How many generations to change skin colour?

As I understand it, all non-African humanity descends from a small group of humans who crossed from modern day Somalia / Ethiopia to modern-day Saudi Arabia about 75000 years ago. Presumably these people were black, as modern-day Somalis are. (Or were they?) Today, 75000 years later, there are vast differences in skin colour, from lily-white to jet black. How long did these changes of skin colour take? This is most apparent in India where in the north they’re almost white but in the south, they’re almost black. Did the Australian aborigines lose their blackness, then regain it? Presumably the process takes over 10K years because the variation in colour in native Americans is much less.

Or could non-blackness result from interbreeding with Neanderthals? Presumably they were extinct in Africa long before homo sapiens left.

It would be nice to point out to racists that X generations ago their forebears were black / whatever.

Changes in melanin distribution could occur with a single generation if the governing genes changed and the modified descendants reproduced successfully.

For the life of me I can’t figure out why this would offer either solace to non-racists or a poke in the eye to racists.

We are the product of our genes and very few of us that I know of think you get to choose your parents. Pretending that one’s genetically-based phenotype is some sort of accomplishment is simply another way of saying “I don’t understand genes.”

It’s not an accomplishment to be born smart or tall or fast or blonde.

I’ve seen the term “racist” used two different ways (broadly speaking) when it comes to genes:

Those who believe various human cohorts have different average prevalences of genes which create average differences in phenotypic expressions for those population cohorts, and

Those who believe various human cohorts have different average prevalences of genes which create average differences in phenotypic expressions for those populatin cohorts which results in some sort of moral superiority pasted upon one cohort by the sole virtue of having been born into that gene pool.

I’m not aware the concept of successful descent with modification applies to either group, since everyone pretty much thinks we all descended from the same ancestor, whether it’s Adam and Eve 5,000 years ago or Mr and Mrs Homo Prime Sapiens 200,000 years ago…

If you want to have fun with the racists, point out that us white folk have the “black gene”, it’s just that ours doesn’t work right. Or, to put it in perspective, white people are really just defective black people.

We have the gene that is supposed to produce melanin in our skin. In white people these gene contains a defect so that the proteins don’t copy properly (or something like that - this is from memory) and melanin isn’t formed.

As far as I am aware, the change from black to white happened fairly quickly, and scientists really aren’t exactly sure why. One of the most convincing (to me) theories that I have heard is that we’re white basically because the climate of northern Europe sucks. Light skin allowed us to produce vitamin D more easily in reduced sunlight.

It’s also interesting to note that lighter skinned Asians have a defect in a different gene, indicating that their light skin evolved separately from ours.

Too many people think natural selection is a steady and even progress. All it takes is one specific change and then a little bit of time for that change to work through the population. If it is a dominant trait, highly favored by the environment and/or sexually selected, it can spread very, very quickly.

It’s also incorrect to assume that all black people are of identical blackness or genetic composition. There are at least six genes involved in skin and hair color and you can tell differences between tribes or people groups in Africa even if they’re normally all lumped into “black.”

Within a single generation, if Michael Jackson was any indication.

Reference the light skinned people of Northern India the local legend is that they are the descendants of Alexanders army.

Though I have met people of the Brahmin caste who are as white as any European.

Just as an aside we were forced to watch an American military training film(Just before lunch coincidentally) called Medecine in Vietnam that was particulary graphic in its content and which showed amongst other things a black bloke who had been hit by Napalm, and under the main layers of skin he was totally white skinned.

Then how come people of African descent who live in Europe and Canada haven’t started producing white babies?

I don’t know where to begin. It’s sort of like asking why near-sightedness isn’t selected against (hint: glasses).

  1. Vitamin D is now readily available from sources other than sunlight (for example, enriched milk)
  2. Vitamin-D deficiencies and rickets are readily identified and easily curable if caught early.

If those were not the case, and the babies people of African descent were dying in droves from such easily preventable and curable things, there would be strong selection pressure on their lighter-skinned offspring and / or those with the above-mentioned genetic defect that inhibits melanin production. However, since cultural advances mean that there is no harm in being dark-skinned in a northern climate, there is no reproductive advantage to having a white baby. I’m sure it happens with the same frequency that it did 10,000 years ago, it’s just that there’s no longer any selection pressure to give that white baby an advantage. (The flip side is also true, otherwise Anglo-Americans in the South and Southwest and Anglo-Australians would be getting darker.)

What about Lamarckian evolution? But seriously, the good doctor is correct.

I remember reading a calculation from an article in, I think, Science News. It was a surprisingly small number of generations. This because there is a significant increase in mortality from vitamin D deficiency among dark-skinned living people in northern latitudes - without supplements, that is.

Strictly speaking the correct answer would be “because evolution happens when a gene which happens to be beneficial appears by chance and survives in the gene pool.” there’s no mechanism in the human body saying “Hmm, there seems to be different levels of sunlight about, best make sure these guys have kids with less menanolin in their skin”.

There are too many variables to say for sure. For example, you might try to set some sort of minimum time based on the observation that the skin color of Native Americans does not vary all that much according to latitude, and most Native Americans (excluding Inuits and a few other groups) are thought to be the descendants of a single NE Asian ethnic group that migrated to the continent somewhere around 12,000-15,000 years ago. But human behavior was very different at that point than it was 100,000 or so years ago when the ancestors of today’s non-Africans were first moving into Asia. There’s no reason to believe that patterns of gene flow between neighboring groups, in particular, were the same at those two points in time.

The legend about old Alexs army refers specifically to Chitralis, who inhabit the most NW district of Pakistan; Chitral

As it is, there have been migrations from the NW to S Asia from millenia, the Aryans, the Scythians, the White Huns etc. In addition you had the Persian and Greek and Arab empires and each contributed to the local genetpool. hard to use the place as a template.

There have also bee migrations from the south via the sea, to this day most of Pakistans coastline west of Karachi is majority black African.

I’m not an anthropologist but descendents of Africa may not have been what we would classify as Negro today. Africa wasn’t always the arid continent it is today so the traits driven by environment may have been different. The Himalayan Mountains are tectonic and as they grew taller they affected the weather patterns over the African continent.

Impossible to tell. It’s at least equally plausible that they were brown skinned like the modern Khoi. That seems to be the primitive condition in humans, with both black and white skin being derived from that form.

Well, there are skin colours ranging from light brown to jet black in sub-Saharan Africa and always have been. So only the lily white shade is plausibly novel, and there is speculation that it is derived from Neanderthal influence. So it’s plausible that all the current skin colours were already common 100, 000 years ago.

India, like most of Southern and SE Asias was originally populated by dark skinned Australoids. So the colour distribution has nothing to do with variationsin skin colour and is solely attributable to immigration patterns.

Unlikely. One hypothesis holds that the original extra-African sapiens were Australoids, and hence black when they left Africa. The other hypothesis is that they were brown skinned, and that it was exposure to the coastal regions of SE Asia that prompted the shift to the darker skin, just as exposure to the open habitats of Africa did.

It is less, but not much less. I’m betting that I could find native Americans that are the same shade as people living in the same latitudes in Africa or Asia. The only thing that’s really lacking are the absolute darkest shades.

Neanderthals seem to have persisted longest in Southern Spain, a matter of miles form Africa. They certainly survived in Israel and Turkey for tens of thousands of years after the first sapiens left Africa. Given that any African exodus must have passed through either Israel or Spain. it’s certainly not implausible that the paler skin tones derive form Neanderthals.

Save your breath. Most racists believe that the first humans were Adam and Eve, and they were white and lived 6, 000 years ago and spoke English, just like Jesus.

Afrikaaners have been living in SA for over 300 years, and they seem every bit as white as modern day Dutch people, so it doesn’t look like the process is even perceptable in that time frame.

Here’s a good place to start to learn about the history of human skin color:

http://en.wikipedia.org/wiki/Human_skin_color

You’ll find links to articles like this:

http://mbe.oxfordjournals.org/cgi/reprint/msl203v1.pdf

But SA is only as far south as North Africa and Saudi Arabia is north. Which doesn’t prove or disprove the point because I don’t know if one people have lived in that latitude for a sufficient length of time to determine what their “natural” color “should” be – perhaps it’s somewhere in between (witness the slightly swarthier South Europeans and Semitic peoples.)

And perhaps that latitude, rather than actively promotes a medium skin tone, is neutral toward skin tone – neither providing a net benefit nor loss no matter what your skin tone is (i.e. the vitamin D could completely balance out the sun exposure.)

Correct. People (all living things, for that matter) don’t evolve because of “where they live.” They mostly evolve because lots of them die without passing on their genes, but some don’t – if that difference happens to be because of where they live, your thesis starts to make sense. Natural selection is a hecatomb; it’s driven by death, death in bulk numbers. Without the croaking you won’t see natural selection, leaving the weaker agents of evolution (sexual selection and genetic drift) in charge.

Why would they change? Dark skin would have to be a reproductive advantage. It might have been a different case when people did not have ready access to permanent shelter, modern fabrics, and sunscreen, but with these things the light-skinned will not die off before they breed in any significant numbers. That’s what has to happen for a population to change: it’s not that a trait is selected FOR, it’s that other traits are selected AGAINST, removing them from the population.

Take away any mitigating cultural factors and leave a group of lily-white Afrikaaners and a group of X!hosa alone for a couple of generations and we’ll see what we get. For fairness’ sake, we’ll fly half of each population to the Yorkshire moors. Biologically, we haven’t changed much in the last 10,000 years, but by keeping nature at a remove we’ve gotten around some of the pressures of natural selection.

(my bolding)
How is this defective? I know you’re trying to kick some racist ass (deservedly) but bieng white is not defective in any scientific definition. Many adaptation could be “defect” by your definitions.