|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 in San 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
remains could 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 to approximately 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
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
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 areas 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
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 Foundation?
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
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 ancestor.
Are any other traditional beliefs
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.