East often meets West and vice versa, but did North frequently meet South when it comes to the history of people and technological innovations migrating across the continents? New genetic analysis suggests the way that continents are oriented may indeed have played a key role in our cultural interactions over time.
For decades scientists have suggested that the east-west orientation of Eurasia helped spread ancient culture and technological innovations such as agriculture and writing more rapidly than occurred in the oppositely oriented Americas, with biologist and ecologist Jared Diamond perhaps most famously making this case in his Pulitzer Prize–winning Gun, Germs and Steel: The Fates of Human Societies (W. W. Norton & Co., 1999). The idea is that populations at comparable latitudes experience largely similar climates, making it easier to adapt crops and domesticated animals and, consequently, humans and technology to new locations east to west. On the other hand, migrating across lines of latitude, north to south, involves adapting to new climates.
Given this notion, genetic analysis might reveal greater differences among human populations north to south than east to west within continents, says population geneticist Sohini Ramachandran at Brown University. If migration is harder across lines of latitude than longitude, then populations would be more isolated north to south, giving them more chances to diversify compared with one another.
To see if this was the case, Ramachandran and her colleague Noah Rosenberg at Stanford University analyzed genetic variation data in 678 genetic markers from 68 populations. This included data from 39 populations from Eurasia—including Europe, southern and central Asia, and the Middle East—collected from the Human Genome Diversity Cell Line Panel, along with 29 populations from Native American groups, such as the Cree, Ojibwa, Maya and Zapotec, gathered by former collaborators of the researchers. "Only recently did we have the kind of genetic data to perform this kind of comparative analysis of Native American and Old World populations," Ramachandran says.
The researchers measured how distant or divergent each population was from one another in terms of both mileage and genetic variation. They discovered more genetic differences north to south in the Americas than over comparable distances east to west in Eurasia, findings detailed online September 13 in the American Journal of Physical Anthropology.
"The findings here are very interesting, with the implications that patterns of technological innovations were influenced by our physical climates and physical environments," says evolutionary anthropologist and human population geneticist Jeffrey Long at the University of New Mexico, who did not take part in this study. "We tend to think of humans as separated from ecology, and in fact there has been ecological structuring of our recent history, and perhaps even in the rise of cosmopolitan society. It's nice for me to see the field of human population genetics maturing out of the who-begat-who phase and into investigating these larger questions."
In the Americas distance north to south explained a good deal of genetic variation. Distance east to west did so as well, although to a lesser extent. This is probably due to the diagonal northwest-southeast positions that North and South America have in relation to each other, respectively.
One potential caveat regarding these findings is that culture and technology could spread from one population to another without them otherwise intermingling and sharing genes, Ramachandran says. As such, although they found evidence that changes in latitude could impede genetic flow, it might not necessarily impair cultural or technological flow.
Future research could go into even finer detail when it comes to ancient obstacles to migrations, including mountain ranges and other physical barriers as well as social barriers such as marriage practices or language differences, Ramachandran suggests. "We can develop new insight into the effects of geography and culture on recent human evolution as we gather more and more genetic data from human populations," she says. Further studies could also look at the impact of patterns of genetic variations on health, Long adds.