There's still a lot we don't know about the first human colonization of the Americas, particularly the exact timing, but we do know that it took tens of thousands of years for humans to make it from Alaska to Chile.
That's a long time, and the archaeological evidence indicates that the spread of populations and technologies in the ancient Americas was far slower than than in Eurasia, a much larger landmass. One theory is that the orientation of the continents is actually a factor here - while Eurasia is primarily an east-west continent, the Americas largely extend north-south, with Central America a particularly good example of an area where there's very little land to the east or west of any given position.
This may seem like a trivial point, but there's a big difference between being able to move into lands that are on the same latitude and being forced to move north or south. Areas that are on the same latitude are likely to have much the same climate, and so ancient humans could move into them without having to rethink their technology or run too much risk of running into disastrous environmental conditions.
It's also not just humans that are forced to re-adapt when moving north-south - domesticated plants and animals might thrive in one latitude, but then prove wholly inadequate to the challenges of a new climate. That can really slow down the spread of a human population, and now Sohini Ramachandran and Noah Rosenberg, of Brown and Stanford respectively, have found genetic evidence to demonstrate just that.
The pair studied genetic variation data from 68 groups, 39 from Eurasia and 29 from the Americas. By examining the genetic differences between the respective groups and then comparing how far apart they were, the researchers could determine how much genes varied with distance on the two continents.
They found that genetic distance - the difference in genes between two groups - is greater along far shorter geographic distances, particularly longitudinal distances, in the Americas than it is in Eurasia. That's more proof that humans settled the Americas far more slowly than their Eurasian counterparts.
"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. 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 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."