Good looks are the target of natural selection because they honestly signal robustness to the slings and arrows of early development. The number one source of uglification for humans is infectious disease. If there's a high pathogen burden, people are going to put a greater emphasis on good looks in a mate -- they want their kids to make it through the gauntlet of bugs, and if you look good in a germ-ridden environment, you must have pretty good genes. (Buss & Gangestad 1993 showed this prediction came true.)
Across the world, where do most infectious diseases come from? Animals. These are zoonotic diseases. This explains the first big split between beautiful and ugly people across the world -- namely between those who've spent all or most of their history as hunter-gatherers, who are uniformly plain or ugly, and those who domesticated animals, who are at least OK to look at and often great. For the former, do a google image search for the Bushmen, the Eskimo, Australian Aborigines, and any Amerindian group such as the Ache or Native Americans.
Still, there's lots of variation within those who've been close to animals. The greater the threat of disease, the greater selection for good looks will be. This threat increases with the number of species you are in contact with, the number of individual animals you're near, how physically close you get to them, how long you're close to them, and in particular to the most disease-spreading parts of their body. Also, some animals may have nastier diseases than others.
This explains the next big split -- between East Asians, who only domesticated animals like the pig and chicken and duck, and most of the remainder of the Old World, which in addition to pigs and small birds also domesticated the largish grazing and browsing animals like horses, sheep, goats, cows, camels, and yaks. The Han (ethnic majority in China) are about as far off in the cultivated agriculture direction as you get, and they're not very good-looking (the world has judged this anyway, looking at magazine covers, beauty pageants, etc.).
The southern parts of India look this way too, where the people tend to look plain or bad and where a primary reliance on animal husbandry never took root. Contrast that to the northwestern parts of the Subcontinent -- there's a much heavier focus on animals, far more people can drink animal milk, and that's where the best looking South Asians tend to come from. (Like the Han, the southern Indians are also famous for being smarter and less rambunctious.)
Why does it matter if the animal is a smallish omnivore like a pig or a largish grazer like a sheep? Sheep herds tend to have more animals in them than swine herds (for say a Chinese farmer). You get a lot closer to sheep to shear them, bleed them, and milk them, whereas you only approach a pig to slaughter it (and that is also true for sheep). This difference in activities also means you spend more time close to sheep than pigs, and especially near their most dangerous areas -- when you're milking a sheep, you've got your hands right on the udders, and you're close to the animal's ass. Assuming you consume the animal's milk, you also risk disease; and people drink sheep's milk but not pig's milk.
There is a final and smaller split that this explains, namely between people who are primarily cultivators of plant foods and who have an odd cow or pig around vs. those who are primarily pastoralists (no matter whether they are sedentary or nomadic). Compared to cultivators, pastoralists are exposed to a wider variety of animals (historically, virtually none herded only one species), they have more of them, they get as close to them, but given how many more of them there are, they spend much more time up close. And the cultivator may or may not be consuming the dairy products -- if they mainly plant seeds, their odd cow or pig could be for processing and trading away on the market. Pastoralists are far more likely to use their animals for subsistence, and that is mainly through dairy (if they killed one every day or two for meat, their flock would be gone very soon).
Pastoralists are found throughout Europe (none of the nomadic kind, though), but especially along the Mediterranean, the Celtic parts of Britain, and other places you'd find listed in a World Atlas of Cheeses. Probably the least pastoralist group is the modern English (either moving more toward cultivation or industrialization), and they're just about as ugly as the Han Chinese, a real exception to otherwise OK or good-looking Europeans.
Moving southward, there are some fine looking nomadic pastoralists across northern Africa, although the real jackpot is around the Horn of Africa. This is probably the greatest concentration of pastoralists in the world, and it's also a supermodel powerhouse.
Western and central Africa has far fewer, as that's the homeland of the Bantu farmer expansion, but there are some in the west, and western Africans look OK. One of the lone groups of southern Africans who are pastoralists are also much nicer to look at than their farmer neighbors -- the Himba.
Just about all of western Asia has a very long history of pastoralism, and again we find some of the best looking people in the world. I've already covered the good-looking people of the northwestern part of the Subcontinent, but the various Persian and other ethnic groups in and around Iran are all overflowing with honey bunnies, not to mention the Levantines further west (as shown by the new phenomenon of the Lebanese babe protester). Women from the Arabian peninsula don't look quite as good, and that's probably due to the fact that they relied almost entirely on the camel rather than lots of other animals as well. See the earlier criteria for how much of a threat animal domestication poses. It also looks like zoonotic diseases from camels aren't as numerous or as disastrous as those from cattle, sheep, goats, pigs, etc. Hard to think of one.
Probably the most famous pastoralists are the horse-riding Turkic groups of central Asia, who produced Genghis Khan. Most of us have no clue what they look like, but they're pretty nice. Here is a random forum thread where someone's posted pictures of girls from various Turkic groups in their traditional attire. They're at least 10 times better looking than the hyper-farmers of eastern Asia. In college, one of the countries that everyone agreed had the cutest girls, based on the international students, was Turkey (the other was Greece -- another case of their long-standing rivalry).
There's not much going on for pastoralism or babe-ness in southeastern Asia, notwithstanding Thailand's sex tourism (it's rare to see Thais as models or on the cover of Playboy, Maxim, etc.). Ditto for the Pacific Islands. Southeastern Asians do look better than the Han to the north, but this is probably just an effect of more tropical diseases, not more animal husbandry. Australia saw a large influx of pastoralists when the Europeans showed up, giving Elle Macpherson to the world, but before that it was only ugly hunter-gatherers.
The same is true for the Americas -- no pastoralism, or even animal husbandry (horses were used to hunt large game, not to milk), and again Amerindians are plain or ugly. One minor exception is the camelid herders of the Andes. I hear that Peruvian Amerindians are better looking than those from, say, Mexico, but you still don't see lots of them on the cover of Vogue magazine, unlike the daughters of goat herders from Mogadishu. As in the Arabian peninsula, there could be some species-specific effect, where the camelids just aren't as big of a threat, not to mention that Andeans don't have huge and diverse flocks. The obvious exceptions in the New World are recent European immigrants.
Aside from selection for good looks based on protecting your offspring from bugs, those who've domesticated herd animals face an additional pressure for disease resistance -- namely, getting the job done here and now. A good-looking woman in a herding society not only promises healthier children, but you'll also be able to put her to work more effectively milking the cows, goats, and sheep. (This pressure is the same if men do the milking.) If she's more vulnerable to the zoonotic diseases spread by the animals she'll be tending to, she'll be weaker on the job and will probably die sooner as a result. Hence a better looking woman will be a better milker.
Then there are the non-genetic reasons that animal raisers tend to look better. They get a much healthier diet than cultivators, who are lucky to get animal food at all. Their diet isn't quite as nice as a hunter-gatherer's, yet they look better than them, so clearly diet is not the only or even primary factor (else the Eskimo, etc., would look best). And as Greg Cochran pointed out, one reason why milkmaids are famed for their beauty is that they were more likely to be exposed to cow pox and thereby become immune to small pox infections, a horribly disfiguring disease. Again, though, the main factor seems to be the genetic / natural selection story.
The strength of the "small pox immunity" theory could be tested by looking at pastoralist groups where the men do most of the milking and other herding activities. If their women still look pretty good, it is not due to greater exposure to cow pox but to sex-blind selection for greater looks as a signal of disease resistance. Even if men are the target of selection, they will pass these good-looking genes on to their daughters, making both sexes look nice. If, across pastoralist groups, only the sex that did most of the milking was good-looking, that would support the "small pox immunity" theory.
One last thing to keep in mind, since I anticipate a lot of very confused comments otherwise: this is all "all else equal." I won't allow any comment that stupidly and sarcastically asks why the pastoralist Himba don't look as good as a European farmer group. Too many other variables differ between them. The relevant comparison is always the group who's just like them in as many ways as possible, except for how they make a living. Scottish vs. English, Himba vs. Zulu, Uyghur vs. Han, etc.