Indians in Tepee
© Wikimedia CommonsThe Interior of the Hut of a Mandan Dipäuch. Mixed media on paper by Karl Bodmer 1833 – 1834.

Indigenous Americans may have had a harder time making a long-distance love connection or returning to an ancestral homeland to marry the descendant of grandfather's girl-next-door because of the difficulty of traveling along the north to south axis of the Americas.

Human populations living on the east to west orientation of Eurasia, however, had better luck.

Evolutionary biologists at Brown and Stanford Universities found that genetic differences in indigenous American peoples were greater than those of indigenous Eurasian populations. This suggested that populations in the Americas tended to stay separated after moving north or south, and the researchers propose geography kept people apart.

Throughout history, people could hypothetically travel from Spain to China along a corridor of relatively similar climates.

But to travel from Cahokia in what is now Illinois to Cuzco, Peru, an indigenous American would have had to leave their accustomed temperate climate. They would then pass through thousands of miles of different environments before finding a climate like the one they left behind.

Silk Road
© Wikimedia CommonsExtent of Silk Route/Silk Road. Red is land route and the blue is the sea/water route.
"It's harder to traverse those distances based on climate than it was in Eurasia," said lead author Sohini Ramachandran of Brown University in a press release. "We find greater genetic differences (in the Americas' populations) because of the difficulty in migration and the increased challenge of reuniting with neighboring populations."

The Brown and Stanford researchers studied the distribution of genetic variation in 68 indigenous Eurasian and American populations. They looked at 678 genetic markers to find evidence of migration patterns.

They found that when populations in the Americas separated, they tended to stay separate and go their own genetic ways. On the other hand, in Eurasia, they found evidence of people returning to lands they once left, a process known as back migration.

"When populations do not share migrants with each other very often their patterns of genetic variation diverge," said co-author Noah Rosenberg.

"Our result that genetic differentiation increases more with latitudinal distance between Native American populations than with longitudinal distance between Eurasian populations supports the hypothesis of a primary influence for continental axes of orientation on the diffusion of technology in Eurasia and the Americas," the authors wrote in the American Journal of Physical Anthropology.

Silk Road_1
© Wikimedia CommonsThe ancient Silk Road, still in use in 1992.
"If a lack of gene flow between populations is an indication of little cultural interaction then a lower latitudinal rate of gene flow suggested for North American populations may partly explain the relatively slower diffusion of crops and technologies through the Americas when compared with the corresponding diffusion in Eurasia," the authors added.

The idea that the geographic orientation of the continents may have had a major impact on human development was popularized by Jared Diamond in Guns, Germs and Steel.

Not only would it have been more difficult for people to travel and migrate in the Americas, Diamond wrote, it would be even more difficult for crops and livestock to adapt to different temperatures, day lengths, and rainfall patterns. Hence, it took longer for corn to travel from its Mexican homeland to Peru, than for wheat to spread from Mesopotamia to Spain.

Animal agriculture has been shown to accompany genetic shifts in human populations due to the different food sources domesticated animals make available, like milk. Indeed milk drinking European adults share a common ancestor with milk-drinkers in India.