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Genetic anthropology is an emerging discipline that combines DNA and physical evidence to reveal the history of ancient human migration. It seeks to answer the questions, "Where did we come from, and how did we get here?"
DNA studies indicate that all modern humans share a common female ancestor who lived in Africa about 140,000 years ago, and all men share a common male ancestor who lived in Africa about 60,000 years ago. These were not the only humans who lived in these eras, and the human genome still contains many genetic traits of their contemporaries. Humanity's most recent common ancestors are identifiable because their lineages have survived by chance in the special pieces of DNA that are passed down the gender lines nearly unaltered from one generation to the next. These ancestors are part of a growing body of fossil and DNA evidence indicating that modern humans arose in sub-Saharan Africa and began migrating, starting about 65,000 years ago, to populate first southern Asia, China, Java, and later Europe. Each of us living today has DNA that contains the story of our ancient ancestors' journeys.
The Story of Human Migration Also is Told in the DNA of Parasites and Pets.
The correlation with human migration is present but less distinct for pets. Studies of domestic cats’ mtDNA reveal that they share a most-recent common ancestor who lived in the Middle East about 70,000 to 100,000 years ago. Wild cats, motivated by the desire to get mice and other food from humanity’s first farmers, seem to have domesticated themselves about 10,000 to 12,000 years ago. Genetic markers in house cats’ mtDNA reveal that the cats followed the same migratory patterns as early human farmers.
When DNA is passed from one generation to the next, most of it is mixed by the processes that make each person unique from his or her parents. Some special pieces of DNA, however, remain virtually unaltered as they pass from parent to child. One of these pieces is carried by the Y chromosome, which is passed only from father to son. Another piece, mitochondrial DNA (mtDNA), is passed (with few exceptions) only from mother to child. Since the DNA in the Y chromosome does not mix with other DNA, it is like a genetic surname that allows men to trace their paternal lineages. Similarly, mtDNA allows both men and women to trace their maternal lineages.
Both the Y chromosome DNA and mtDNA are subject to occasional harmless mutations that become inheritable genetic markers. After several generations, a particular genetic marker is carried by almost all male and female inhabitants of the region in which it arose. When people leave that region, they carry the marker with them. By studying the genes of many different indigenous populations, scientists can trace when and where a particular marker arose. Each marker contained in a person’s DNA represents a location and migration pattern of that person’s ancient ancestors.
For example, roughly 70% of English men, 95% of Spanish men, and 95% of Irish men have a distinctive Y-chromosome mutation known as M173. The distribution of people with this mutation, in conjunction with other DNA analyses, indicates that the men's ancestors moved north out of Spain into England and Ireland at the end of the last ice age.
Neanderthals inhabited Europe and parts of western Asia starting about 230,000 years ago. They cohabited with modern humans for thousands of years before becoming extinct about 29,000 years ago. Since 1999, several DNA samples have been extracted from Neanderthal fossils and sequenced, allowing scientists to compare large sections of the Neanderthal genome with that of modern humans. This is possible because DNA fragments can survive for 50,000 to 100,000 years.
Neanderthal DNA sequences were found to be about 99.5% similar to the modern human genome, indicating that modern humans and Neanderthals had a common ancestor about 700,000 years ago. The genetic difference between Neanderthals and modern humans, however, was on average about three times greater than the genetic difference between any two modern humans. Studies of the mtDNA of nearly 80,000 people found no traces of mutations known to be common in Neanderthal mtDNA. These differences indicate no significant interbreeding between Neanderthals and modern humans after the two species diverged.
Scientists assume that random mutations in specific pieces of human DNA occur at a constant rate. A comparative study of mtDNA from people of many different geographical regions reveals the number and order of mutations that have occurred since the most recent common ancestor. Once the rate of mutations is determined, the time at which any given mutation arose can be determined by counting the number of mutations that have occurred since its appearance. Estimates of the mtDNA mutation rate were made by studying the mtDNA of groups of people whose ancestors migrated at known times. New research shows that some regions of mtDNA mutate much faster than others. This discovery makes the mtDNA mutation rate uncertain and gives a broad margin of error for ages determined with mtDNA.
There are rare occurrences of paternal transfer of mtDNA in humans and other animals. This happens during conception when some mtDNA from the sperm tail mixes with the egg. So far there is only one proven case of this in human mtDNA. Therefore, it is not considered to be a significant contribution to the overall mutation rate of human mtDNA.
The Y chromosome transfers only from father to son, so it cannot be used to study the ancestry of paternal grandmothers. Similarly, mtDNA cannot be used to study the ancestry of maternal grandfathers. The genetic markers from these relatives reside in our autosomal DNA, composed of the 22 pairs of nonsex chromosomes found within the nucleus of every cell. Autosomal DNA tests look for SNPs, or alleles, located throughout the DNA. Since autosomal DNA is a random mixture of contributions from each parent, these tests cannot determine from which side of the family the alleles came unless the family members in question also have donated DNA. Because many generations of a family are represented in autosomal DNA, it provides a broad picture of an individual's heritage rather than a trail of specific ancestry.
Many companies sell autosomal DNA tests as a means of determining an individual's "biogeographic ethnicity." These ethnicities are defined by geographical region or by ancient human migration patterns and vary arbitrarily in composition depending on the company offering the test. No scientific definitions for genetic ethnicity are universally accepted.
DNA studies do not indicate that separate classifiable subspecies (races) exist within modern humans. While different genes for physical traits such as skin and hair color can be identified between individuals, no consistent patterns of genes across the human genome exist to distinguish one race from another. There also is no genetic basis for divisions of human ethnicity. People who have lived in the same geographic region for many generations may have some alleles in common, but no allele will be found in all members of one population and in no members of any other. Indeed, it has been proven that there is more genetic variation within races than exists between them.
The U.S. Department of Energy's (DOE) Human Genome Project (HGP) devoted 3% of its annual budget toward studying the ethical, legal, and social issues (ELSI) surrounding the availability of genetic information. In 2004, DOE sponsored a Nature Genetics supplement called Genetics for the Human Race. This supplement originated from a May 2003 workshop held by the National Human Genome Center at Howard University in Washington, D.C. The workshop, Human Genome Variation and 'Race,' and the special issue of Nature Genetics were proposed by scientists at Howard University and financially supported by DOE's HGP through its Office of Science; Irving Harris Foundation; National Institutes of Health through the National Human Genome Research Institute; and Howard University. The supplement contains articles based on presentations at this workshop.
Most of what is known about anthropological genetics is based on DNA samples donated by about 10,000 indigenous and traditional people from around the world. Several efforts currently are under way to generate larger or more detailed databases. Three of these efforts are listed below.
In 2006, the National Geographic Society, IBM, geneticist Spencer Wells, and the Waitt Family Foundation launched the Genographic Project, a 5-year nonprofit study that will produce the largest DNA database ever collected for genetic anthropology. The project focuses on obtaining DNA samples from "key populations" composed of people who have lived in a particular region for several generations and maintained the same culture. An estimated 5000 of these so-called key populations live on earth. The project will attempt to choose a subset of these who represent current human genetic diversity. Each person sampled must first give informed consent, and issues such as participants' privacy and the cultural and physical impact of the project are considered. No medical research will be conducted during the study, and no genetic data will be patented. The project's database will be open to the public.
The Genographic Project has begun sampling key populations in:
When the project is finished, its database will contain DNA samples from more than 100,000 people from key populations. The Genographic Project also is selling public participation kits to all who want to contribute their DNA. Researchers already have released an open source database of 78,590 contributed mtDNA variants. See PLoS Genetics.
The goal of the International HapMap Project is to develop a haplotype map of the human genome. HapMap will describe the common patterns of human DNA sequence variation by comparing individuals' genetic sequences to identify chromosomal regions where genetic variants are shared. HapMap is expected to be a key resource for researchers to find genes affecting health, disease, and responses to drugs and environmental factors. Information produced by the project will be released to the public and freely available. The project is a collaboration among scientists in Japan, the United Kingdom, Canada, China, Nigeria, and the United States. DNA samples for HapMap will come from 270 people: the Yoruba people in Ibadan, Nigeria, Japanese in Tokyo, Han Chinese in Beijing, and the genetic samples in the Paris Centre d'Etude du Polymorphisme Humain (CEPH) collection. HapMap started in 2002 and was scheduled to run for only 3 years, but it has been extended.
Human Genome Diversity Project
The Human Genome Diversity Project (HGDP) was started by Stanford University's Morrison Institute in 1993.The HGDP is not related to the Human Genome Project or Genographic Project. It is an effort by researchers to document the genetic variation of the human species worldwide. The project's goal is to collect renewable biological samples from different population groups to build a representative database of human genetic diversity. Data also will be used to learn about human biological history and the biological relationships among different groups. Information from the project could be useful in understanding the causes of and determining the treatment for particular human diseases.
Early in the study, political and ethical issues associated with the protocols used to collect genetic samples from indigenous populations stalled research. Since the project's aim was to collect infinitely renewable lymphoblastoid cell lines rather than simple DNA samples, potential donors were afraid their cells lines would be exploited in the future. From 1994 to 1997, the HGDP was at a virtual standstill while a committee formed by the National Research Council of the National Academy of Sciences met to discuss the project’s feasibly and ethics. In 1997 the committee recommended HGDP proceed, with particular attention to donors' privacy, informed consent, and other related ethical issues. Also in 1997, several preexisting cell lines from other studies of human evolution were contributed to the project. This gave the HGDP a core of renewable genetic data from 1064 people from 52 indigenous populations that spanned all continents. The first studies using this data were published in 2002.
The Human Genome Project (HGP) was an international 13-year effort, 1990 to 2003. Primary goals were to discover the complete set of human genes and make them accessible for further biological study, and determine the complete sequence of DNA bases in the human genome. See Timeline for more HGP history.
Published from 1989 until 2002, this newsletter facilitated HGP communication, helped prevent duplication of research effort, and informed persons interested in genome research.