Recently I came across several examples of how genetic information has greatly helped us understand the past. Quite often this data is gathered in the most unexpected places. Consider these examples.
In an article published in December 2009, an international group of scientists addresses the issue of the extinction of North American megafauna traditionally dated some 11,000 years ago. A skeleton of a woolly mammoth on display in a museum never fails to impress us. At the same time, most of us would agree that it is a good thing we don’t have to worry any more about these lumbering giants messing up our evening commute. However, what most of us stop worrying about is when these animals became extinct and why. Most of us, that is, but not all of us. This is where the story of ancient DNA retrieved from perennially frozen soil comes in.
A traditional approach to estimate when and where a species became extinct has been to map and date the last known survivors. The thinking was that knowing when and where the last specimens lived would automatically clue us in as to why they died out. Is this true, however? Do these last known survivors really represent that last ones left standing? Or did we miss them and make wrong assumptions?
The party line about woolly mammoths was that they survived on remote islands in the far northern regions of Alaska and Siberia, a region referred to as Beringia. That is now old hat. New genetic data tells us a different story. Mammoths may have survived much longer than originally thought in the Alaskan interior. Scratch 13,000 years ago. Now it looks like woolly mammoths may have survived for an additional 2,600 to 3,700 years in parts of Alaska. Mitochondrial DNA was retrieved from perennially frozen soil near Stevens Village in the Yukon Flat. What we have here then is a suggestion that we can establish the presence of certain animals in a region at a certain time in the past. No bones needed. Just animal DNA left behind in the soil and preserved because of the permanent frozen condition of the dirt.
While the researchers were careful to address any shortcomings of their approach (contamination of the soil, migration of more ancient DNA from lower-lying areas to more recent layers closer to the surface), there will be reactions from other members in the scientific community. This may be frustrating to those among us who like to see “the final answer” to questions like these, this dialogue is part of what science is all about. We will have to see if these new results will stand up to the criticism that will come.
As megafauna slowly disappeared from the North American landscape, human settlers were making their presence felt. I have written about the questions of where these earlier migrants into the Americas may have come from. There is very good evidence that the Paleoindians migrated from parts of Asia.
It turns out that we all have a copy of this virus residing within us. It is harmless to most of us, unless your immune system is compromised. Geneticists studying this virus found that it was remarkably stable and very rarely mutated into a new variety. Moreover, the strain of the JC virus carried by the Navajo today is nearly identical to that carried by the modern inhabitants of Tokyo. The JC virus bolsters an Asian origin theory for the First Americans.
|Beringia - Image courtesy of NASA.|
As to how Paleoindians arrived into the Americas, genetics can help us focus that picture as well. If one accepts that the Bering Strait was an ancient migration route – and most people have no problem accepting this – then the issue is: exactly what route did they follow? A coastal route and an interior, overland route, often suggested by archaeologists, now both seem to have been used.
North American mitochondrial DNA, collected from contemporary populations, points to two migration routes. In a paper published Jan. 19, 2009, scientists studied various mitochondrial DNA haplogroups, zooming in on two rare groups. One of these (known as D4h3) is found only along the Pacific coast and is mostly in South America, while the second group (X2a) is restricted to northern North America.
The presence of X2a in North America east of the Rocky Mountains may support the idea of an ice-free corridor between two ice sheets covering Canada and parts of the US. Some of the earliest migrants may have followed that route which would have taken them into the Great Plains, where the glacial corridor would have ended. The presence of D4h3 along the Pacific may represent a coastal migration route.
Woolly mammoths leaving their DNA in the soil, viruses carried by all of us and DNA shared through the mother’s family line all help us refine and refute some of the ideas on how the first immigrants arrived in the Americas. I am sure genetics will continue to add to our understanding of this momentous period in human history. Stay tuned.