Britons embraced a "convenience food" lifestyle around 6,000 years ago when they replaced hunting and fishing with dairy farming, scientists say.
Studies of old rubbish dumps and dirty dishes found our ancient ancestors gave up their passion for fish and wild meats to begin a love affair with milk.
Early hunters feasted on venison and wild boar and ate large quantities of seafood, including seals and shellfish, research by the University of Bristol and Cardiff University found.
But when experienced immigrants introduced domestic animals 6,000 years ago, Britons quickly gave up wild foods and fishing was largely abandoned.
Seafood was shunned for the next 4,000 years, reappearing in the British diet during the Iron Age and becoming a significant part of it only with the arrival of the Vikings.
The findings come from a large-scale investigation of British archaeological sites dating from around 4,600 BC to 1,400 AD.
Researchers examined millions of fragments of bone and analysed more than 1,000 cooking pots to ascertain how ancient Britons ate.
More than 99 per cent of the earliest farmer's cooking pots lacked sea food residues, while analysis of skeletons also showed a drop in fish consumption in the period.
Dr Lucy Cramp, a lecturer in Archaeology and Anthropology at the University of Bristol, was lead author of the study, published today in Proceedings of the Royal Society B.
"The absence of lipid residues of marine foods in hundreds of cooking pots is really significant," Dr Cramp said. "It certainly stacks up with the skeletal isotope evidence to provide a clear picture that seafood was of little importance in the diets of the Neolithic farmers of the region.
"Amazingly, it was another 4,000 years before sea food remains appeared in pots again, during the Iron Age, and it was only with the arrival of the Vikings that fish became a significant part of our diet."
A team led by Professor Richard Evershed of the University of Bristol's School of Chemistry developed new techniques in an effort to identity fish oils in the cooking pots.
The group used a compound-specific carbon isotope technique developed to identify the actual fats preserved in the cooking pots.
Results showed a lack of fish oils once cattle and sheep arrived, with dairy products dominating the menu right across Britain and Ireland.
Researchers from Cardiff University, led by Dr Jacqui Mulville, examined human bones for a unique chemical signature passed on to the skeletons of those eating seafood.
Early British hunters were found to possess this signature but it was lacking from later farmers.
"Whilst we like to think of ourselves as a nation of fish eaters, with fish and chips as our national dish, it seems that early British farmers preferred beef, mutton and milk," Dr Mulville said.
The ability to milk animals was a revolution in food production for our early ancestors, as it was the first time they did not have to kill animals to obtain food.
Milking stock requires a high level of skill and knowledge, which experts believe came into Britain by immigrants.
Alison Sheridan, from the National Museum of Scotland, said: "The use of cattle for dairy products from the earliest Neolithic confirms the view that farming was introduced by experienced immigrants."
Professor Richard Evershed, of the University of Bristol, said the abrupt change from a seafood to a farming diet remains a mystery.
"Since such a clear transition is not seen in the Baltic region, perhaps the hazardous North Atlantic waters were simply too difficult to fish effectively until new technologies arrived, making dairying the only sustainable option," Professor Evershed said.
The study was funded by the National Environmental Research Council (NERC) with research on the Irish material funded by the European Union EU FP7 (Marie Curie Actions).
'Immediate replacement of fishing with dairying by the earliest farmers of the northeast Atlantic archipelagos' is published in Proceedings of the Royal Society B.