This course is about what we can learn from examining the human skeleton, and how we can use this knowledge to reconstruct the lives of people who lived in the past. In archaeology and anthropology, human skeletal remains can provide unique insights into the past and the present; insights that cannot be gained otherwise. These insights are explored in five main themes spread out over five weeks of learning. First, it is shown how age-at-death, sex and stature can be estimated by the close examination of (archaeological) skeletal remains. In subsequent modules it is shown how human bones can provide information about the diseases and injuries that people suffered from and what they ate. Also, it is shown how the human skeleton provides information about the kinds of activities that people engaged in and about how they migrated and moved around their landscapes. In this course, you will examine all aspects of the human skeleton that can provide us with information about these different facets of life. Together we will explore the scientific field that is known as human osteoarchaeology. - Human, because it is about us and our ancestors, - Osteo, because it is about our bones, - Archaeology, because we use this information to better understand the behaviors and events experienced by past people. During the course, you will decipher the clues left behind in the skeletons of past peoples with the methods and techniques that are presented. You may also discover some clues hidden in your own skeleton and what they reveal about the life that you are living. Want to know more? You can take a look at the course trailer here: https://www.youtube.com/watch?v=fJi22TxzpRw
Mobility and Migration: Strontium Isotopes
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Del curso dictado por Universiteit Leiden
Osteoarchaeology: The Truth in Our Bones
253 calificaciones
De la lección
Bones to Mobility and Migration
Welcome to the last module of this course!
This week is all about mobility and migration, so people moving through the landscape. How much did people relocate, and where did they go? We can answer these questions by looking at the shape of longbones, and by studying the isotopes and the DNA that can be found in human bones. Stick with us to find out how exactly these methods work, and what they reveal about population movements in the past.
Conoce a los instructores
Andrea Waters-RistAssistant Professor
Faculty of Archaeology
Welcome to the second case study of the mobility and migration module.
In this video we will learn about strontium isotopes and how they have been
increasingly used to infer patterns of migration in past populations.
Recall that video 3.2 reviews the basics about stable isotopes.
What they are, how they work and
how osteoarchaeologists can detect different isotopes.
That video was focused on the isotopes that are commonly used for
the reconstruction of diet.
But as you will learn, there are also isotopes that are used to tell if
a person moved from one geographic area to another in their lifetime.
Perhaps the strontium isotope values of your bones and
teeth would reveal that you had moved from one part of the world to another.
You can tell us on the online discussion forum.
Strontium is a trace element found in rocks which through weathering gets into
water and soil.
Which there by becomes incorporated into the tissues of plants and animals.
There are four naturally occurring strontium isotopes
present in different proportions as shown here.
Osteoarchaeologists use the ratio of two of these,
strontium 87 to strontium 86 for migration research.
That's because the amount of strontium 87 varies in rocks according
to the age of the rock.
For example, very old rocks, such as granites and
shales, have higher strontium 87 to 86 ratios than younger rocks.
So the main type or types of rocks underlying where you live,
referred to as bedrock, will give you a strontium isotope signature that
varies from geographic areas composed of different bedrock.
This figure shows the strontium-87 to 86 values of bedrock from the United States.
Such maps are called isoscapes.
Importantly, the world's oceans have a near constant strontium-87
to 86 ratio of 0.7092.
Areas along the coast will have the same or at least a very similar value.
Moving inland and away from coastal environments,
bedrock strontium-87 to 86 ratios have been found
to vary between 0.703 and 0.760.
In 1985, Ericson was the first archeologist to demonstrate
that the strontium-87 to 86 ratio in human bones and
teeth reflects the strontium ratio of the geographic area that
a person lived in while that tissue was being formed.
One may ask how does strontium even get into our bones and teeth.
Well, strontium has a molecular structure similar to calcium.
Calcium, as you know, is a major component of the mineral of both bones and teeth.
And quite simply a very small amount of strontium
isotopes will replace calcium in hydroxyapatite.
Now recall that tooth enamel forms during infancy and childhood.
The last bit of your enamel in your third molars forms at 12 to 13 years of age.
And after this, the chemical composition of your enamel does not change.
So for your entire life, this enamel holds a record of your childhood home.
In contrast, your bones as you know are always turning over, remodeling.
And so, always changing and reflecting the most recent period of life.
So the strontium-87 to 86 value of an adult's bone mineral
will reflect their adult home.
Different strontium-87 to 86 values in
a person's enamel compared to their bone can therefore indicate a geographic
change of residence from childhood to adulthood that is a migration event.
Another common approach is to characterize the local bio-available range
of these strontium isotope ratios and
consider anyone who falls within two standard deviations of the average as local,
and anyone who falls beyond this 95% range is non-local.
This is shown in a figure here by Boric and Price,
where a relatively high number of non-locals entered the Danube Gorge's
region in the North Central Balkans around 8,200 years ago,
which they interpret as evidence for early farmers entering the region.
The non-local could come from an adjacent, close-by region that has different
bedrock, or from hundreds or even thousands of kilometres away.
Matching the non-local strontium isotope value to that of geographic regions with
the same value can allow us to see what possible homelands they came from.
And sometimes in combination with other isotopes,
such as oxygen and sulphur, as well as clues from the skeleton itself and
material culture items, we can deduce which homeland was most likely.
Now the principle underlying the use of strontium isotopes
to reconstruct migration in past populations is quite straight forward,
but the reality is seldom as simple.
And we have to keep a few things in mind that impact the feasibility
of this kind of research.
First, in order to use this method there has to be sufficient isotopic
variation in the geographic area under study.
There are large areas of the world underlain by the same bedrock or
with bedrocks are very similar strontium isotope values,
such that migration within and between this areas simply cannot be detected.
Second, one's diet can influence if their strontium isotope
values are useful to track migration.
One example of this is the consumption of a lot of seafood.
As regardless of where one lives, if they eat a lot of seafood they will
have a strontium-87 to 86 value close to or of sea water.
Another example concerns modern day and some select past groups and
individuals who ate a lot of imported foods.
If you eat a lot of non-locally grown food as is the case for
many of us today in western developed nations, your strontium-87 to 86 value
will not be reflective of where you live, but rather where your food came from.
Third, the accuracy of the method of determining what the local strontium
isotope value of a region is is important to consider.
Archaeologists have characterized the strontium isotope value of an area in
different ways including by direct measurements of exposed bedrock or soil;
or by determining the value in plants; or from the bones of small mammals; or
the shells of land snails,
animals that can only roam over a small area.
Each of these methods has it's own advantages and
disadvantages that have to be kept in mind.
And finally bone strontium ratios can be potentially altered due to chemical,
physical or biological changes after death
particularly from the soil, if the bone was buried,
and the assessment of any such alteration must be done for each and every sample.
But if all of these factors can be taken into account,
which is often the case, strontium isotope analyses of past peoples
can be a powerful method to reconstruct migration.
Now to talk to us about one example from the Caribbean region is
Dr Jason Laffoon is a postdoctoral researcher with Leiden University, and
the Free University Amsterdam specializing in isotope bio-archaeology.
Jason, thank you very much for joining us today.
So to begin, I'd like to ask you to tell us about your research area, and
what some of the main goals of this research are.
>> Yes, thank you for having me.
The research area is the Caribbean region which is consisted of series of island
archipelagoes and the surrounding costal mainland of Central and South America.
The earliest element of the islands of the Caribbean occurred for about 5,000 BC.
But not very much known about this early inhabitants
mostly because of issues of preservation.
The lack of well preserved archaeological or osteological materials to study.
My research primarily focuses on the later occupation
of the islands of the Caribbean.
What we call the late ceramic age that dates from approximately AD 500 to
AD 1500.
And the previous research in the region had revealed the widespread
circulation of various aspects of material culture agent from ceramic
styles to shared lithic resources, to similar manufacturing techniques.
So we knew the people were probably moving from the mainland to the island,
in between the islands and my research question focused on where
exactly people were moving from and what were the patterns of migration over time.
>> So in order to investigate the patterns of migration over time,
you use strontium isotopes.
As our users have been learning, the first step in such research is to characterize
the strontium isotope variability of the region that you want to investigate.
So can you tell us a bit how you undertook that task?
>> Certainly.
The method that we found that worked best in the Caribbean region
was the bioavailable method.
And that's where you characterize the variation for
a site or population based on independent sampling of biological samples.
>> Okay. >> So we focused out primarily
on modern plants and archaeological and modern animal remains.
And we analyzed more than 300 samples from all the different islands and
island groups in the Caribbean region.
>> Fantastic, so that's a very good base with which to start.
The next step then being the analysis of the humans from these
different archaeological sites in different regions and time periods.
Can you tell us a bit about the sample size that you were able to look at,
and also what tissue you analyzed?
>> Certainly, in terms of the tissues, we focused solely on dental enamel.
And that's for two primary reasons.
Dental enamel is the most resistant to diagenic alteration.
That's changes to the tissue based on being buried in the ground for so long.
And we tried to develop a very large data set.
So what we did is we focused on as many individuals as we could, and for
my dissertation we analyzed more than 360 human individuals,
from 26 different sites on 12 different islands.
>> Amazing, very, very large sample size.
So with all of that data, you must have been able to say quite a bit.
If you could tell us, then, about some of the more remarkable or
spectacular results that came out of this research.
>> Yes, of course.
So the first thing that we found that was one of the most interesting results
actually was that, for every single population that we analyzed,
we identified non-local immigrants.
And the proportions of immigrants at each population were highly variable,
ranging up to more than 30% for some populations.
The second very interesting result that we found is that these non-local
immigrants for each population were not all originating from the same place, and
they displayed a high degree of isotopic diversity, indicating diverse origins for
different immigrants.
Other very interesting results included that there was
no sex-based differences in immigration, so that an equal number of male and
female immigrants proportionally over time.
And that suggested that was there was no migration
linked to post marital residents, such as patrilocal or matrilocal residents.
For some populations but not others there were clear differences between local and
non local immigrants in terms of both dietary practices and burial treatment.
The overall chronological pattern that we observed was a
decrease migration over time.
And we think this is most likely linked to demographic variables such as
increasing population densities and
subsequent decrease in social distances between settlements.
And lastly for the sole early colonial period of population that we analyzed,
we identified several very long distance migrants, individuals originating
from the Maya regions of Mesoamerica and from Western Africa.
>> Wow, very interesting.
Thank you very much for taking the time to share your research with us today.
>> Thank you for having me.
>> In this video,
you learned about strontium isotope analyses of the mineral in teeth and
bone as a method to reconstruct aspects of an individual's migratory history.
With this method, it is in some cases possible
to determine that an individual was born far away from where they died.
You also heard about the strontium isotope research in the Caribbean, and
how such data have improved our knowledge of the prehistoric colonization and
migration of the area.
Coming up next is the final case study for this module where you'll hear about
how the genetic information contained in each and every one of us and
in fossils going back hundreds, thousands, and even hundreds of thousands of years,
has revealed a tremendous amount about our migration.
Both at global and continental scales, as well as at smaller regional scales.
See you soon.
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