Did the orientation of the continents hinder ancient settlement of the Americas?

The research, published in the American Journal of Physical Anthropology, is part of a special section which explores who the first Americans were and how they were able to settle in the last great unexplored habitat.

The research, by Sohini Ramachandran and Noah Rosenberg, from Brown University and Stanford University respectively, uses genetic information to explore the effects of continental axes and climates on human migration and adaptation across the Americas.

"It has been proposed that the east-west orientation of the Eurasian landmass aided the rapid spread of ancient technological innovations, while the north-south orientation of the Americas led to a slower diffusion of technology there," said Ramachandran. "Our research develops this idea, arguing that continental orientation influenced migration patterns and played an important role in determining the structure of human genetic variation and the distribution and spread of cultural traits."

The research supports the idea that technological diffusion was accelerated across Eurasia because populations with the same latitude experience similar climates, making adaptation to new locations easier for domesticated animals, plants and consequently humans. Alternatively, migrating along lines of longitude involves adapting to new climates.

"The idea that technology was diffused along latitudinal lines was proposed by Jared Diamond in 1997, but if this is correct and the spread of technology was accompanied by human migrations it follows that a comparative study into genetic variation would reveal a signature of greater genetic differences between populations along lines of longitude in the Americas than that in Eurasia along lines of latitude," said Ramachandran.

To test this hypothesis the team analysed genetic variation data from 68 populations, 39 from Eurasia and 29 from Native Americans. The data were used to identify relationships between the genetic and geographic distances between populations on each continent.

The results confirmed that the increase in genetic distances along lines of longitude in the Americas occurs over shorter geographic distances than the increase in genetic distances in Eurasia along lines of latitude.

"For many years anthropologists have asked who the first Americans were and how they were able to settle in the last major habitat open to humans," said Jeff Long, guest editor of the special section. "These six papers use genetics to answer these questions, not only confirming the genetic signatures of historic relationships between Native Americans and Eastern Asia, but also providing evidence for prehistoric migration and adaptation of settlers to the new world."