Bird-shaped pestle (made 4 - 8,000 years ago). Stone, found in Papua New Guinea

Lunch here at the British Museum staff canteen has always been a pretty international affair. It's not just the scholars and the curators who come from all round the world; it's also of course the food. As today is one of my healthy days, I'm at the salad bar, looking in among the greens at a fairly standard run of potato salad, rice, sweetcorn and kidney beans. What I find interesting about these vegetables is not just that they come originally from all over the world, but that none of them would exist in the form they do today if the plants they come from hadn't been carefully chosen, cherished and modified in a long process that began about ten thousand years ago with some intrepid and ingenious Ice Age cooks.

'Everyone did these things, and the families existed as families because you did it not for yourself but for the family.' (Madhur Jaffrey)

'We need some new evolutionary tricks in order to spread out into increasingly hostile environments.' (Martin Jones)

In previous programmes, we have looked at how our ancestors moved around the world; in this week's programmes I'm going to be focussing on what happened when they settled down. This is a week full of ancient animals, powerful gods, dangerous weather, good sex and even better food.

Around 11,000 years ago, the world underwent a violent and rapid period of climate change, leading to the end of the last Ice Age. Temperatures increased by as much as seven degrees centigrade in a hundred years, and sea levels rose by over 300 feet (or 100 metres). Ice turned to water and snow gave way to grass, and the result was slow but profound changes in the way that humans lived. Over the course of this week, I will be covering about seven thousand years of human history when, as the Ice Age ended, people in many different parts of the world began to breed animals, grow plants and eat differently.

Ten thousand years ago, the sound of daily life began to change across the world, as new rhythms of grinding and pounding prepared the new foods that were going to change our diets and our landscapes. For a long time, our ancestors had used fire to roast meat but, in the sense that we would use the word today, they were now 'cooking'.

There's an enormous range of objects in the British Museum that we could have chosen to illustrate this particular moment in human history, when people literally start putting down roots and cultivating plants that will feed them all year round. The beginning of this sort of farming is a process that seems to have happened in many different places at more or less the same time, and we've recently discovered that one of these places was Papua New Guinea, that huge island just to the north of Australia, where this bird-shaped stone pestle comes from. We think it's about eight thousand years old, but a pestle then would have been used exactly as it is now - to grind food in a mortar and break it down, so that you can make it edible. It's a big pestle, it stands just over a foot tall (about 35 cm), and the business end is a stone bulb, about the size of a cricket ball, and you can feel that it's been much used. Above the bulb, the shaft is very easy to grasp and the upper part of the handle, or the shaft, has been shaped in a way that's got nothing to do with making food at all - it looks like a slender, elongated bird with wings outstretched and a long neck dipping forward; it looks a bit like Concorde.

We're very familiar now with the idea that making food unites us, either as a family or a society, and we all know how much family memory and emotion is bound up in the pots and pans and wooden spoons of childhood. These sorts of associations seem to date from around ten thousand years ago; from the very beginning of cooking implements, roughly the period of our pestle. The chef Madhur Jaffrey is still very attached to her pestle, which her mother gave her when she left India, and she came to look at ours:

'I just thought it was beautiful to look at and had a well-honed, worn look, and a patina that made me feel that it was used - and used again and again. It is a fundamental act both of cooking and of living, and living with a family and passing on, at least in India.

'When I left India, which was a long long time ago, my mother gave me certain utensils to take with me, and they were all heavy, I remember that. There was a wok, a grinding stone, and a huge mortar and pestle, so those are what I left with, and I have all of them, and I use my mortar and pestle to this day.'

Other stone pestles and mortars have also been found in New Guinea, and what they show is that there were farmers growing crops in the tropical forests and grasslands here in ancient times, around ten thousand years ago. This relatively recent discovery has upset the conventional view that farming began in the Middle East, in what's called the Fertile Crescent, and from there spread across the world. We now know that in fact this particular bit of the history of humanity happened simultaneously in many different places. Clearly a lot of us became farmers at the same time, and wherever people were farming, they began to concentrate on a small number of plants, selectively harvesting these from the wild, planting and tending them. In the Middle East, they chose particular grasses - early forms of wheat; in China, wild dry rice; in Africa, sorghum - a grain that looks a bit like grass; and in Papua New Guinea, the starchy tuber, taro. For me, the most surprising thing about these new plants is that in their natural state you very often can't eat them at all, or at least they taste pretty filthy if you do. Why would you choose to grow food that you can eat only once it's been soaked or boiled or ground to make it edible at all? Martin Jones, Professor of Archaeological Science at Cambridge University, sees this alchemy of food as an essential part of human evolution:

'As the human species expanded across the globe, we had to have a competitive edge over other animals going for the easy food. So we went for the difficult food, we went for things like the small hard grass seeds we call cereals, that are indigestible if eaten raw and may even be poisonous, and we have to pulp them up and turn them into things like bread and dough. And we went into the poisonous giant tubers, like the yam and the taro, which also had to be leeched, ground up and cooked before we could eat them. And that was our competitive advantage - other animals that didn't have the big brain couldn't think several steps ahead to do that.'

So it takes brains to get to cookery. We don't know what gender the cooks were who used our pestle to grind taro in New Guinea, but we do know from archaeological research in the Middle East that cookery there was primarily a woman's activity. From examining burial sites of this period, scientists have discovered that the hips, ankles and knees of mature women are generally severely worn - the grinding of wheat then would have been done kneeling down, rocking back and forth to crush the kernels between two heavy stones. This arthritis-inducing activity must have been very tough, but the women of the Middle East and the new cooks everywhere were cultivating a small range of nourishing basic foods that could sustain much larger groups of people than had been possible before. Most of these new foods were pretty bland, but the pestle and mortar can also play a key part here in making them more interesting. Madhur Jaffrey again:

'If you take mustard seeds, which were known in ancient times, if you leave them whole they have one taste, but if you crush them, they're like Jekyll and Hyde; they become pungent and bitter, so you change the very nature of a seasoning by crushing it.'

And Martin Jones believes that in due course, this early farming changed the whole pattern of our society:

'We specialise in those smaller number of foods - not stopping the others - but emphasising a smaller number of foods, and that's how agriculture emerges, and it allows people to interact in different ways, to share food in different ways and to make contact with each other in different ways.'

These new crops helped create new kinds of communities because, if you were lucky with the weather, they could produce surpluses which could then be stored, exchanged or simply consumed in a great feast. Our pestle's long, thin elegant body looks far too delicate to have been able to withstand the vigorous daily pummelling of taro, so we should perhaps think of it more as being used to prepare special meals; meals where people gathered, as we might do today, to trade, to dance, or to celebrate key moments in life. Sharing food is one of the most basic ways of binding people together.

Looking at the food in the British Museum canteen, so much of it flown across the world, I'm struck by the fact that while we all travel more and more freely, we depend on food grown by people who cannot move, who must stay on the same piece of land. We're all increasingly aware of how vulnerable this makes farmers across the world to any change in climate, and this dependence on regular predictable weather led the farmers of ten thousand years ago to identify gods of food and climate, who needed constant placation and prayer in order to ensure the cycle of nature and safe, good harvests. Nowadays, most people look to governments, and to campaigners like Sir Bob Geldof:

'The whole psychology of food, where it places us, is I think more important than almost any other aspect of our lives. Essentially, the necessity to work comes out of the necessity to eat. So, this central idea of food is the fundamental in all human existence. It's clear that no animal can exist without being able to eat, but right now, at the beginning of the 21st century, it is clearly in the top three of priorities for the global powers to address. Upon their success or not will depend the future of huge sections of the world population. There simply isn't enough food for the world at the moment. There are several factors, but the predominant one is climate change.'

So a change in climate, like the one that brought us agriculture in the first place, is now threatening our global survival. Just after the Ice Age, the growing population that the new foods allowed was of course not a problem but a positive advantage. The first settled societies increased quickly in numbers and, as long as the weather allowed, they developed new, stable communities.

Tomorrow I'll be focussing on the fertility not of the land, but of the people farming it. I'll be looking at a stone sculpture that's the first representation, anywhere, of a couple making love.