Tressa Jamison
The Australian Aboriginal People:
Dating the Colonization of Australia
Abstract
The colonization of each continent by modern human populations remains an important question in our history as a species. Studies of variations in mitochondrial genomes, Y-chromosomes, satellite DNA, and other genetic markers can be used to estimate the time of divergence of one population from another. Recent advancements in technology have advanced our capabilities in genetic analysis. In particular, PCR can be used to amplify, study, and sequence DNA from long-deceased specimens.
Ingman and Gyllensten studied 101 complete mitochondrial genomes from contemporary
populations in Australia and many other
regions. The mitochondrial genetic diversity of Aboriginals is
remarkably high, similar to that found in Asia. Ingman and Gyllesten estimated colonization of Australia at
40,000-70,000 years ago and supported multiple waves of migration. Another study by Adcock and others
indicates that anatomically modern humans were present in Australia before
complete fixation of the mtDNA lineage, but does not establish a colonization
date.
A study of Y-chromosome variation by
Vandenburg and others in 1999 revealed two haplotypes unique to Australian
Aboriginals. Most (78%) of
Aboriginal haplotypes fell into two clusters, possibly indicating two original,
separate lineages of founding Aboriginal Australians.
As recently as the 1960Õs, anthropologists
predicted the colonization of Australia at less than 10,000 years ago. Advancments in thermoluminescence dating
pushed back the estimated colonization time to 50,000-60,000 years ago in the
early 1990Õs. Improvements in
thermoluminescence and carbon dating techniques adjusted previous colonization
dates of 50,000-60,000 years to approximately 45,000 years.
The results of morphological dating best-support the multiregional evolution hypothesis of modern humans, and further complicate the possible ancestry of modern Australian Aboriginals.
Mitochondrial DNA analysis, recent thermoluminescence dates, and the history of ocean levels coincide roughly to indicate a colonization of Australia at approximately 50,000 years ago.
Introduction
Throughout recorded history, humans have been defined by a desire to know ourselves: Where did we come from? How did we get here? Where are we going? Many questions are directed towards our appearance as a species and subsequent populating of the earth, -- piece by piece. One particularly large piece of the earth is Australia: a continent, a country, and a mystery in human history.
Though the major events in Australian natural history have been revealed, for the most part, human history remains a great topic of debate. Population geneticists, anthropologists, archaeologists, biologists, and theologians have agonized over the arrival of the founders of the Australian Aboriginal population since Europeans colonized Australia in the 18th century (Brown, 1997). This paper seeks to present, discuss, and scrutinize evidence from various scientific ÒcampsÓ explaining the origin of the Australian Aboriginal people.
AustraliaÕs history has been influenced by many circumstances and events. In the last 600 million years, the continent of Australia has drifted around the globe, encountering vastly different climates, ocean levels, continental neighbors, and resident organisms. Most recently, Australia has come to reside with neighbors of New Zealand, Papua New Guinea, various islands of the South Pacific, and Tasmania. Focusing on the last 100,000 years, AustraliaÕs native species have been shaped and pressured by AustraliaÕs considerable size and extreme arid interior.
The circumstances of the environment, along with population isolation and the necessary time interval, allow species on separate continents to be distinguished from one another via slight variations. Species isolated in Australia for thousands of years display traits and variations not seen in their counterparts on other continents. Indigenous human populations are no exception to the rule. While one population may be phenotypically* similar to another, variations exist in genotypes which are not evident to the outward observer (Strachan and Read, 2004). Studies of variations in mitochondrial genomes*, Y-chromosomes, satellite DNA*, and other genetic markers can be used to estimate the time of divergence of one population from another (Strachan and Read, 2004).
In addition to genetic studies, archaeological and anthropological evidence can provide clues and a context on which the genetic information may be applied (Brown, 1997). Some human remains in Australia have been dated to correlate with other evidence, or in some cases, contradict it (Brown, 1997). Genetic and anthropological/archaeological evidence taken together may begin to explain the origin of the Aboriginal Australian people.
The
Arrival of the Australian Aboriginal People
Introduction to Human Population
Genetics
Recent discoveries in molecular biology and genetics have made it possible to use DNA, as opposed to morphological* features, to describe the relatedness of species and populations. By comparing the relative similarities and differences in the DNA sequence, the divergence of species can be plotted on an evolutionary tree (Strachan and Read, 2004). More recent population divergence can also be studied using slight variations in the genetic material. In particular, PCR* can be used to amplify, study, and sequence DNA from long-deceased specimens (Strachan and Read, 2004; Adcock et al., 2001).
*Indicates a particular term is defined in the
glossary.
Among the most commonly studied human sequences is the mitochondrial genome. Mitochondrial DNA (mtDNA) is significant because it is passed from mother to offspring (never father to offspring) and evolves at a different rate than the nuclear genome (Strachan and Read, 2004; Ingman and Gyllesten, 2003). Living humans are remarkably homogenous in regards to mtDNA in comparison to other species (Strachan and Read, 2004; Ingman and Gyllesten, 2003). Still, there are some differences between populations which may be compared.
Sequence variation in the Y-chromosome may also be used to study human populations (Vandenburg et al., 1999). The Y-chromosome can be seen as the male analog of the mtDNA, as the Y-chromosome is only passed from father to son (Vandenburg et al., 1999). The Y-chromosome evolves at a different pace than mtDNA, but the two taken together may yield powerful information.
Population Genetics Applied to Ancient Australia
Recent advances in DNA technology and molecular biology have provided new insights into the colonization of Australia via examination of genetic material. Specifically, todayÕs living Aboriginal Australians may be compared to ancient anthropological specimens unearthed in Australia, New Guinea, Africa, and a number of other contemporary populations to propose a time of divergence of the founding population of Australia.
*Indicates a particular term is defined in the
glossary.
Studies of mtDNA and Y-chromosome variation are among the most
commonly studied and informative.
Of particular importance is a study conducted by Ingman and Gyllensten in
2003. Ingman and Gyllensten
studied 101 complete mitochondrial genomes from contemporary populations in Australia, New Guinea, Asia, India, Europe,
Melanesia, and Polynesia (Ingman and Gyllesten, 2003). Samples were selected from a wide range
of subpopulations. Based on an estimated substitution rate for the
mitochondrial coding region of 1.7x10-8
substitutions per site per year, and using modern computational/statistical
methods* to develop a likely evolutionary tree, the deepest genetic divergence occurred
approximately 71,000±12,000 years ago (Ingman and Gyllesten,
2003). Notably, the mitochondrial
genetic diversity of Aboriginals is remarkably high, similar to that found in
Asia (Ingman and Gyllesten, 2003) This supports the hypothesis that Aboriginals
migrated to Australia earlier in human history than first suspected (Ingman and
Gyllesten, 2003). Mitochondrial
genome data indicates that Australia was colonized between 40,000 and 70,000
years ago from a heterogeneous source population or by multiple migrations of
smaller groups (Ingman and Gyllesten, 2003).
*Indicates a particular term is defined in the
glossary.
Another type of study by Adcock
and others in 2001 examined mtDNA from 10 ancient Australians from several archaeological sites (Adcock et al., 2001).
The study included 4 specimens within the skeletal range of living
Australians and 6 with morphologies outside the range of contemporary
indigenous Australians (Adcock et al.,
2001). The Lake Mungo 3 specimen (see
map next section) is the oldest (approximately 60,000 years ago, by
thermoluminescence dating) Òanatomically modernÓ human from which DNA has been
recovered. This ancient manÕs
mtDNA only survives as a segment inserted* into chromosome 11 of the nuclear
genome, which is now widespread among human populations (Adcock et al., 2001).
This indicates that that Lake Mungo 3Õs lineage probably diverged before
the most recent common ancestor of contemporary human populations. The other ancient Australian
individuals have mtDNA sequences descended from the most recent common ancestor
of living humans (Adcock et al.,
2001). Results indicate that
anatomically modern humans were present in Australia before complete fixation
of the mtDNA lineage (Adcock et al.,
2001). While this result does not
establish a definite colonization period of Australia, it is a challenge to
current opinions on human origins, and remains an interesting piece in the
human evolutionary puzzle (Adcock et al.,
2001).
In contrast to
Australian mtDNA, Australian Y-chromosome diversity is surprisingly limited. One would expect that an isolated
population would develop variations in Y-chromosome DNA in parallel with mtDNA
variations. A study by Vandenburg
and others in 1999 revealed two haplotypes unique to Australian Aboriginals
(Vandenburg et al., 1999). The frequency distributions of 4 highly
polymorphic Y-chromosome specific microsatellites* were determined in 79
unrelated Aboriginal males from the Northern Territory thought to be the site
of earliest colonization (Vandenburg et al., 1999).
These results were compared with other worldwide populations at the
locus* and haplotype* level.
Combing all 4 microsatellites produced 41 unique haplotypes. Most (78%) of Aboriginal
haplotypes fell into two clusters, possibly indicating two original, separate lineages
of Aboriginal Australians (Vandenburg et al., 1999).
This is in contrast to the study by Ingman and Gyllesten, which
supported multiple migrations of Aboriginal Australians.
*Indicates a particular term is defined in the
glossary.
Archaeology and
Anthropology Applied to Ancient Australia
Anthropological and
archaeological evidence may support or contradict the estimated time of
colonization of Australia obtained by genetic analysis. As recently as the 1960Õs,
anthropologists predicted the colonization of Australia at less than 10,000
years ago (OÕConnell and Allen, 2004).
In the 1980Õs, Carbon 14* dating pushed back the estimate to
approximately 40,000 years ago. Archaeologist
Rhys Jones observed that this was suspiciously close to the limits of Carbon 14
dating techniques (OÕConnell and Allen, 2004). JonesÕ suspicions proved justified when thermoluminescence*
dating pushed back the estimated colonization time to 50,000-60,000 years ago
in the early 1990Õs (OÕConnell and Allen, 2004; Roberts et al., 1994).
One particular broad
and very recent study by OÕConnell and Allen reviewed data from over 30
archaeological sites, focusing on 5 sites with proposed ages of greater than
45,000 years (OÕConnell and Allen, 2004).
Improvements in thermoluminescence and carbon dating techniques pushed forward
previous colonization dates of 50,000-60,000 years to approximately 45,000
years (OÕConnell and Allen, 2004).
ÒOn current
evidence, 45,000 calendar years is the best outside date for the colonization
of Pleistocene Sahul. It requires more than the mere announcement of yet
another old date, without adequate consideration of context, to change this
conclusionÓ (OÕConnell and Allen, 2004).
*Indicates a particular term is defined in the
glossary.
To review, archaeologists
in the 1960Õs dated the colonization of Australia at about 10,000 years ago (OÕConnell
and Allen, 2004). Carbon 14 dating
pushed this estimate back to about 40,000 years ago in the 1980Õs (OÕConnell
and Allen, 2004).
Thermoluminescence dating in the 1990Õs pushed the colonization back
further to approximately 50,000-60,000 years ago (Roberts et al., 1994).
And most recently, improvements in dating techniques adjusted the
estimate to approximately 45,000 years ago (OÕConnell and Allen, 2004). Perhaps further advances in
archaeological dating techniques are required to solve this mystery. See Figure 1 below for prominent
archaeological sites in Australia and Southeast Asia, including the Lake Mungo
site mentioned in the previous section.
Figure 1: Significant archaeological
sites in Sahul* and adjacent parts of southeast Asia (Brown, 1997).
*Indicates a particular term is defined in the
glossary.
In addition to dating specimens, anthropologists look to morphological features to date human remains. One particular specimen of importance is WLH50, recovered from a deflating land surface in the Garnpung/Leaghur Lakes region of southwestern New South Wales (Brown, 2004; Hawks et al., 2000). This fragmentary cranial vault is the source of considerable controversy. In 1987 the specimen was Carbon 14 dated as ÒmodernÓ (Brown, 1997). In 1991 the specimen was dated older than Lake Mungo remains on morphological grounds (Brown, 1997). Electron spin resonance estimates the specimen to be 30,000 years old (Brown, 1997). This specimen is of considerable importance because various theories of human evolution make different predictions about its ancestry (Hawks et al., 2000), namely the complete replacement hypothesis and the regional evolution hypothesis. Researchers used metric and non-metric data from three specimens that are potential ancestors of WLH50 to test predictions of WLH50Õs ancestry (Hawks et al., 2000). The results best-support the multiregional evolution hypothesis of modern humans, and further complicate the possible ancestry of modern Australian Aboriginals (Hawks et al., 2000).
*Indicates a particular term is defined in the
glossary.
In direct disparity, a study conducted in 1999 (previous to
the study by Hawks and others) asserts that evidence collected from WLH50 as
supportive of multiregional evolution is unreliable (Ant—n and Weinstein,
1999). This study concludes that
it is unwise to use non-metric traits, such as cranial contours, to infer
relatedness between fossil Australians and other hominids of that time (Ant—n and
Weinstein, 1999). According to the
study, the frequency of non-metric traits may be influenced by artificial
deformation and pathological hypervascularity/hyperostosis* (Ant—n and Weinstein,
1999). Peculiarly, the study
by Ant—n and Weinstein preemptively and specifically contradicted the study by
Hawks and others. This is just one
indication of the disagreement among archaeologists/anthropologists on the
colonization date of Australia and related human history.
Conclusions
Though genetic,
archeological, and anthropological studies on the colonization date of Australia
are contradictory at times, some general conclusions may be reached when the
evidence is considered collectively.
Based on mtDNA
analysis, Ingman and Gyllesten proposed a colonization date of 71,000±12,000
years ago and suggested multiple migrations to Australia or a
heterogeneous source population. Vandenburg and others did not suppose a
colonization date, but based on Y-chromosome analysis suggested two distinct
waves or colonies of founding Aboriginal Australians. The most recent and reliable thermoluminescence dates
support the estimated colonization of Australia at approximately 45,000 years
ago (OÕConnell and Allen, 2004).
Morphological studies in general have supported the multiregional model
of human evolution (Hawks et al.,
2000), and hence have not supported a colonization date because these models
assert that humans arose independently in Australia.
*Indicates a particular term is defined in the
glossary.
The range of IngmanÕs and
GyllestenÕs study (40,000-70,000 years ago) coincides roughly with OÕConnellÕs
and AllenÕs estimate of colonization at 45,000 years ago. Brown points out that sea levels
dropped around 50,000 years ago (Brown, 1997) and would enable humans to
migrate to Australia without large ocean obstacles (see Figure 1 on page 8). Although more evidence is necessary to
declare a definite establishment date of AustraliaÕs native human population,
Aboriginal Australians may justifiably presume a history of about 50,000
years. Improvements in genetic
analysis (undoubtedly on the way), advancements in thermoluminescence dating,
and the discovery of additional human specimens will help to clarify this
colonization date.
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GLOSSARY
Carbon 14 dating- the determination of the age of old material (as an archaeological or paleontological specimen) by means of the content of carbon 14 (Merriam-Webster Online).
Computational/statistical methods to develop evolutionary trees-involves generating most likely trees, most simplistic trees, and statistical techniques to predict an evolutionary tree from a set of data (Ingman and Gyllesten, 2003).
Haplotype- a set of genes that determine different antigens but are closely enough linked to be inherited as a unit; also : the antigenic phenotype determined by a haplotype (Merriam-Webster Online).
Hypervascularity- excessive growth
relating to a channel for the conveyance of a body fluid, especially blood
vessels (Merriam-Webster Online)
Hyperostosis- excessive growth or
thickening of bone tissue (Merriam-Webster Online)
Locus-
the position in a chromosome of a particular gene or allele (Merriam-Webster
Online).
Mitochondrial genome-the complete set of DNA contained and maintained by mitochondria in the cells of eukaryotes.
Morphology- Òa : a branch of biology that deals with the form and structure of animals and plants b : the form and structure of an organism or any of its parts.Ó (Merriam-Webster Online)
Nuclear inserts-DNA inserted into the nuclear genome and perpetuated
by replication, may consist of transposons or other sequences (Strachan and
Read, 2004).
PCR-Polymerase Chain Reaction: ÒUsually designed to permit selective amplification of a specific target DNA sequence within a heterogeneous collection of DNA sequences.Ó (Strachan and Read, 2004)
Phenotype- Òthe visible properties of an organism that are produced by the interaction of the genotype and the environment.Ó (Merriam-Webster Online)
Sahul- ÒSahul is the continent formed when glacio-eustatically lowered sea levels exposed dry land connections between Australia, New Guinea and Tasmania.Ó (OÕConnell and Allen, 2004).
Satellite DNA-Òsequences whose base composition is significantly different to that of the bulk DNAÓ (often repeated sequences). (Strachan and Read, 2004)
Also mini- and microsatellite DNA (smaller repeated sequences)
Thermoluminescence dating-