I’ve started to post this a couple of times, but couldn’t think of the right words to say what I mean, so I didn’t. This is my third attempt, and I’m gonna post it no matter how it comes out!
Humans are a peculiar lot when it comes to evolution. We have the ability to modify our environments to a degree that no other life form on earth has. In a sense, we are simply dictating our own conditions for natural selection. In another sense, we are practicing artificial selection on ourselves. And, in yet another sense, we are only fooling ourselves to think we have elevated ourselves beyond ‘natural’ natural selection.
With reference to the latter: we are becoming increasingly dependent upon technology for the ‘advancement’ of our species. Because of this, we are creating a sort of insulatory bubble, from which we shield ourselves from the ‘real’ world. Within our techno-bubble, we can advance / evolve / develop / whatever you choose to call it. Outside of the bubble, however, we are little better off now than we were 10,000 years ago. The same diseases would still kill us without our medicines; the sick and affirmed would still get picked off by predators and the like. If any of us were to be dropped buck-naked into the Congo, we would probably not even have as good a chance of surviving as our ancestors did.
So, we have basically two options. The first is to abandon all of our technology and submit ourselves to REAL natural selection (thereby actually increasing immune systems, tolerance of temperature extremes, etc.), or commit ourselves to techno-evolution and be forced to create artificial environments for the rest of the duration of our species, and artificially enhance ourselves to overcome biological limitations (wherein anything detrimental that might weed a given individual out of the population is countered by technology).
Having said that, the ‘next steps’ in human evolution are not likely to occur on Earth. Most instances of speciation involve peripheral isolates - groups which become separate from the main population and are thus able to overcome the damping effect large populations have on gene frequencies. So, my personal prediction is that humans will largely remain as we are (from a biological standpoint) unless we begin to colonize other systems. These colonies, then, would become the next pool for evolutionary change. However, it is nearly impossible to predict how we might evolve, since that would depend on whatever environments these colonies might find themselves in.
Neither the depletion of the Earth’s ozone layers (note the plural), nor the link between ozone depletion and melanoma, are as firmly established as you seem to believe.
Ozone primarily blocks light in the ultraviolet-B frequency range, while letting the lower-frequency ultraviolet-A pass through pretty much unattenuated. Some recent studies have shown that, while ultraviolet-B causes sunburns, it’s ultraviolet-A that has the stronger link with skin cancer (melanoma). If that’s true, we’re in far bigger danger of getting melanoma from tanning booths than we are from ozone depletion.
Sorry, didn’t do my PubMed search. I stand corrected about the nitpick. It doesn’t change my argument, however. We will get darker, as darker skinned populations grow and cross population breeding increases with more travel. Add this to the fact that skin cancer is the most prevalent type of cancer in many parts of the world, and has an increasing incidence for whatever reason. Granted, it is far from the largest cancer killer due to ease of detection, but there is still a selective advantage to not getting cancer.
So:
-There is a decrease in fitness from being fair skinned and blond.
-There is a very sizable segment of the population who has a selective advantage by carrying more melanin in their skin.
It would stand to reason that there is now a selective pressure to darken (assuming the rate of death from skin cancer stays high, and aggressive melanoma is still a pretty terrible thing to have).
If we all came out of the Olduvai Gorge 150,000 years ago and migrated to all the corners of the world, I can’t readily explain why we lost our skin pigmentation if we started with it in the first place. There don’t seem to be any disadvantages associated with it. It was probably sexual selection – maybe the ancient Norsemen liked their women to match the snow.
Now that we have entered the age of the blonde Australians and intermingling populations, I bet these sexual preferences will quickly lose out to good old natural selection.
Edwino: The trouble with your “darker skin” prediction is that most people who die from skin cancer have already had most of their children. Skin cancer isn’t typically a disease of the young. You usually get skin cancer in your 40s, 50s and beyond. So even if we had a severe skin cancer problem it wouldn’t have that much effect on fitness. And I imagine that Africa is going to have negative population growth very soon. The AIDS epidemic is just getting started over there, in some countries 25% of the population is infected. I don’t see any way to avoid a Black Plague style die-off even if we had a vaccine tomorrow. I agree that we will see a more homogenous population, which means fewer pureblooded europeans, which means fewer extremely white individuals. But the average level of pigmentation probably won’t change significantly in the next couple of thousand years barring some sort of racial artificial selection (like race wars and ethnic cleansing).
Skin cancer incidence is going up. Age of onset is going down. I have many friends who have had pre-cancerous growths (nevi or solar keratoses) or actual cancers (basal cell carcinomas, which I know is rarely life-threatening) removed in their early 20s. Granted, we are medical students and of high vigilance. If you can’t readily pin this on ozone depletion, and the studies are still basically out on this one, I don’t know what you can pin it on.
I was in Europe 2 summers ago with a bunch of Australians and New Zealanders. Out of about 30 of them I met, I know at least 3 or 4 of them had cancers removed already. One of them already had a small melanoma removed from her shoulder. None of them were over 30.
With the AIDS population, your movement still holds. AIDS necessarily doesn’t kill until breeding age (except for blood-borne and perinatal transmission). Perinatal transmission will soon be reduced (hopefully) by prophylactic AZT to pregnant women. Within a few generations, education hopefully will reduce the AIDS epidemic to manageable levels. Granted, it will dent the population growth curves for the next century or so – as did the Black Plague in Europe. I don’t think it will be enough to push it below ZPG however. Also, the two most populous countries on the planet (India and China) have smaller AIDS crises (except in parts of South India where it is close to 33% carriage) and have darker skin.
With such population movement, in the overall scheme of things, it will again become advantageous for us to get darker skin, even if 1:1000 ends up having reduced fitness due to early death from cancer. That’s quite a big difference in fitness for evolution. I’m pale. My wife is pale. I live in Houston. If my descendants continue to live here for another 10,000 or 100,000 years, I should hope that they get darker again.