Native American origin - A review

You might be surprised to learn that the peopling of the Americas (just in case you get confused I'm talking about North and South America throughout this post) occurred extremely late in the day compared to other continents. It wasn't until around 17,000 years ago that the first pioneers arrived, compared to Australia 50,000 ya and Europe 40,000 ya. The reason for this colonisation being so late in our history is due to the prior isolation of North and South America to the rest of the world. This isolation allowed many extraordinary species to evolve which are not found anywhere else including the megafaunal giant ground sloth, the saber tooth cat Smilodon and short faced bears, however it prevented Homo sapiens from entering.

Figure 1 Species endemic to the Amerias - Short faced bear, Smilodon and Giant ground sloths  

The theory generally accepted these days is that the entry route into the Americas was via the Bering Strait land bridge. This bridge was originally created during the last glacial maximum 26,000-19,000 ya a time when ice sheets were at their most extensive, but the bridge lasted right up until 11,000 ya. Over the course of many generations hunters would have migrated across the land bridge following herds of caribou from Siberia into Alaska, from here they followed the coast down the west of North America and eventually into South America, this route can be traced by an increasing decline in genetic diversity displayed by native populations as you travel south.

Figure 2 - humans migrated from Siberia into Alaska

Other theories which have been dismissed include entry to the Americas by South Asia and Polynesia on multiple occasions by means of the sea and by land. The land bridge theory was finally accepted as the most likely in 1997 thanks to genetic evidence linking the Siberian and Alaskan populations. However although the location of migration was agreed upon, the number of migrations which took place were still open for debate. The original 1997 study proposed that the whole population of the Americas stem from a single migration from East Asia.

A new paper coordinated by Professor Andres Ruiz-Linares of UCL and published in Nature in August of this year may shed some light into what really happened. Through more extensive evaluation of the genetics of both North American populations and Siberians they concluded that the migration actually occurred in three waves of migration. Although the first wave of migration did populate the majority of the two continents, some Arctic populations trace their heritage back to a second and third migration occurring later in history. The study also highlights how complex dispersal patterns can be; some Central American populations are equally related to both North and South American populations, suggesting that some back migration would have occurred (people travelling from South to North America). This kind of migration pattern can muddy the waters and make it confusing to see the real picture.

Figure 3 - The new three wave migration route

For the time being the new 2012 paper seems to be accepted by the scientific community as the most likely scenario,  however it should be remembered that studies such as these contain limitations. In this case it is a lack in genetic data, due in part to a reluctance among native populations to partake in the study but also to uncertainty whether skeletal remains are ancestors of tribes or not. I think it will be interesting to see whether scientists manage to obtain more data in future and if so what new evidence may emerge on the settling of the Americas, there may have been many more migrations yet to be discovered.