Chemistry Unlocks Ancient Secrets

Some ancient civilizations left writings and drawings that archaeologists and anthropologists decipher to uncover clues to long-ago practices—daily activities, traditions, technologies, and travels. But even societies that didn’t leave comprehensive written accounts invariably left details in another form—chemical records—and cutting-edge analytical chemistry methods are getting better at interpreting these molecular clues.

“There’s a lot of information to be had from archaeological chemistry that just cannot be discovered by visually examining artifacts or archaeological sites,” says T. Douglas Price, an emeritus professor of anthropology at the University of Wisconsin, Madison. Price, who directed Wisconsin’s Laboratory for Archaeological Chemistry, adds that it is often difficult or impossible to ferret out details of ancient technologies, diets, and customs, or the travels and exchanges between ancient peoples, without the benefit of modern analytical chemistry techniques.

Alan K. Outram, a professor and head of archaeology at the University of Exeter, in England, notes that “there is a vast role for chemistry to play” in helping to answer some of the most fundamental questions regarding changes in the way humans lived their lives. The list includes questions about the origins of farming, domestication of animals, and the development of large permanent communities.

One group of methods of growing importance is based on measuring ratios of stable isotopes of certain elements. This mass-analysis technique often involves comparing the relative abundances of ¹²C and ¹³C or isotopes of other elements in bones and other archaeological samples to uncover trends in people’s diets and in the ways they exploited food resources.

The supporting idea is that because body tissues are constructed from the components of food, and the stable isotopes of carbon and nitrogen, for example, are fractionated by natural chemical and physical processes, analyzing isotopes in human remains can provide important clues to the composition of foods consumed long ago.

A quick sampling of studies published in just the past few weeks shows that those kinds of analyses underlie numerous investigations in archaeology and anthropology. For example, researchers at the San Rafael Museum of Natural History, in Argentina, conducted isotope studies of teeth and bone samples up to 6,000 years old to understand geographical patterns in the development of pre-Hispanic maize farming. The results show that for most of the period under study, people in central western Argentina had a diet that was largely free of maize (J. Archaeol. Sci., DOI: 10.1016/j.jas.2011.01.010).

Similar diet-analysis methods are also key to a just-published study led by researchers at the Smithsonian Institution, in Washington, D.C. Samples up to 2,000 years old collected from California’s Channel Islands show that Native Americans and their dogs in that coastal region subsisted almost exclusively on fin fish, marine mammals, and other marine resources. The similarities in diet confirm the assumed age-old close relationship between domesticated dogs and people. In contrast, the data show that island foxes, no friend of humans, ate primarily terrestrial foods (J. Archaeol. Sci., DOI: 10.1016/j.jas.2011.02.008).

Isotope ratio methods also play a central role in a study by Outram and coworkers in the current issue of Antiquity (2011, 85, 116), but the emphasis there is not on local economy and food gathering but rather on the role of horses in funeral rites.

Horses are known to have been a key part of the fabric of life in Kazakhstan during the Bronze Age, Outram explains. Various lines of evidence show that they were prominent in culture and funerary rituals in the early part of that period. In some cases, individuals believed to have been prominent warriors were laid to rest over 4,000 years ago together with chariots, weaponry, and sacrificed horses, he says. With time, the chariot-burying tradition ended, but the practice of interring honored people with horses continued.

By the late Bronze Age, that custom too had all but ended, or so it would appear from a conventional examination of graves. But by analyzing bone fragments and by applying an isotope-analysis method to lipid residues collected from pottery samples excavated from grave sites of that era, a different picture emerges.

Horses no longer made their way to human graves in an iconic capacity. Yet their chemical signatures on mortuary ceramics indicate that their presence was still important at burials—perhaps as the main course of funeral feasts. In this late period, “we still see a clear association between human death and horses,” Outram says. He adds that even today in parts of central Asia where horses are tied to economic and social status, it is still the food of important feasts.

Teasing out the origins of fat residues on ceramics that have been buried for millennia is no small feat. Richard P. Evershed of the University of Bristol, in England, a coauthor of the horses-in-rituals study, has been developing the method and its applications for more than a decade. In the past couple of years, he has also exploited the technique to help uncover the origins of dairying and horse domestication.

Lipids from milk and carcass fats can be distinguished on the basis of their chemical compositions when the samples are relatively fresh. For long-buried samples, however, geological processes degrade the fat components, making straightforward identifications impossible. But by comparing the ¹³C-to-¹²C ratio in the C₁₆ and C₁₈ fatty acids of an archaeological sample to the same ratio in a standard reference material, Evershed showed that milk and carcass fats from ruminant (cud-chewing, cloven-hoofed) animals could be distinguished. The distinction rests on differences in biosynthetic mechanisms that convert carbohydrates in the animals’ diets to milk and carcass fats.

In practice, the analysis calls for extracting lipids from fragments of pottery vessels (potsherds) collected from archaeological dig sites, derivatizing the lipids for separation via gas chromatography, and detecting the combustion products of the analytes in an isotope-ratio mass spectrometer.

Work based on that lipid-analysis method drew considerable attention when Evershed teamed up with a large multinational group to analyze more than 2,000 ancient potsherds collected from the Middle East and parts of Europe. One of the group’s key findings, reported about two years ago, was that a large fraction of sherds found in the Sea of Marmara region of Anatolia (Turkey) were infused with milk residue. That discovery indicates that dairying was widely practiced there around 6000 B.C., which pushes back the origins of that practice by two millennia (Nature, DOI: 10.1038/ nature07180).

Milk residues analyzed recently by Hadi Özbal of Bo˘gaziçi University, in Istanbul, push back the start of dairying in Anatolia by an additional few hundred years. The latest results come from pottery uncovered at Barcin Höyük, a Neolithic settlement that’s being excavated by Fokke Gerritsen of the Istanbul-based Netherlands Institute in Turkey; Rana Özbal of Koç University, also in Istanbul; and coworkers.

Archaeologists have known for years that cattle, sheep, and goats were domesticated in the Ancient Near East more than 9,000 years ago mainly for their meat, Hadi Özbal says. But until the milk residues were found, there was little credible evidence that dairying began any earlier than 2,000 to 3,000 years after domestication. Now, with data supporting a much earlier start to dairying, researchers are stepping up efforts to answer questions on a related subject: the evolution of human lactose tolerance.

As Özbal explains, about 65% of the world’s population today does not produce intestinal lactase after weaning and cannot consume some milk products without becoming ill. Some researchers propose that the genetic coding that protects lactose-tolerant milk drinkers was introduced to Europe thousands of years ago by migrating populations from Anatolia and the Ancient Near East. “Lactase persistence would have provided those populations with tremendous dietary advantages and increased survival rates,” Özbal says. Details of that genetic migration are not yet well understood. But they may begin to emerge soon from a recently launched European Union study of ancient DNA.

Kazakh pottery has followed suit in being coaxed to give up its milky secrets. Following the Nature dairying study, Outram and Sandra L. Olsen, head of anthropology at the Carnegie Museum of Natural History, in Pittsburgh, teamed up with Evershed and others to investigate early horse domestication in Kazakhstan. That phenomenon revolutionized transportation, communication, and ancient warfare, Olsen says, but its origins are not well understood.

As part of a multiyear archaeological study of the Botai culture of northern Kazakhstan, which dates to 3500 B.C., the team analyzed excavated ceramics for the presence of lipid residues. Because of metabolic and biosynthetic differences between ruminant and nonruminant animals, the Botai study required carbon and hydrogen isotope-ratio measurements for unambiguous identification of horse milk fat. The results revealed that these ancient people were processing horse milk (and meat) in their ceramic vessels. Olsen notes that the practice of milking mares to produce koumiss, “an extremely sour vitamin-rich fermented drink,” continues today in parts of Kazakhstan. Taken together with physical evidence of harnessing, the study shows that in Botai culture, horses were domestic animals (Science, DOI: 10.1126/science.1168594).

Strontium is another element that can be tapped to reveal long-buried secrets. Wisconsin’s Price explains that strontium in rocks makes its way into bones and teeth through the food chain. But because tooth enamel changes little after it is formed during childhood, a difference between a Sr-isotope ratio in skeletal teeth and the region surrounding the grave documents relocation from the deceased’s birthplace to his or her grave site.

That’s exactly the type of analysis Price and coworkers applied to human remains discovered at the Mayan Copán Acropolis in Honduras. Archaeologists disagreed about the birthplace of the site’s most prominent figure, who was identified as K’inich Yax K’uk’ Mo’, the first ruler of the Copan dynasty. “The strontium data prove that Yax wasn’t a local,” Price says (J. Anthropol. Archaeol., DOI: 10.1016/j.jaa.2009.10.001). Judging by his teeth, it appears Yax traveled almost 200 miles to reach Copán, Price adds.

Meanwhile, disagreements about the authenticity of the Malinaltepec funerary mask, named for the area in southwest Mexico where it was discovered in 1921, led José Luis Ruvalcaba of the UNAM Institute of Physics, in Mexico City, to analyze many of its 762 colorful tiles with X-ray fluorescence and vibrational spectroscopy methods. His results, coupled with those from earlier studies, show that the mask is indeed authentic and that it was created in pre-Hispanic Mexico between A.D. 200 and 900 and decorated with turquoise, shell, and amazonite some 700 years later and used again in a funeral rite. The study also shows that some of the materials had to have been collected via long-distance trade.

Applying analytical chemistry methods to problems in archaeology and anthropology “sometimes helps answer questions that are otherwise completely intractable,” Evershed says. But more than that, it uncovers fascinating details of human history, he adds.