Profiles Vol. 1 No. 2 August 15, 2000
Recent Research Contributions of Genetics to the Studies of Population
History and Anthropology in Puerto Rico
Juan Carlos Martínez Cruzado is Professor of Genetics at the University of
Puerto Rico, Mayagüez Campus. We invited him to talk to us about his
research project, funded by the National Science Foundation, to determine
the continental origin of the mtDNA of Puerto Ricans--a project spurred by
the surprise finding of a much larger-than-expected number of Puerto Ricans
testing positive for Amerindian ancestry.
Some important research contributions of Genetics to the study of Population
History and Anthropology in Puerto Rico: An interview with Dr. Juan Carlos
Martínez Cruzado, Dept. of Biology, University of Puerto Rico at Mayagüez
Some time during the 1980s, the Institute of Puerto Rican Culture inSan
Juan, Puerto Rico, received four skeletal remains from a small burial ground
that was accidentally discovered during the construction of a boardwalk in
Arecibo, Puerto Rico. Having been disinterred out of context by a
construction crew, these remains were of little archaeological value, soand
as a result, the Director of the Program of Archaeology of the IPRC, Juan
José Ortiz Aguilú, gave the remains to Dr. Juan Carlos Martínez Cruzado, a
Molecular Biologist at the University of Puerto Rico at Mayagüez (UPRM), to
analyze for their mitochondrial DNA (mtDNA) content. Ortiz Aguilú’s interest
in mtDNA analysis of remains in burial sites was spurred by the peculiar
positions in which he had found some skeletal remains, including one male
interred holding a head in his hands, and another in which two skeletons
were interred in one pit. An analysis of the mtDNA of these remainscould
shed light on the reasons for these peculiar positions by indicating whether
the interred might have been parent and child, brothers, or, lacking a
filial maternal relationship, victor and vanquished.
For Dr. Martínez Cruzado, the project had important administrative as well
as research implications. Since returning to the UPRM in 1989 after
receiving his Ph.D. at Harvard, he has worked with his departmental
colleagues in order to develop initiatives designed to improve UPRM’s pool
of undergraduates who pursue Ph.D. degrees in the field of biomedical
sciences. The dramatic success of these initiatives resulted in the need for
improvement in the infrastructure of the Department of Biology since it
could not meet the students’ growing demand for research opportunities. The
mtDNA project has helped amelliorate the situation by expanding the scope of
the research projects in the department and by affording six students lab
experience in identifying Puerto Rican mtDNA. Another six students from the
Departments of Sociology and Psychology have also benefitted from the
project, gaining field work experience by collecting genetic samples (hair
roots) and interviewing donors.
Even more important, the identification of the Puerto Rican mtDNA could
support or challenge--at least regarding the evolutionary contribution of
females--the conventional wisdom that, because the indigenous population had
disappeared by the end of the sixteenth century, there was little Amerindian
contribution to the Puerto Rican gene pool. Should the results of the mtDNA
analyses challenge the conventional wisdom, the stage would be set for
Y-chromosome studies to assess the male contribution to the ethnic evolution
of Puerto Ricans.
The results of Carbon 14 analysis, which dated the skeletal remains
toapproximately 645AD, was just the first of many exciting discoveries that
this project has generated. The seventh century was a time of great change
on the island because some great natural disaster paved the way for a
fundamental change in the native population, most importantly, in the
evolution of ceramic cultures. Before the disaster the inhabitants of the
island were organized into egalitarian communities; but evidence indicates
that, after the disaster, a hierarchichal social structuralization of the
native population evolved.
What exactly is mitochondrial DNA, and what does its analysis reveal? An
analysis of mitochondrial DNA can positively identify female ancestors
because the mitochondrion is an organelle--a cell organ--that does not
recombine as it passes from one generation to another down the female line;
that is, it passes intact, without combining with the male mtDNA which is
not transmitted from one generation to another. Nevertheless, the
mitochondrion has a fast mutation rate, thus making it possible to trace
ancestry within short periods of evolutionary time. These two
characteristics of the mtDNA make it a highly informative genetic unit and
the darling of human evolutionary geneticists.
What were the results of the mtDNA analyses of the skeletal remains that you
received from Ortiz Aguilú? The results were surprising: all 4 skeletans
possessed identical mtDNA.
Why are these results surprising?
Though a high incidence of homogeneity within particular ethnic groups
(referred to as the bottleneck effect) is not uncommon--and indeed, previous
studies suggest the occurrence of just such an effect in the Pre-Columbian
colonization of Puerto Rico through the Lesser Antilles--such homogeneity
makes it impossible to identify the filial relationship of the interred. The
results indicated that these people were definitely Amerindian, but it was
not possible to determine whether a filial, as well as a cultural,
relationship existed between them, even when the most hypervariable region
of the mtDNA was analyzed. In order to determine that relationship we would
have to examine the mtDNA of contemporary descendants of these people in
search of variable sites in the mtDNA.
How could this be achieved?
Because the mitochondrion remains genetically intact through the maternal
line, analysis of the mitochondria of contemporary Puerto Ricans who were
likely to be of Amerindian ancestry could scientifically reveal such
ancestry. Ideally, a study of a large group of Amerindian mtDNA should make
it easier to determine the variable sites within that mtDNA, and so help us
trace relationships back in time.
Considering that the history of Puerto Rico suggests that there were no
Amerindians on the island by the end of the 16th century, how did you
identify such descendants?
According to historian Salvador Brau, the censuses of 1777 and 1787 recorded
the existence of some 2,000 Amerindians in the areas of Indiera Alta,
Indiera Baja and Indiera Fría. These were descendants of a group of Tainos
who, in 1570, decided to intern themselves in the mountainous regions of
central Puerto Rico in order to protect themselves from Spanish
colonization. Also, it is popular belief in the area around the city of
Mayagüez that the barrio Miraflores of the town of Añasco was populated by
many indians and "negros cimarrones" fleeing slavery. We went to these
and obtained a total of 23 samples of hair roots (18 from the Indieras, 5
from Miraflores) to analyze. We also sent a general e-mail to the staff,
faculty and students of the UPRM requesting sample donations from anyone who
had a mother or a grandmother who had Amerindian traits. This request
resulted in 33 samples.
What did the analyses of these samples reveal?
More surprises. Of the 18 samples from the Indieras, 10 presented Amerindian
mtDNA (55%); of the 5 samples from Miraflores, 4 were Amerindian (80%); of
the 33 from the UPRM, 25 were Amerindian (76%). The high incidence of
Amerindian mtDNA among these three groups was not in itself surprising
because we had intentionally sought out those people who had reason to
believe they were of Amerindian ancestry; but it was surprising to find that
there was a higher incidence among the university students and personnel
than among the inhabitants of the Indieras--who were considered "pure"
Amerindians by the census of 1777 and 1787. This led us to request hair root
samples from additional students regardless of their ancestry. Of the 38
samples obtained in this collection, 20 (53%) presented Amerindian mtDNA.
Such a high incidence in the general student population suggested that,
contrary to the prevailing view, some 53% of Puerto Ricans were of
Amerindian ancestry exclusively through their maternal line. These findings
made it clear that we needed to extend the study by analyzing a
representative sample of the mtDNA of contemporary Puerto Ricans
It was at this point that you requested a grant from the National Science
Yes. In August, 1999, I received a grant from the National Science
Foundation to determine the continental origin of the mtDNA of Puerto Ricans
through the analysis of a representative sample. To select the sample, we
used a computer program that made a random selection of the total population
of Puerto Rico based on the census of 1990. When corrected to take into
account population growth in the last 10 years, the original 872 households
chosen by the program became 1,073. To further insure the randomness of the
sample, we requested hair root samples only from the adult in the household
whose birthday most closely followed the date of the interview. We also
interviewed the donors requesting information about their mothers,
grandmothers, and great-grandmothers, as far back as they could remember, to
learn of their origin. To date, 92% of the potential donors have agreed to
participate, so that we have been able to collect hair samples from 781
What do the analyses of these samples suggest?
The results of the analyses of approximately 300 of these samples identify
62% as Amerindian, 30% as African blacks and 8% Caucasian. So these results
confirm your original findings and cast doubt on the notion that the Tainos
disappeared from Puerto Rico by the end of the sixteenth century. It seems
so, for the moment, especially considering that similar studies in other
countries have yielded similar results. In Belen, Brazil, for example, mtDNA
analysis identifies 59% of the contemporary population as Amerindian, while
Y-chromosome analysis identifies less than 5% as Amerindian. This indicates
that 59% of the population of Belen has an Amerindian mother somewhere down
the ancestral line, while less than 5% of them have a male Amerindian
Are any other traditional beliefs affected?
Yes. Our findings also indicate that the conventional wisdom that
Amerindians would be concentrated in the mountains while African blacks
would be concentrated in the coasts, is not accurate. A strong Amerindian
presence has been found in the southern coastal city of Ponce, for example,
while African black mtDNA is present in the central mountains of Puerto
Rico. Undoubtedly, African slaves must have fled from the coasts to the
mountains even though history does not record such a flight. Loiza Aldea, an
area east of San Juan populated mostly by blacks, presents an interesting
example. By a crown decree from Spain, the colonial government of Puerto
Rico was instructed to place runaway slaves from the British colonies in
what is today Loiza Aldea. This area was chosen by the Crown because it was
the weakest flank of defense of the island, and they hoped that the freed
blacks would help defend the island against British invaders. This is a
historical fact, but what history cannot explain is the great quantity of
fishermen among the blacks of Loiza Aldea. Fishing by blacks is considered
an aberration because black slaves were traditionally taught a fear of the
sea as a way to keep them enslaved. Some historians have argued that the
blacks of Loiza developed their fishing skills through direct contact with
the Tainos of Puerto Rico. The presence of Amerindian mtDNA in Loiza,
supports this hypothesis. In general, the project underlines the fact that
biology can help reveal ethnic origins as well as population growth and
migration in the development of a people.
Sounds like a true meeting of the arts and sciences. What comes next?
Our findings to date are of great interest to historians and Puerto Ricans
in general, but another important goal of our study of the continental
origin of the mtDNA of Puerto Ricans is to determine the variability sites
within the mtDNA so that the filial relationship among the remains found in
Amerindian interments can be ascertained. A detailed characterization of
Amerindian mtDNA will identify variable sites that will facilitate the
design and execution of ancient DNA studies. Ancient DNA studies are
necessary to relate the succesive historic ceramic cultures found in Puerto
Rico to Pre-Columbian migrations and population expansions. They may also be
conducted to study the relationship of prehistoric Puerto Ricans to their
neighbors, as well as their burial and religious practices. So, as you can
see, we have only just begun our research. Fortunately, the large number of
samples of contemporary Puerto Rican mtDNA that we have been able to collect
is giving us a good basis for accomplishing our research goals; and the
results of our analyses to date have set the stage for Y-chromosome studies
that will allow us to estimate with precision the complete ethnic
composition of the various geographic regions of Puerto Rico and define the
contribution of both sexes to this composition. We have also found a number
of variable sites in the mtDNA of Puerto Ricans, so we may some day be able
to tell maternal relationships among our ancestors.
(c) Delaware Review of Latin American Studies
From the website of the Delaware Review of Latin American Studies
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