweat, water, and other debris from falling down into the eye socket, but they are also important to human communication and facial expression. It is not uncommon for people to modify their eyebrows by means of hair addition, removal and makeup.
    - njarboe 9y ago
      Nice to finally see this comment after much text implying that eyebrows are some crazy useless feature of humans. The ability of humans to sweat for cooling is one critical feature that has helped the species survive. The full body sweating that allowed humans higher endurance than any other animal on the African plains would have been much less useful if they were continuously blinded by the salty sweat pouring down off their foreheads.
      - toasterlovin 9y ago
        FWIW, I was specifically trying to leave the actual purpose of eyebrows out of the argument. We will probably never have a definitive answer to the question of why we have eyebrows. The best we'll ever do is some decent hypotheses. So if the argument is that eyebrows serve function X, but we find out in the future that eyebrows do not, in fact, serve function X, then a naive observer will think that eyebrows did not evolve to serve any purpose and are, in fact, just a random feature. IMO, a more thorough argument is to point out that the only way species-wide, complex morphological features like eyebrows can possibly exist is if they serve an important purpose, even if that purpose has a miniscule impact on the outcomes of any given organism. This argument is resistant to any given explanation for the purpose of eyebrows turning out to be wrong. Plus it gets to a deeper understanding of evolution. P.S. I'm skeptical about the sweat in the eyes explanation for eyebrows. Anecdotally, my eyebrows have never kept sweat from getting in my eyes. And less anecdotally, if you spend a few minutes looking at somebody's face and imaging sweat rolling down their forehead, you'll see immediately that eyebrows aren't very effective for sweat diversion. My money is on protecting our eyes, which are our most important sense organs. If you google image search "mma black eye", you'll see what I think is kind of remarkable: despite serious trauma to the surrounding areas, most people's eyes are intact.

lurquer 9y ago

Is there a correlation between reproduction and eyebrows? Seems like a simple thing to study. I doubt anyone has. (And, I doubt anyone will.) It's easier to simply conjure up stories about how eyebrows must have had some beneficial effect and then hand-wave while muttering 'millions and millions.' Given the assumed population levels, the timescales involved, and the minuscule statistical effect of something like eyebrow-ness on survival, I expect 'survival of the fittest' had nothing to do with eyebrows. Rather, it's just a random bit of 'noise' in our DNA.

toasterlovin 9y ago
A couple of things: 1. Eyebrows are a pretty elaborate structure to have evolved purely by chance. You don't get from not having eyebrows to having eyebrows via a single mutation. It seems reasonably likely that the specific shape of eyebrows (as in, one person has bushy eyebrows and another has thinner eyebrows) might be tied to sexual selection (as in, the variations don't serve a functional purpose other than being attractive to the opposite sex). But that eyebrows exist at all is probably tied to some survival/reproductive benefit. 2. I think you underestimate the impact that something as a small as, say, a 3% improvement in an outcome can have on the evolution of a specific trait. You can use a spreadsheet to model this yourself. Start with a population where 1% of people possess some trait that allows that subpopulation to have 3% more children per generation that the other 99% of the population. I'm showing that it takes a little over 150 generations for the descendants of that first group to comprise 50% of the population. As a point of comparison, there have been roughly 7,000 generations of humans. So, you can see that natural selection will act even on minute details of anatomy because when you play them out for many generations they matter.
- Daniel_sk 9y ago
  I guess no one has mentioned this - but they definitely have a function. Their main function is to prevent sweat, water, and other debris from falling down into the eye socket, but they are also important to human communication and facial expression. It is not uncommon for people to modify their eyebrows by means of hair addition, removal and makeup.
  - njarboe 9y ago
    Nice to finally see this comment after much text implying that eyebrows are some crazy useless feature of humans. The ability of humans to sweat for cooling is one critical feature that has helped the species survive. The full body sweating that allowed humans higher endurance than any other animal on the African plains would have been much less useful if they were continuously blinded by the salty sweat pouring down off their foreheads.
    - toasterlovin 9y ago
      FWIW, I was specifically trying to leave the actual purpose of eyebrows out of the argument. We will probably never have a definitive answer to the question of why we have eyebrows. The best we'll ever do is some decent hypotheses. So if the argument is that eyebrows serve function X, but we find out in the future that eyebrows do not, in fact, serve function X, then a naive observer will think that eyebrows did not evolve to serve any purpose and are, in fact, just a random feature. IMO, a more thorough argument is to point out that the only way species-wide, complex morphological features like eyebrows can possibly exist is if they serve an important purpose, even if that purpose has a miniscule impact on the outcomes of any given organism. This argument is resistant to any given explanation for the purpose of eyebrows turning out to be wrong. Plus it gets to a deeper understanding of evolution. P.S. I'm skeptical about the sweat in the eyes explanation for eyebrows. Anecdotally, my eyebrows have never kept sweat from getting in my eyes. And less anecdotally, if you spend a few minutes looking at somebody's face and imaging sweat rolling down their forehead, you'll see immediately that eyebrows aren't very effective for sweat diversion. My money is on protecting our eyes, which are our most important sense organs. If you google image search "mma black eye", you'll see what I think is kind of remarkable: despite serious trauma to the surrounding areas, most people's eyes are intact.