UK researchers have uncovered the first direct evidence that milk has been part of our diet for thousands of years.

Scientists working at Bristol University in the west of England have found microscopic stains of milk on Iron Age cooking pots.

Although it is thought animals were domesticated as far back as 9,000 BC, there has been no real proof that they were milked. Archaeologists have relied on artefacts like ancient cheese strainers to get some clues as to the origin of dairy farming.

Milk is an important part of the human diet

Now the Bristol team has resorted to chemistry to study the milk fats which can still be found in ancient pots dating back 2,500 to 3,000 years.

"They're very porous and that's where the fats and waxes are trapped," says research assistant Stephanie Dudd. "During cooking, during the heating process and maybe mixing, the fats and waxes that float on the surface of the water during cooking are absorbed into the matrix of the pot.

"We take a small piece of pottery, approximately thumbnail size, which we grind in a pestle and mortar.

"We grind it to a fine powder and then extract the fats."

Animal fats

Once the fats have been extracted, the team have to identify whether they come from dairy products or another source, such as the animal fat contained in meat.

Stephanie Dudd: New testing method

"Recognising fats is no problem at all, but starting to ascribe those fats to particular species of animal, or then saying whether they are dairy fats is much more difficult," says Stephanie Dudd.

"The problem that we've shown is that over time degradation of milk fat gives you a signature that looks very much like animal fat, which is why we've then had to exploit another property of the fats to recognise dairy fats in archaeological pottery."

The scientists have developed a testing method that checks the ratio of isotopes, or types, of carbon atoms that exist in different fats.

The team have shown that the ratios are very different in milk and the flesh of animals - the adipose tissues.

"What we've recognised is that during the production of adipose fat in the cow, the carbon source for that fat is mainly the carbohydrate in the pasture which the animal is grazing on," says Stephanie Dudd.

"However, when the cow goes into lactation, a large proportion of the carbon that appears in the milk fat is derived directly from the fatty acids in the diet. So the adipose fat and the milk fat reflect that, and that is the basis of our identification of milk fats."

Domesticated animals

From the earliest days of human evolution, the relationship between man and beast has been close. Initially, animals were hunted for their flesh which became a vital source of energy.

As cultures became more complex, animals were domesticated and at some time in the past, humans began to exploit animals for their milk.

When were domesticated animals first used for their milk?

This new research will help us determine more precisely when that change occurred.

"The pictorial records dating to about 3,000 or 4,000 BC are currently the earliest evidence we have and that comes from Egypt and the Sahara," says Dr Richard Evershed, who leads the Bristol team.

"The interesting question is going to be to start to find when milk became exploited as a commodity from domestic animals in the very earliest stages in the Neolithic period [about 8,000 BC].

"That is our next step - to look at this very ancient pottery. Also we want to extend that abroad and look at evidence for dairying in other cultures."

The new research is published in the journal Science.