Two articles published last month in the American Journal of Physical Anthropology are starting to greatly complicate bioarchaeologists' use and interpretation of stable oxygen isotope ratios in an attempt to understand migration and mobility in the past. Science is constantly progressing, and it can be challenging to keep up with the latest research. The real challenge for me, though, is in interpreting the isotope analyses I have done on populations from Imperial Rome - first because only one other oxygen isotope study has been done in the entire Italian peninsula, and second because the ancient Romans were quite unlike other archaeological populations in their mobility and importation of food and water. This post, then, works through some of the ideas I laid out in my dissertation and adds to them in light of recent articles on oxygen isotope analysis.
Oxygen Isotope Redux
Studying the relative amounts of stable oxygen isotopes in ancient organic material seems to have begun with palaeoclimate studies, as oxygen isotopes are related to various climatological factors that affect the elemental composition of water. The relative amounts of oxygen isotopes of both meteoric (rain, snow) and environmental water (rivers, springs, lakes) vary by region in relation to factors such as temperature, humidity, distance from the coast, latitude, rainfall, and elevation. This means that different water sources in different areas have different ratios of stable oxygen isotopes.
|Graph showing variations in oxygen and hydrogen isotopes|
(credit: fig. 6 from the SAHRA website)
The mammalian body needs oxygen to survive - not just in the form of air (inspired oxygen) but also in the form of water and food. Researchers became interested in looking at the amount of oxygen trapped in mammalian tissue decades ago and began measuring the abundance of two different isotopes of oxygen: 18O and 16O. The ratio between these two measurements - written as δ18O ‰ (per mil) - results from fractionation of the two isotopes, which is caused by different metabolic processes.
Starting in the 1990s, though, bioarchaeologists began studying stable oxygen isotopes in order to investigate ancient migration. If the majority of the oxygen that a person ingested or inspired while his teeth were forming came from local water sources, the measured δ18O value from his hard tissue would be characteristic of the geographical peculiarities of that water, taking into account metabolic fractionation processes. It should be possible, then, to use δ18O numbers to identify individuals who accessed either local or nonlocal water sources and, by inference, locals and immigrants. In the early 2000s, oxygen isotope analysis became widely used in answering questions about past human mobility, including identifying immigrants and pinpointing their geographical homelands.
Problems with Finding Immigrants' Homelands
Within the past decade, though, researchers have learned much more about oxygen isotope ratios, including the metabolic processes that affect fractionation. In fact, two articles published online on May 3 by the American Journal of Physical Anthropology demonstrate just how complicated it can be to interpret oxygen isotope ratios from past human populations - a topic that I've been thinking about a lot lately as I write up for publication my own oxygen data from Imperial Rome.
|δ18Ow contour lines in Italy|
(credit: Longinelli & Selmo 2003)
Sickle-Cell Disease Affects Oxygen Isotopes
Reitsema and Crews (2011) throw another wrench into oxygen isotope analysis with their paper, "Oxygen isotopes as a biomarker for sickle-cell disease?" Oxygen isotope fractionation results from various metabolic processes, as I mentioned above. In humans, differing oxygen isotope ratios can result from smoking, exercise, and disease. For example, there is increased fractionation in smokers, due to their compromised ability to diffuse oxygen through the pulmonary membranes. Conversely, people who engage in routine exercise appear to have a decrease in fractionation because of the increased rate of respiration. It has also been established that oxygen isotope fractionation is lower in people suffering from anemia, as the binding of oxygen to hemoglobin is a fractioning process.
Because of these known fractionation effects, Reitsema and Crews hypothesized that the bone tissue of an organism with sickle-cell disease would have a different oxygen isotope ratio than a healthy organism. They studied bone apatite from 24 mice - 8 control and 16 transgenic mice with human HbS (mutant hemoglobin S) genes that cause sickle-cell disease. The bones of the sick mice had a significantly (p=.002) lower oxygen isotope ratio than those of the healthy, control mice. In fact, the sickest mice had the lowest oxygen isotope ratios.
This study has far-reaching implications for bioarchaeological analysis because it is currently quite difficult to identify individuals with sickle-cell disease (and other kinds of anemia) within an archaeological population. We tend to use frequencies of observable pathologies - like cribra orbitalia and porotic hyperostosis - to infer anemia in an individual, but these are non-specific disease processes and cannot be directly tied to sickle-cell disease or another condition.
Sickle-Cell and Malaria
|Three normal and one sickled RBC|
(found online here)
If the HbS allele is so dangerous to humans, why hasn't it been selected out of the population? Well, researchers who first studied the prevalence of the HbA and HbS alleles discovered that several populations in Africa, India, and the Mediterranean had fairly high frequencies of the HbS allele - up to 20% of the populations they studied had one copy of the HbS allele, meaning those people were heterozygous. The advantage of having one copy of the HbS allele was discovered to be a partial resistance to malaria, a disease vectored by the mosquito in warm areas like central Africa, the Mediterranean, and southern Asia. This can be illustrated in the following map of Africa:
|Frequency of malaria (left) and sickle-cell trait (right) in Africa|
(credit: Wikimedia Commons)
The heterozygote advantage here is that people with one copy of the HbS allele have enough normal hemoglobin to survive into adulthood yet enough sickled red blood cells to fight off malaria (as the malaria parasites don't survive in the sickled red blood cells).
Malarial Romans or Immigrants?
One of the major questions facing bioarchaeologists who work in ancient Italy is, understandably: What was the prevalence of malaria in the population? This question has been tackled primarily by Robert Sallares using historical data in his book Malaria and Rome: a History of Malaria in Ancient Italy (2002) and by David Soren using osteological data in papers like, "Can archaeologists excavate evidence of malaria?" (2003).
|Incidence of malaria in 1940s Italy|
(found online here)
So, what do the Pollard et al. and Reitsema and Crews articles mean for our understanding of the ancient Romans? Let's take a look at this graph that I've been pondering for the past couple of weeks:
|Oxygen isotope ratios from Portus (Prowse et al. 2007) and Rome (Killgrove 2010)|
This histogram shows the distribution of oxygen isotope ratios in two Imperial-period populations: one from the cemetery of Isola Sacra between Ostia and Portus Romae (Prowse et al. 2007) and one from two cemeteries (Casal Bertone and Castellaccio Europarco) located just outside the city walls of Rome (Killgrove 2010). There are outliers in both populations - and a quite dramatic one from Portus - but the distributions themselves are different. The population from Portus Romae has lower oxygen isotope ratios on average than the population from Rome itself.
Based on what we've learned about oxygen isotope ratios, there are a number of possible explanations for this variation:
1. Water Sources. The city of Rome had a vast network of aqueducts, whereas Portus had only one. The aqueducts from Rome, though, were fed by springs at the base of the Apennine mountain range. With increasing elevation, we actually get a concomitant decrease in oxygen isotope ratios. So I don't think that water sources alone can explain the difference, since we would expect oxygen isotopes from Rome to be lower than from Portus.
3. Homeland. Portus and Rome were both immigrant-receiving areas. This is not a stretch of the imagination, since slavery was widespread in the Empire and both cities would also have attracted merchants, students, and travellers. But suppose each city attracted immigrants or slaves from specific homelands. Prowse and colleagues interpreted their oxygen isotope data to suggest that people may have been coming to Portus from areas with higher altitude. I interpreted my oxygen data to suggest that, while many people seem to have been local, there were plenty of people with lower and higher oxygen isotope ratios, who came to Rome from areas with higher elevation/colder climates and lower elevation/warmer climates. This interpretation has been my favorite so far - I don't know of any historical evidence that shows that different cities in the Empire pulled slaves or other immigrants from certain areas, and I think this histogram lets me at least suggest that it's a plausible scenario.
4. Malaria. The Reitsema and Crews article - although it's about bone apatite in mice rather than human enamel - brings up another possibility, though: that the people at Portus were more affected by sickle-cell disease than the people from Rome. Both areas were fairly malarial, but Ostia and Portus were famously abandoned several times as a result of malaria. A greater prevalence of malaria could have had an influence on the ancient population of Portus/Ostia, as the people fittest to survive the marshy area would have been those with at least one copy of the HbS allele. Or perhaps Portus attracted more people from Africa - another area with high frequency of the HbS allele - than did Rome, as merchants or traders.
I don't have a concrete interpretation of this graph yet. Perhaps I'm making too much out of the variation - after all, oxygen isotope analyses have only been done by Prowse and colleagues at Portus and by me at Rome. It's difficult to say with only two large samples what the variation means, and I'm not sure when I'll be able to tease out the effects of diet, water source, immigration, and disease on the oxygen isotope ratios of the Romans.
UPDATE - 9/23/11. This post has been translated into Ukranian by Sofya Kravchuk and can be found here. I'm happy that Sofya wanted to translate this post, and I hope I gain some Ukranian readers!
Killgrove K. (2010). Migration and Mobility in Imperial Rome. PhD dissertation, University of North Carolina at Chapel Hill.
Pollard AM, Pellegrini M & Lee-Thorp JA (2011). Technical note: Some observations on the conversion of dental enamel δ18Op values to δ18Ow to determine human mobility. American Journal of Physical Anthropology PMID: 21541927.
Prowse TL, Schwarcz HP, Garnsey P, Knyf M, Macchiarelli R, & Bondioli L (2007). Isotopic evidence for age-related immigration to Imperial Rome. American Journal of Physical Anthropology, 132 (4), 510-9 PMID: 17205550.
Reitsema LJ & Crews DE (2011). Brief communication: Oxygen isotopes as a biomarker for sickle-cell disease? Results from transgenic mice expressing human hemoglobin S genes. American Journal of Physical Anthropology PMID: 21541922.
Sallares R. (2002). Malaria and Rome: a History of Malaria in Ancient Italy. Oxford University Press.
Soren, D. (2003). Can archaeologists excavate evidence of malaria? World Archaeology, 35 (2), 193-209 DOI: 10.1080/0043824032000111371.