Chamorro DNA Studies and the Origin of the Chamorro People
From where do the original inhabitants of the Marianas originate? How long ago did they first settle the islands? What kind of migration pattern describes the settlement or settlements of the Mariana Islands? These are some of the questions that researchers are trying to answer regarding the origins and relationships of the Chamorro people.
Miguel Vilar of the University of Pennsylvania is leading a team of scientists in collecting and analyzing DNA samples from modern Chamorro populations to trace the movement patterns and connections with other populations to discover what may have happened in the Marianas. While archeological evidence supports the idea that the Marianas may have been settled as early as 4,000 years ago by people from Island Southeast Asia (modern day Indonesia and the Philippines), there remain many questions about the genetic origins and gene flow of the Chamorro people.
Vilar’s research on Chamorro genetics builds on previous research conducted by J.K. (Koji) Lum into mitochondrial DNA. While most of the “stuff of heredity” (DNA) is found in the nucleus of our cells, the mitochondiron–responsible for cellular respiration or energy production–also contains DNA. Known as mitochondrial DNA, or mtDNA, this genetic material is remarkable in that it is passed though the maternal, or mother’s, line. Since sperm cells do not contribute mitochondria during fertilization, daughters and sons inherit mtDNA exclusively from their mothers, and only daughters can pass it on to the next generation.
This current research is part of National Geographic’s Genographic Project, an effort to work with indigenous communities worldwide and analyze DNA to answer questions about human origins and to understand how humans populated the earth. Vilar, one of the managers of the project, is working on collecting and analyzing samples of Chamorro male lineages to complement the work he has already done on female blood lines.
DNA and gene flow
DNA is short for deoxyribonucleic acid, the molecule that carries the information and instructions for the development and functioning of an organism. DNA is found in nearly every cell of the human body and is responsible for the unique characteristics each human being possesses. The genetic information contained in a person’s DNA is hereditary, meaning it is passed down from one generation to the next.
DNA is a long strand that has a characteristic “double helix” shape, like a twisted ladder. Most of it is found in the nucleus, a specific structure inside a cell that is surrounded by a membrane. When a cell is ready to divide, the DNA makes a copy of itself, and coils up into a bar-shaped form called a chromosome. Because of the extra copy, the chromosome looks like an “X;” each leg of the “X” is called a chromatid. When the cell divides, the chromosomes separate, with each new cell receiving an identical copy of the chromosome. The resulting daughter cells are genetically identical.
Chromosomes vary in number and shape among different organisms. Most species are generally characterized by having a specific number of chromosomes. For example, mosquitoes have 6, dogs have 78, an amoeba has 12, and shrimp have 254. In some organisms, chromosomes occur in pairs. These are called homologous chromosomes or homologues–they are not identical, but carry the same type of information. Human beings, for example, have 23 pairs of homologous chromosomes for a total of 46. Muscle cells, skin cells, nerve cells and other body cells have 46 chromosomes. Chromosomes carry information for traits like hair color, eye color, height, skin tone, etc., as directed by DNA.
However, sex cells–eggs and sperm which are used in reproduction, only have half the number of chromosomes, or 23. The parents of an individual contribute one of each pair of homologous chromosomes to bring the number of chromosomes back up to 23 pairs (46 in total). Twenty-two of the chromosome pairs are called autosomes. The sex chromosomes, labeled “X” or “Y,” determine the sex of the offspring. Females inherit an “X” chromosome from both the mother and father, while males inherit an “X” chromosome from the mother and a “Y” chromosome from the father. Females do not have a “Y” chromosome. Therefore, the “Y” chromosome is only passed on through the paternal or father’s line.
When sex cells form, their genetic information can undergo changes in a process known as “crossing over.” Here, pieces of DNA may move from one homologue to another, resulting in new variations of genetic material. This explains, in part, why siblings who have the same parents would be genetically and physically different from each other.
Because of the variations in genetic material that occur during reproduction, it is possible to map the movement of certain kinds of genes (and the characteristics they code for) among populations. Gene flow is the movement of genes from one population to another. Scientists measure the frequency certain kinds of genes appear in a given population. This is how they determine if a gene (or a population that carries the gene) has moved from one place to another. Most often, scientists look for changes in DNA, or mutations, because these stand out and are easier to track.
Research questions in relation to previous studies
An early study of Chamorro genetics in 1967 by C.C. Plato and M. Cruz, and in 1998 by J.K. Lum and R.L. Cann, demonstrated that Chamorros are distinctive in relation to neighboring Micronesian populations. The Lum and Cann study reported that more than 85 percent of the Chamorros studied belong to mtDNA haplogroups E1 and E2, which are relatively common in populations from Island Southeast Asia, but rare in other groups of Pacific islands. Most of the remaining Chamorro lineage groups belonged to a unique haplogroup labeled B4a1a1a, which is common in Island Southeast Asia and Melanesia. Haplogroup B4a1a1a is also the most common group in Central and Eastern Micronesia and Polynesia, and is called the “Polynesia Motif.”
The term “haplogroup” refers to a specific genetic population that shares a common ancestor directly from their maternal or paternal lines. They are usually designated by letters of the alphabet and numbers to indicate more specific haplogroups. The most common changes in DNA that scientists look at for each haplogroup are genetic markers known as “SNPs” or “single nucleotide polymorphisms,” which occur naturally over time. When an SNP occurs, it becomes a marker that can be passed on from one generation to the next. Humans who descend from the same genetic line will share a common SNP. Haplogroup lineages can then be mapped out like a tree, showing more diversity over time through the various branches that emerge.
The fundamental research question that the Vilar group is trying to answer is whether Chamorros are the direct descendants of a single migration from Island Southeast Asia around 4,000 years ago, or the descendants of a second wave of migrations from Island Southeast Asia about 1,000 years ago. The question arises from an understanding of the two distinct periods of Marianas history–the pre-Latte Era and the Latte Era. The Vilar team is also interested in finding if the Chamorro gene pool shows evidence of gene flow from neighboring islands.
Archeological evidence points to settlement of the Marianas as early as 3,600 years ago, and the appearance of a unique clay pottery style, referred to as Marianas redware. Marianas redware shows similarities with other pottery styles of the same period in Southeast Asia and what is known as the Lapita Cultural Complex. But while Lapita pottery is widely dispersed throughout Eastern Melanesia, Polynesia and Central-Eastern Micronesia, Marianas redware is unique to the Mariana Islands.
Unique to the Mariana Islands (in relation to all other islands of the “remote” Pacific) is rice cultivation, and utterly unique to the Marianas is the construction of latte, large stone columns with hemisphere-shaped capstones. However, both rice cultivation and latte construction do not appear until the Latte Era, beginning about 1,000 years ago. Genetic study of modern Chamorros may provide clues to test competing theories as to whether Latte Era people are descendants of earlier pre-Latte people, or are largely descended from a second migration wave, which brought latte technology and rice agriculture to these islands.
Genetic findings regarding distinctiveness of Chamorros in the Mariana Islands
Vilar’s research team analyzed blood and tissue samples of 105 self-identified Chamorro volunteers and 17 Saipan islanders with Carolinian ancestry on their mother’s side. They also reassessed the 210 samples from neighboring islands collected during the Lum and Cann study, but with additional information. The scientists also analyzed the complete mtDNA of 32 individual Chamorros.
Vilar’s team found limited genetic diversity among the lineages of the Chamorro subjects. The 122 individuals tested in the sample population together showed 19 unique haplotypes (groups of traits inherited together), while the 105 Chamorros showed 14 haplotypes. The Chamorro group also shared mtDNA mutations characteristic of the E1 and E2 haplogroups, which none of the 17 individuals with Carolinian ancestry showed. About 65 percent of the Chamorro individuals had the E2 haplogroup; the low frequency of variations resembled the pattern seen in a young expanding population. About 28 percent of the Chamorro lineages belonged to the E1 haplogroup, which also resembled a young expanding population.
Analysis of the specific E1 and E2 haplogroups of the Chamorros showed they were not present outside the Mariana Islands. This pattern indicates that a founder effect occurred. A founder effect is when an original founding population arrives, becomes isolated, and over time, results in a new population that has only a small amount of the genetic variation seen in the original population. In the Marianas, the founding E1 and E2 haplogroup lineages arrived from Island Southeast Asia around 4,000 years ago and after 3,500 years in isolation, the two lineages acquired the mutations that gave rise to the unique genetic lineage seen in the Marianas. The scientists also believe the initial arrival of the Marianas population may have occurred even earlier, maybe 5,000 years ago.
The B4 haplogroup was found in only 8 percent of the Chamorro lineages, but 100 percent of the individuals with Carolinian ancestry. Most of the Saipan Carolinians in the sample shared a particular B4a1a1a lineage, but another B4a1a1a lineage with a specific mutation known as C16114T was revealed only in Chamorros on Guam and Rota.
Analysis of the 32 complete mtDNA samples also pointed out the uniqueness of the lineages found in the Chamorro individuals. Like the E1 and E2 haplogroups, the Chamorro mtDNA of the Marianas is found only in these islands, but shows strong links to Island Southeast Asia that possibly date back 4,000-5,000 years ago.
The uniqueness of the B4 haplogroups with the C16114T mutation suggests that this lineage is a recent introduction to the Marianas. Perhaps a second migration occurred, bringing knowledge of rice cultivation and latte technology to the islands about 1,000 years ago. However, the Vilar research team also suggests the possibility that the B4 haplogroup may have arrived from neighboring Micronesian islands sometime in the last 2,000 years, and the mutation may have been acquired in the Marianas.
The limited genetic diversity is consistent with the depopulation of the Marianas which occurred with the arrival and conquest by the Spanish in the 17th century. The Spanish colonial policy of reducción resulted in the forced removal of the native population of the northern islands (except Rota) and the northern villages of Guam. The people were placed into districts organized by the Spanish in central and southern Guam. The northern islands remained empty until they were repopulated by Caroline Islanders in the early 1800s; Chamorros finally were allowed to move back several decades later. There is also evidence of gene flow: people from Saipan of Carolinian ancestry today share lineages with other Caroline islanders and not with Guam or Rota or other Chamorros from Saipan. This is possibly from the resettlement by Caroline islanders. Meanwhile, the E1 and E2 haplogroups of Chamorros in Saipan may reflect the movement of Chamorros from Guam and Rota back to Saipan in the mid-1800s.
Vilar’s research on Chamorro male lineages
Dr. Vilar is currently (2013) conducting research that focuses on Y-Chromosome DNA from male lineages, as well as carrying out a particular type of analysis using DNA inherited from both male and female lines, known as autosomal STR analysis. He expects to measure the magnitude of gene flow from Spanish, Mexican and Filipino populations that settled on Guam during the Spanish administration (1668-1898). Such determination of “mixture” could not be revealed by mtDNA studies, owing to male “bias” regarding mating and intermarriage, which largely involved foreign men with Chamorro women, in turn, related to the island’s transition into a male dominated society under Spanish colonialism.
Vilar’s research brought him to Guam in August and September 2013 when he participated in the 2nd Marianas History Conference and presented his findings on female lineages which he published in 2012. In an interview with the Pacific Daily News, Vilar expressed that while he expects to find mixtures of Spanish, Mexican and Filipino genes, he also suspects that he will see a “retaining of this ancestral, indigenous [Chamorro] Y-chromosome,” but how much of that Y-chromosome still exists in the current population is uncertain.
Vilar expects that his research will provide answers as to when different populations arrived and settled in the Marianas. The next step will be to compare the Chamorro samples with samples from the Philippines and other Micronesian islands. The Marianas are unique because people migrated to the islands and stayed. But because of the colonial history of the Marianas, he expects to find a wide range of genetic contributions, including Filipino, Mexican, Japanese and American.
While eager to work with Chamorros who are interested in their genetic origins and connections, Vilar understands the necessity of being respectful and sensitive to the needs of the community. As research is not done in a vacuum, researchers should always work under a strict code of ethics that includes treating people who provide samples with respect and dignity. Vilar works only with live individuals and not ancient skeletal remains. In addition, he plans to share his research with his informants before he publishes his findings, perhaps in another two years.
|Editor’s note:||Guampedia thanks Dr. Gary M. Heathcote for his kind assistance in completing this entry.|
For further reading
Lum J. Koji and Rebecca L. Cann. 1998. “mtDNA and language support a common origin of Micronesians and Polynesians in island Southeast Asia.” American Journal of Physical Anthropology. 105:109–119.
Lum J. Koji and Rebecca L. Cann. 2000. “mtDNA lineage analyses: origins and migrations of Micronesians and Polynesians.” American Journal of Physical Anthropology. 113:151–168.
Miculka, Cameron. 2013. “History written in Chamorro DNA: National Geographic project seeks to trace migration.” Pacific Daily News. 14 Oct 2013. Accessed 14 Oct 2014.
Plato Chris C. and M.T. Cruz. 1967. “Blood groups and haptoglobin frequencies of the Chamorros of Guam.” American Journal of Human Genetics. 19:722–731.
Vilar, Miguel G., Chim W. Chan, Dana R. Santos, Daniel Lynch, Rita Spathis, Ralph M. Garruto and J. Koji Lum. 2012. “The origins and genetic distinctiveness of the Chamorros of the Mariana Islands: An mtDNA perspective,” American Journal of Human Biology. Wiley Periodicals, Inc. Published online in Wiley Online Library.