Human Heredity 11th Edition by Michael Cummings

Exercise 9

Tracing Ancient Migrations
Genomic information is being used to trace the paths followed by ancient migrations of our species out of Africa to all parts of the Earth. A logical question is: How can we map out events that occurred thousands of years ago that left no written records? The answer is written in the genomes of present-day populations. To work out these routes, geneticists use genetic markers. The markers used in this work are Y chromosome sequences, which are passed directly from father to son, and mitochondrial sequences, which are passed from a mother to all her children. These markers allow men to trace their paternal heritage and men and women to trace their maternal heritage. Because these DNA markers do not undergo recombination during meiosis, mutations that arise in these DNA sequences become heritable markers. These new mutations spread through the population from generation to generation. After many generations, a specific marker will be carried by most members of a population living in a particular geographical region. If people leave that region, they carry that marker with them, and along the way pass it on to their offspring, making its path traceable. The relative ages of markers can be established by assuming that mutations in the markers are random and occur at a constant rate. This assumption is more reliable for Y chromosome markers than for mitochondrial markers but is still useful for establishing the relative ages of each marker.
Ancient migration routes are traced by cataloging the markers present in existing indigenous populations. Knowing the markers characteristic of many indigenous populations provides a starting point from which researchers can work back to track the markers through different populations. DNA samples donated by about 10,000 members of indigenous and traditional peoples from around the world form the starting-point database. Each set of markers we carry represents an ancient point of origin and an end point (where we are now) along a path of migration. By surveying many people in present-day populations, the track of each marker can be reconstructed.
What this means for all of us is that it is now possible to trace our heritage far beyond grandparents and great-grandparents to ancestors who lived thousands of years ago, and to follow the path of their ancient migrations that lead to us and where we live now. The Genographic Project is assembling the largest database for these studies. Part of the database is made up of DNA samples from the 5,000-or-so indigenous populations that have lived in particular regions for many generations and have maintained their languages and cultures. However, the project is also selling kits to those who wish to contribute their DNA, using swabs to collect cheek cells. Online vendors offer similar kits. Others offer autosomal DNA testing using SNPs to provide a large-scale view of someone's heritage, but these tests do not have the specificity of tests using Y chromosome and mitochondrial markers.
Modern forms of H. sapiens spread through central Asia some 50,000 to 70,000 years ago and into Southeast Asia and Australia about 40,000 to 60,000 years ago. H. sapiens moved into Europe some 40,000 to 50,000 years ago, displacing the Neanderthals who had lived there from about 100,000 years ago to about 30,000 years ago.
Genetic data and recent archaeological findings indicate that North America and South America were populated by three or four waves of migration that occurred 15,000 to 30,000 years ago. Migrations from Asia across the Bering Sea are well supported by archaeological and genetic findings, but some researchers feel that Asia may not have been the only source of the first Americans. Some skeletal remains, such as Kennewick Man and the Spirit Cave mummy, have anatomical features that more closely resemble Europeans than Asians. Evidence from mitochondrial DNA haplotype X, found in western Eurasians but not East Asians, and a reinterpretation of stone-tool technology make it seem possible that Europeans migrated to North America more than 10,000 years ago. Some genetic evidence argues against this possibility, but based on the analysis of mitochondrial, Y chromosome, and autosomal DNA from remains excavated in Central Siberia, the results suggest that Native American genomes are of mixed origins, derived from east Asians and western Eurasians.
Further evidence from genetics, anthropology, archeology, and linguistics will be required to provide the final answers about the origins and migrations of the first people to reach the Americas.
The presence of haplotype X in Native American genomes could have originated from gene flow from western Eurasians into Siberians or by migration of early Europeans across the Atlantic Ocean. What kind of evidence would help resolve this issue?