DNA helps map human history
Last week, we were talking about DNA analysis and barcoding. It has certainly moved the science of taxonomy forward by leaps and bounds. We now identify species on a molecular basis. But that is not all DNA analysis can do.
It can also tell us who our ancestors were and where we come from. Or, at least, it can to a certain extent. After all, we are all African at some distant point in our family tree as we are all related.
The latter is easy to show as each generation going backwards doubles the number of our direct ancestors. That is, you have two parents and they had two parents giving you four grandparents, and they had two parents giving you eight great-grandparents, and sixteen great-great-grandparents, and so on. In just 35 generations or about 700 years, your direct ancestors total 34.4 billion people, which is more than the total number of humans who have ever been alive.
DNA analysis has been used to map out lineages. There are commercial enterprises to determine your ancestry. They do this using portions of the genetic code with mutations specific to certain sub-groups.
Mutations happen all the time when populations are separated. Some are subtle. Some are not. But for the most part, a population of individuals do develop some unique and identifiable aspects to their DNA. It goes well beyond the simple phenotypical traits such as hair colour or facial features but it is important to remember we are all humans with just slight genetic variations.
The rate at which base pairs change in DNA is reasonably constant and predictable. This allows molecular geneticists to not only tell if two populations are related but how long ago they split from one another.
For example, archaeologists and anthropologists have long debated the population of the Pacific Ocean by seafaring Polynesians. We know that about 3000 years ago, people on the eastern edge of Asia began sailing east and discovering some of the island groups. About 1,000 years ago, their descendants invented the double-hulled canoe which allow them to travel even further, crossing thousands of miles of ocean. They settled in places like Hawaii, Rapa Nui, and New Zealand.
The Polynesians were superb navigators. They developed an understanding of waves, wind, and weather that allowed these daunting voyages to occur with relative frequency. There was an extensive trade network amongst the islands.
But the question has remained – did they make it all the way to the Americas?
Considering the length of some of the voyages they undertook and their familiarity with the ocean’s currents, it has always seemed likely there should have been a connection. For example, sweet potatoes which originated in the Andean highlands were grown across many islands in eastern Polynesian well before European contact. Samples of Polynesian sweet potatoes from the 1700s share genetic markers with coastal South American varieties. As the Pacific islands were never part of South America, the only mechanism by which sweet potatoes could have arrived is via some form of transportation. But the debate over the issue remains contentious.
A new study might finally settle the issue. Published in Nature, Alexander G. Ioannidis and co-workers have analyzed the DNA of 807 individuals from 17 island populations and 15 Pacific coast Native American groups. According to the authors “we find conclusive evidence for prehistoric contact of Polynesian individuals with Native American individuals (around AD 1200) contemporaneous with the settlement of remote Oceania.”
Essentially, the research team collected samples and analyzed for specific sequences within each individual. They were then able to map their DNA against markers from other populations. For example, the individuals from the Cook Islands in the western Pacific all had a genetic make-up consistent with populations further to the west, in Vanuatu. For the most part, the individuals were solely Polynesian.
However, individuals from the North and South Marquesas showed a consistent amount of DNA with markers for the Zenu or indigenous Colombians. The consistent concentration of these markers across the whole population is indicative of contact a long time ago. More recent admixtures of DNA show variation in concentration for particular individuals depending upon their recent ancestry.
The DNA of Native Americans found in the Polynesian people can be dated to the different islands – beginning in South Marquesas around AD 1150 and finally reaching Rapa Nui around AD 1380. The DNA data tells us there was traffic across the Pacific and the two peoples interbred in some fashion. It doesn’t tell us whether it was Polynesian sailors visiting South America for a period of time or South American people venturing out into the vast Pacific Ocean.
Tracking specific DNA markers is helping scientists unravel the story behind the diaspora of human beings from Africa to every place on the planet. Even the remote islands of the Pacific Ocean.