The field of anthropology is concerned with the scientific study of humans and their close relatives, the non-human primates. Genetic methods are now being applied to studies of human-ape divergence, the size and geographic origin of early hominid populations, and the earliest migrations of anatomically modern humans. Our work focuses on much more recent events in human history. This has typically been the domain of cultural anthropologists, who have studied the relationships among human populations by detailed analyses of cultural practices called ethnographies, with parallel work on their linguistics, archaeology and physical anthropology. Recently, however, a new dimension has been added to these studies with the inclusion of genetic analyses. The field of anthropological genetics uses patterns of genetic similarity among different human populations to infer demographic history, including mating structure, the history of migration and admixture with surrounding groups, and population size fluctuations.
Sample collection. The raw material for our genetic work is DNA, the chemical "blueprint" found inside every living cell in the human body. DNA samples are collected with informed consent from self-identified linguistic and/or ethnic groups. Only healthy adults are sampled. Typically, 5 ml of blood is collected by venipuncture, and the red cells lysed, leaving a white cell pellet. This is then suspended in a buffer containing high concentrations of EDTA, Tris-Cl and sodium chloride for DNA preservation, as well as a detergent for lysis and sodium azide to retard bacterial and mold growth. The resulting white cell lysate is stable for weeks to months under unrefrigerated field conditions, and the preserved DNA is of high quality and usable for all molecular biological applications.
Click here for DNA extraction protocol
We try to collect at least 50 samples from each group - half males and half females, if possible. With the markers we are using to study Central Asian diversity, this sample size appears to be sufficient for accurate assessments of allele frequencies.
After returning to the laboratory, final DNA extractions are performed, and polymorphisms are studied, or "typed," using techniques based on the polymerase chain reaction (PCR), which requires very little starting DNA. From 5 ml of whole blood, we routinely obtain 70-100 µg of DNA, which provides sufficient material for typing over 3,000 polymorphic markers. As we share the collected samples with many researchers, some thrift is necessary, but this amount of material should suffice for the typing of all relevant polymorphisms. Archival DNA samples are stored at -70°C for future use.
Choice of genetic markers. Modern genetic research has shown that traditional racial groups have almost no meaning at the genetic level; at polymorphic loci, 85% of the variation is found among individuals within populations, 7% is found among populations within a racial group, and only 8% is found among traditional racial groups (Lewontin, 1972, Evol. Biol. 6:381-398). This apportionment of variation has been confirmed by recent studies using more sensitive DNA markers. Clearly, far more variation is found among individuals than among groups.
While most genetic variation in humans is found within groups, some inter-group differences can be found in alleles and allele frequencies. In particular, extremely polymorphic genes, such as short tandem repeat (microsatellite) loci, human leukocyte antigen (HLA) and mitochondrial DNA (mtDNA) D-loop sequences, provide anthropological geneticists with powerful tools for the inference of population history. Studies of world populations using microsatellite loci (nucleotide repeats, such as CACACACA..., found throughout mammalian genomes) have shown that they are immensely useful for inferring the history of human migration. HLA loci have also proven to be a very sensitive tool for studies of this type. These loci encode cell-surface molecules which present fragments of degraded proteins to the "killer" and "helper" T-cells of the immune system. In this way, the immune system can recognize a change in cellular function (caused by viruses, cancerous mutations, etc.) and destroy the affected cells. The action of natural selection, through the adaptation to geographically isolated pathogens, can change HLA allele frequencies in an irregular way over long periods of time. For this reason, HLA loci may not be as useful as other loci for studies of extremely ancient demographic events; however, they can provide a wealth of detail about the short-term history of populations (on the order of a few thousand to tens of thousands of years). MtDNA appears to provide the least resolution of the loci we study, but is nonetheless important because of the large database of sequences with which to compare, and the relative ease with which it can be amplified from ancient materials.
Our genetic studies of the samples collected during EurAsia '98 will involve the comprehensive analysis of autosomal microsatellite, HLA, mtDNA and Y-chromosome variation using PCR-based methods. Much of the technology currently used in molecular genetics is based on fluorescently-labelled tags which allow us to detect small quantities of the molecules of interest. Automation allows the number of individuals analyzed in a laboratory study to be increased significantly - perhaps by 100-1000 fold over the manual methods in use only a few years ago. In this way, we anticipate being able to collect the large amount of data necessary for complete studies of these populations. This portion of the project will be carried out in several laboratories in the United States, Europe and the FSU.