Academic Reading — Test 122

Reading passage

When archaeologists excavate a burial ground, the skeletons they recover are far more than inert relics. Locked within the mineral and protein of ancient bone is a chemical archive that records, in remarkable detail, what a person ate and where they lived. The technique that unlocks this archive is stable isotope analysis. Isotopes are variants of the same chemical element that differ only in the number of neutrons in their nuclei, and therefore in their mass. Unlike radioactive isotopes, stable isotopes do not decay over time, so the proportions measured in a bone today reflect the proportions that were laid down while the individual was alive. By comparing the ratio of a heavier isotope to a lighter one, researchers can reconstruct aspects of a person's life that no written document could ever supply. The dietary signal comes principally from carbon and nitrogen, the two elements that dominate the protein collagen preserved in bone. Plants fall into two broad photosynthetic categories that fix carbon in chemically distinct ways. The first group, which includes wheat, barley and most temperate trees, produces tissue with a comparatively low proportion of the heavier carbon isotope. The second group, which includes maize, millet and many tropical grasses, produces tissue noticeably enriched in that heavier isotope. Because the carbon signature of a plant passes up the food chain into the animals and people who consume it, a bone unusually rich in heavy carbon points strongly to a diet built around the second group of plants. This is how researchers have traced the spread of maize agriculture across the Americas without relying on a single surviving seed. Nitrogen tells a complementary story about the position a person occupied in the food web. With each step up the chain, from plant to herbivore to carnivore, the heavier nitrogen isotope becomes more concentrated in body tissue, an effect known as trophic enrichment. A largely vegetarian population therefore carries a lower nitrogen value than one that consumes substantial quantities of meat. Marine foods complicate the picture in a useful way, because aquatic food chains are longer and consequently push nitrogen values higher still; a coastal community living on fish and shellfish can be distinguished from an inland farming community on this basis alone. Even the diet of infants leaves a trace, since breastfeeding effectively places a baby one trophic level above its mother, producing an elevated nitrogen signal that falls away once weaning is complete. Migration is reconstructed using a different element altogether. Strontium, and to a lesser extent oxygen, enters the body through food and water and is incorporated into the hard tissues of the teeth. Crucially, the strontium ratio in any region is governed by the age and type of the underlying bedrock, so it varies from one landscape to another. Tooth enamel forms during childhood and, once formed, is not remodelled, meaning it preserves a chemical fingerprint of the place where a person grew up. Bone, by contrast, is living tissue that is continually rebuilt throughout life, so its isotope values reflect the most recent years before death. When the strontium signal in an individual's enamel does not match the local geology of the place where they were buried, the most economical explanation is that the person moved there from elsewhere after childhood. These methods are powerful, but they are not infallible, and responsible researchers treat their results with caution. Bone collagen can degrade in acidic soils, and contamination from groundwater can shift a measurement in misleading ways, so samples are screened rigorously before any conclusion is drawn. A single isotope value rarely identifies a precise homeland or a specific meal; instead it narrows the range of possibilities, and it is most persuasive when several independent lines of evidence converge. For this reason isotope data are normally interpreted alongside the broader archaeological context, including grave goods, the style of burial and the surrounding settlement. Used carefully and in combination, the chemistry of the dead allows researchers to glimpse the journeys and the meals of people who left behind no written record of either.