I saw a culinary map of Europe some years ago which had, amongst other things, the olive oil contour marked. North of this line frying was done traditionally in butter; south of it olive oil did the job. It's obvious when you think about it - climate is a major determinant of our diet. Rice is a staple in countries where it grows, dairy produce in countries rich in pasture.

The connection between a people's diet and the exigencies of their everyday life has always fascinated me. Southern Italy has traditionally eaten the staple of pasta and pulse - pasta e fagioli - because it was cheap and nourishing.

The peasant diet wasn't based so much on choice as on necessity. Pulses produce a lot of protein per acre. All over Europe different regions have different specialities, bound inextricably to their local produce. Parma hams and Parma cheese - parmesan - are traditional in this area because it's close to the plains of the Po Valley, and because its dry air allows the successful salt curing of the hams. None of this accidental; food has its origins in the local agriculture and is therefore bound up with climate.

Sometimes dishes evolve from different circumstances. My favourite example of this is pasta alla carbonara. This is a dish that was made by the charcoal burners who gave it its name. Before gas was commonplace in Italian kitchens, charcoal was the cooking fuel and therefore there was a need for its production. Because Italy is so heavily populated and cultivated, forests are largely confined to the high mountains. The charcoal burners would take their mules in a caravan to the high places in the early spring and would spend the summer coppicing the hornbeam, birch and beech. All through the summer they made piles of faggots which would then be covered with sods and fired, so that the combustion would take place with no air and therefore no flame - carbonizing the wood. In the autumn they returned to the valley, mules laden with charcoal for sale.

Life in the mountains was spartan and pasta alla carbonara evolved from this way of life. One pot, one fire, dried pasta, salt-cured sausage - these define the dish. In its pure mountain form the pasta was boiled and drained, eggs and chopped sausage were tossed into the pot and the pasta was stirred. That's it. If you're an Italian charcoal burner that may be fine, but in a modern kitchen the dish can be refined a little and I think, improved. While a packet of cut pasta, preferably penne, is on the boil, take four egg yolks and beat them well with half an eggshell of water, ground black pepper and grated parmesan. If the parmesan thickens it up, add a little water. If no salsiccie are to hand, cube a little collar of bacon and fry it lightly in olive oil. When the spaghetti is cooked, strain and return it to the pot. Pour in the bacon and oil, stir, pour in the egg. Stir once more. The residual heat of the spaghetti will cook the egg and the dish is made.