Then her skin gradually darkens until she resembles one of those "blackface" comedians in early 20th-century America. She looks down at herself in dismay, then at another Thai beauty, very white indeed, who magically appears beside her. "If I was white, I would win," Horwang says to herself.

But salvation is at hand: her young rival generously points to a package of Snowz that magically appears between them. Cris brightens up, and so does her skin. The ad ends with her smiling again and saying: "Eternally white, I'm confident."

After an eruption of protests the ad was withdrawn with "heartfelt apologies" from Seoul Secret. But they didn't withdraw the product.

They would have been crazy to take it off the market, because Asian women spend $13 billion a year on skin-whitening products. Africa is even more extreme, with 35 percent of South African women using skin-whitening creams, and an astonishing 75 percent of Nigerian women.

Several hundred million women are using these products regularly, despite a range of possible side-effects that run from ochronosis (which causes the skin to turn a dark purple shade) to leukemia, diabetes and cancers of the liver and kidneys. But why? What's wrong with dark skin?

Nothing, obviously. Originally, several million years ago, all our ancestors were "white", but they weren't actually human yet. They had pale skin, like most animals with fur, because the fur blocked most of the incoming sunlight, and pale skin is six times more efficient in turning the ultraviolet light (UV) from the sun into the vitamin D that they all need.

When modern humans evolved, they lost their fur – and since they evolved in equatorial Africa, where there is an abundance of sunlight, their bare skin was actually getting too much UV. So early humans developed dark skin to cut down on their UV intake, and early humans were all "black". Then some of them migrated out of Africa and colonised the rest of the planet, including the parts in the high latitudes where there is much less UV in the sunlight.

The ones who ended up in northern Eurasia went back to having pale skin – Europeans in the west, North Asians in the northeast – in order to make better use of the limited UV that was available. And that's the end of the story: we ended up with the skin colour that suited where we lived.

The details are more complicated, of course. Bare skin was getting plenty of the UV that triggers the production of Vitamin D, which helps calcium absorption (for bones and teeth) and is needed for various metabolic processes. But so much UV also depleted another vitamin, B12, which is essential for a healthy nervous system and other key metabolic processes. It also risked causing severe sunburn. Dark skin solved both those problems.

When the migrants moved north, there was no risk of sunburn and no threat to Vitamin B12, because far away from the equator the UV rays come in at a slant through the atmosphere, not straight down, and most are absorbed before they reach the surface. In fact, there was not enough UV in the north to make Vitamin D, especially in winter.

So evolution went into reverse, and by 10 or 20,000 years ago practically everybody living north of the Mediterranean and the Himalayas was pale-skinned, to make better use of the limited UV that was available. And that's all there is to know about skin colour. It's not good or bad; it just suits the geographical circumstances.

So why the prejudice against dark skin? It's all about history, but it's much older than the European conquest of the world in 1500-1900. That left particularly deep psychological scars, but light-skinned people from the north have been conquering dark-skinned people further south for thousands of years.

The reasons are too complex to go into here, but it had nothing to do with skin colour. (See Jared Diamond's classic book Guns, Germs and Steel for a plausible explanation.) Nevertheless, dark-skinned people have been the historical losers for thousands of years, and people tend to blame themselves for losing.

Add in a few details like the European and Arab slave trade in Africa, and the fact that people who work outdoors, and therefore have skin darkened by the sun, tend to be in the lower social classes, and you have an explanation for the internalised prejudice against dark skin even in many dark-skinned people.

But this prejudice really is on the way out at last. The most important thing about that awful advertisement is not the fact that it was made. It's the fact that the outcry in Thailand forced it off the air.

Gwynne Dyer is an independent journalist whose articles are published in 45 countries.