It really happened: Six generations of inbreeding spanning the years 1800 to 1960 caused an isolated population of humans living in the hills of Kentucky to become blue-skinned.

The startlingly blue people, all descendants of a French immigrant named Martin Fugate and still living near his original settlement on the banks of Troublesome Creek when hematologists studied them in the 1960s, turned out to have a rare blood condition called methemoglobinemia. A recessive gene was pairing with itself to change the molecular composition of their blood, making it brown as opposed to red, which tinted their skin blue.

The hematologists' attempt to trace the history of the mutant gene revealed a gnarly Fugate family tree, contorted by many an intermarriage between first cousins, aunts and nephews, and the like over the generations. Dennis Stacy, whose great-great-grandfather on both his mother's and father's sides was the same person — Henley Fugate — offered a simple explanation for the rampant interbreeding: In the old days in eastern Kentucky, Stacy said, "There was no roads."

It sounds sordid at worst and lazy at best, but in fact, the Fugates' tale is a miniature version of the story of human coupling since time immemorial. Local populations interbreed, causing a sharing of genes, a resulting in-group physical resemblance and, eventually, identification as a distinct race or ethnic group.

According to Stephen Stearns, a Yale professor of ecology and evolutionary biology, before the invention of the bicycle, the average distance between the birthplaces of spouses in England was 1 mile (1.6 kilometers). During the latter half of the 19th century, bikes upped the distance men went courting to 30 miles (48 km), on average. Scholars have identified similar patterns in other European countries. Widespread use of bicycles stimulated the grading and paving of roads, lending credence to the Fugate clan's excuse and making way for the introduction of automobiles. Love's horizons have kept expanding ever since.

"The distance between the birthplaces of parents has continued to increase since the invention of the bicycle, making it now easy, if not standard, for parents to have been born on different continents," Stearns told Life's Little Mysteries.

Stearns says globalization, immigration, cultural diffusion and the ease of modern travel will gradually homogenize the human population, averaging out more and more people's traits. Because recessive traits depend on two copies of the same gene pairing up in order to get expressed, these traits will express themselves more rarely, and dominant traits will become the norm. In short, blue skin is out. Brown skin is in.

Already in the United States, another recessive trait, blue eyes, has grown far less common. A 2002 study by the epidemiologists Mark Grant and Diane Lauderdale found that only 1 in 6 non-Hispanic white Americans has blue eyes, down from more than half of the U.S. white population being blue-eyed just 100 years ago. [One Common Ancestor Behind Blue Eyes]

"The only explanation for the observed pattern that was consistent with the data (that we could think of) was that assortative mating had changed," Lauderdale told Life's Little Mysteries. Assortative mating is the tendency of people to mate with members of their same ancestral group — a tendency that has seemingly lessened over time. "This was consistent with a birth year-related increase in the proportions of individuals who listed more than one ancestry in the 1980 census."

Lauderdale says blue eyes won't die out completely; they'll simply stabilize at a low level that reflects the chance of mating between two individuals possessing recessive blue-eye genes.

Other recessive traits will drop to low levels too, according to University of Delaware evolutionary biologist John McDonald. "Because most immigrants to the U.S. are from Asia, Africa or Latin America, any trait that is more common in northern Europeans than in the rest of the world will get less common over the near future in the U.S. due to immigration," McDonald wrote in an email. "Examples would include red and blond hair, blue eyes, and freckles. Traits that are recessive and are more common in some groups than others will decrease due to blending; blue eyes, sickle cell anemia, and cystic fibrosis are examples of this."

Meanwhile, many other physical traits will simply blend together. "Most of the traits that we think of as distinguishing different groups (hair color, skin color, hair curliness, facial features, eye shape) are controlled by multiple genes, so they don't follow a simple dominant/recessive pattern," McDonald explained. "In those cases, blending will make people look more similar over time."

It's not straightforward to predict how blending of genes affects physical appearances, but McDonald said the tendency is for such traits to average out. The average American skin and hair color will probably darken slightly, and there will be fewer people with very dark or very pale skin and hair.

The genetic mixing under way in the United States is also happening to a greater or lesser degree in other parts of the world, the researchers said. In some places, unique physical traits tailored to the habitat still confer an evolutionary advantage and thus might not bow out so easily; in other places, immigration happens much more slowly than it does elsewhere. According to Stearns, perfect homogenization of the human race will probably never occur, but in general, Earth is becoming more and more of a melting pot.

A population forged from the long-term mixing of Africans, Native Americans and Europeans serves as an archetype for the future of humanity, Stearns said: A few centuries from now, we're all going to look like Brazilians.

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