Skeleton keys

By studying ancient bones, scientists in Yorkshire have gained an insight into our Iron Age diet. So why did we stop eating seafood for 4,000 years? Simon Hadlington investigates

Beneath the bright white lights of the isotope laboratory in the department of archaeological sciences at Bradford University, Mandy Jay rummages through a drawer and produces a small, sealed polythene sack containing half-a-dozen shards of a pitted, yellowing material. They are bones; fragments of femur excavated from an Iron Age burial site in east Yorkshire.

For Jay and her fellow researchers in the laboratory, bones such as these contain a key piece of information about ancient societies: what people ate. Remarkably, bones retain a chemical signature of what went into making them in the first place: what it was in the diet that provided the raw materials for the bone to grow.

By examining bone in this way, the Bradford researchers, led by Dr Mike Richards, have made a number of significant discoveries. The most intriguing is that around 6,000 years ago Stone Age man in Britain seems suddenly to have stopped eating fish and shellfish. This dietary restriction persisted for the better part of 4,000 years, until the Romans arrived.

Using bones to obtain information about diet is complicated, and relies on a technique called stable-isotope analysis. Bone consists mainly of a mineral component containing calcium and a protein component - collagen. Proteins are made up largely of carbon and nitrogen. In nature carbon and nitrogen can exist in a number of forms, called isotopes. These are chemically identical but have slightly differing atomic masses. The most common form of nitrogen has an atomic mass of 14, but a small amount of nitrogen with a mass of 15 also exists. Most carbon has an atomic mass of 12, but a small proportion is carbon-13.

These isotopes are present naturally in the environment and as such they get incorporated into green plants as they grow. When the plants are eaten by herbivores, the carbon and nitrogen become incorporated into the animals' tissues. When the herbivores are eaten by carnivores, including man, the isotopes become assimilated into the carnivores' tissues, including the collagen in the bone.

The ratios of the isotopes (carbon-12 to carbon-13, and nitrogen-14 to nitrogen-15) are specific to the source of the food. In other words, it is possible to examine the pattern of the carbon isotopes and nitrogen isotopes and from this to ascribe the source of the food which was consumed to make the bone collagen - whether the food was from plants, animals or a mixture of the two.

The bone is steeped in dilute acid to dissolve away the mineral component, leaving the collagen. This is then subjected to a technique called mass spectrometry, which measures the quantity of each nitrogen and carbon isotope in the sample.

"What you are able to say after analysing the isotope ratios is where the person was in the food chain," says Jay. "Whether they were mainly carnivorous, omnivorous or whether they were vegans."

Jay has been examining the diet of people buried at the largest Iron Age cemetery in Britain at Wetwang, on the Yorkshire Wolds. The cemetery dates from the 3rd or 4th century BC, and contains around 450 people. "The cemetery was used over a period of about 200 years, and there is a very particular pattern to the burials," says Jay. "There are five chariot burials, where bodies have been buried with chariots. It is assumed that these were the highest-status individuals." There are remains of bodies that were buried under specially constructed mounds, or barrows, which presumably was also indicative of status, and finally bodies buried in the ditches surrounding the barrows - suspected to be the lower status.

"The question I wanted to ask is whether we could see a difference in diet depending on the assumed status of the individuals," says Jay. "Were the chariot people eating more meat, for example?"

Following isotope-ratio analysis on almost 50 samples, Jay has concluded that there is no difference between the three groups in terms of the source of their protein. "All of the samples showed quite a lot of animal protein in the diet," she says. "We cannot say if the chariot people were getting fillet steak and the ditch people scrag end, but in terms of protein they were pretty much eating the same thing."

The proportion of animal and plant protein remained similar throughout the period that the cemetery was being used - 200 years. This suggests that the community was highly economically stable over this time, with the same farming practices persisting for two centuries.

"The other thing that we can say with some confidence is that there is no evidence of any marine protein having been consumed," says Jay. "Things like fish and shellfish were absent from the diet." While this may not be too surprising, given that Wetwang is about 20 kilometres from the coast, it fits in with a recent finding by Dr Richards that people simply did not eat seafood at this point in history.

"We have examined hundreds of bones of different ages from sites across Britain," says Dr Richards. "We know that about 6,000 years ago, during the Neolithic period, there was a revolution in the way people lived. People stopped being nomadic hunter-gatherers and started to farm animals and crops, and live in villages."

There were big cultural as well as economic changes at this time. Domesticated animals were brought over from the Continent, and wheat and barley appeared. Pottery began to be made, and elaborate burial monuments started to appear.

"From a dietary point, before this time there was only wild food," says Dr Richards. "If you do isotope analysis of bones found at coastal sites, you find evidence of a large amount of marine food in the diet. But after about 4,000BC suddenly there is no marine food in the diet. People simply stopped eating fish and shellfish."

The reasons for this are not clear. One school of thought suggests that a shift in climate at that time, causing sea levels to rise, made fishing difficult. Other archaeologists think that the advent of farming made the food resource much more secure - there was no need to harvest wild food.

Dr Richards believes that the radical change in diet reflected larger changes in society. "It coincides with the appearance of pottery and of big monuments and new burial practices," he says. "My hunch is that there was a spread of a new kind of belief system, a new way of looking at the world, and a big part of that could have been a change in diet. But it is rare that you see such sudden changes."

Fish seems not to have appeared again on the menu until the Romans arrived, 4,000 years later. The pattern is confirmed in Jay's findings. She has looked at samples of Iron Age bone from two coastal sites, in Cornwall and East Lothian. These, too, are devoid of any evidence of a marine diet.

"It seems odd from a commonsense point of view," says Jay. "We know that the technology for fishing existed and you would have thought that a ready source of food would be exploited. It might have been that seafood in some way became taboo. Even now there are dietary taboos - for example we balk at the thought of eating horsemeat or dog, but these are eaten in some societies. In fact we know that people in the Iron Age did eat dogs and horses."

Interestingly, another sample of Iron Age bone that Jay has been examining, from Hampshire, shows no evidence of any animal protein having been consumed at all. The bone's owner was apparently vegan. Jay stresses that she has yet to confirm unequivocally that the bone is from a human, but that experts who have examined it so far say that it is. "There are two main possibilities," Jay says. "Either the person was indeed vegan, which would be quite unusual, or it might be that they had a disease which affected their metabolism in such a way to alter the isotope ratios in the collagen. We don't yet know enough about this to draw an absolute conclusion. But either way, it is an intriguing piece of bone."