Fat can be fatal. Obesity is the great new global health scare. Heart disease and late-onset diabetes grow out of the grease. The danger is baffling because it is paradoxical. For ours is the most diet-conscious era and diet-obsessed culture in the history of the world. We think thin and we get fat.

This is more than a cultural peculiarity: it bucks the whole trend of human evolution. Our species has long been conspicuously more successful in absorbing fat than any other land-based animal - why is that going wrong now?

The experts' favourite explanations are all ideologically biased. Some blame capitalism for forcefeeding us sugar and starch, or industrialisation and urbanisation for distancing millions from healthy food. Dieting, say others, makes you fat by disturbing the metabolism and encouraging faddish eating. Some blame poverty, some blame abundance. Some of these explanations are wrong; the rest are inadequate. Really, fat is a function of deeper disturbances in our eating habits. It's the outward and visible sign of a profound social disaster: the decline of the meal. We have to face this threat if we want to face it down.

Mealtimes are our oldest rituals. The companionable effects of eating together help to make us human. The little links which bind households together are forged at the table. The stability of our homes probably depends more on regular mealtimes than on sexual fidelity or filial piety. Now it is in danger. Food is being desocialised. The demise of mealtimes means unstructured days and undisciplined appetites.

The loneliness of the fast-food eater is uncivilising. In microwave households, family life fragments. The end of home cooking has long been both tearfully predicted and ardently desired. The anti-cooking movement started, rather feebly, more than 100 years ago, among socialists who wanted to liberate women from the kitchen and replace the family with a wider community. In 1887, Edward Bellamy imagined a paradise of kitchenless homes. Workers would order dinner from menus printed in newspapers and eat them in people's palaces. Twenty years later, Charlotte Perkins wanted to make cookery "scientific": in effect, eliminating it from most lives, while professionals in meal-making factories maintained energy levels for a world of work. It would have been insufferably dull - institutional eating can never beat home cooking. But at least it was nobly conceived, with socialising effects in mind.

Now capitalism has succeeded where socialism failed. We are facing a nightmare version of Perkins' vision: a dystopia in which cooking has surrendered to "convenience" and family break-ups start at the fridge. The eateries Bellamy imagined have materialised but they are supplied by private enterprise in fast-food outlets, serving uniform pabulum. The scientific cooks Perkins predicted are now found in processed food factories, stuffing tinfoil with gloop. People still eat at home - but mealtimes are atomised: different family members choose different meals at different times.

People no longer learn cooking at home. They need Delia to show them how to boil an egg and instruction from Nigella on How to Eat. Mealtimes have adjusted to new patterns of work. In Britain and America, they are vanishing from weekday lives. Lunch has disappeared in favour of daytime "grazing". People eat while they are doing other things, with eyes averted from company. They snack in the street, trailing litter, spreading smell pollution and dropping fodder for rats. Office workers forage for impersonal sandwiches, grab ready-made from refrigerated shelves and bolt them down in isolation. Before leaving home in the morning they do not share breakfast with loved ones. Family breakfast has been crowded out of daily routines. In the evening there may be no meal to share - or, if there is, there may be a shortage of sharers. Latchkey kids come home alone and fall ravenously on instantly infused pot noodles or beans eaten straight from the tin.

Microwaves erode society. In these machines, eaters can heat up whatever ready-mades are to hand. No reference to community of taste needs to be made. No mummy or daddy can arbitrate for a whole family. No one in a household has to defer to anyone else. This new way of cooking reverses the cooking revolution which made eating sociable, and threatens to return us to a presocial phase of evolution.

Part of the result of the snacking society is undermined health, as eating disorders multiply. People alienated from the comradeship and discipline of the common table starve and stuff themselves into extremes of emaciation and obesity. The obesity pandemic has coincided with the decline of the meal. A new kind of malnutrition has emerged - engorgement on deadly diets and lethal lipids. The new eating habits multiply microbes while spreading fat. When foods are mass-produced, one mistake can poison many people. Every time prepared foods are unfrozen or chilled meals heated, an eco-niche opens for microbial infestation.

The raw food movement is not a healthy alternative. Raw food freaks seem to prefer ruminants to humans. This is psychologically unhealthy - however salubrious bean sprouts may be: romantic primitivism allied with ecological anxiety. Modern urbanites head for the raw bar seeking readmission to Eden. When the African-American elite dumps the fat-rich dishes of Southern tradition - collard greens suppurating with pork fat, pigs' feet with black-eyed peas - in favour of the raw vegetables of the "new soul food", a sacrifice of culture accompanies a loss of girth. The raw movement is not a solution, but part of the threat, dividing families by taste and diet.

So the family mealtime looks irretrievably dead. The future, however, usually turns out to be surprisingly like the past. We are in a blip, not a trend. Cooking will revive, because it is inseparable from humanity: a future without it is impossible. Communal feeding is essential to social life: we shall come to value it more highly in awareness of the present threat. There is bound to be a reaction in favour of traditional eating habits, as nostalgia turns into fashion and evidence builds up of the deleterious effects of snacking. The advertisers are already beginning to re-romanticise family feeding. Some convenience foods can be adapted as friends of family values: fast preparation time can make fixed mealtimes possible.

A return to the table is inevitable because, as Carlyle once said, "the soul is a kind of stomach, and spiritual communion an eating together". We seem incapable of socialising without food. Among people who like to enjoy other's company, every meal is a love feast. We eat to commune with our gods. The discreetly lit table is our favourite romantic rendezvous. At state banquets, diplomatic alliances are forged. Deals are done at business lunches. Family reunions still take place at mealtimes. Home is a place which smells of cooking. If we want relationships that work, we shall get back to eating together. Along the way, we shall conquer obesity: if we stop grazing, we shall stop gorging.

· Felipe Fernandez-Armesto is a professorial fellow at Queen Mary, University of London, and author of Food: A History comment@theguardian.com