We human beings are dividing, all over the developed world, into two. I'm not talking about the obvious division - between the rich and poor - but about the fat and the thin. You would think the two are similar. The rich get fat and the poor starve, and are therefore thin. But that's not the case.

Certainly, everyone in the African countries where there is famine and war - the two so often go together - is thin, and the only way to help them is somehow to organise the world so that wars can't happen and food can be made available to all.

But what about poorer countries where there are no wars, the rains have been plentiful, and there is enough to eat? And what about the poor in developed countries like ours, where everyone has enough to eat, even if they are at the bottom of the income scale?

The bare facts are that wherever there is enough to eat, everyone, poor and wealthy alike, have the potential to become fat.

And when you take away the necessity to have to work physically hard to earn a crust, it's very difficult not to put on unwanted stones without becoming very physically active.

The facts are simple. Only two generations ago, everyone spent most hours of the day in physical effort. If we didn't do it at work itself we cycled or walked to work. And food prices, in comparison to our wages, were high, so that we didn't have a surplus that we could spend on abundant food in the shops.

Now we have bigger wage packets, have much more choice of food at almost rock-bottom prices, and we don't have to stint.

Our work has been made far easier and less energy-sapping, and our longer leisure hours are far more likely to be spent sitting in front of a video or TV screen than moving about. While we sit, we munch.

Our big problem is that for the last four million years or so, our ancestors had to face long periods of starvation - we had either a feast or a famine.

The ones who survived to send their genes down to us had to adapt to those conditions. That meant that they had to be able to convert what they ate in the bountiful times into fat stores that they could use when food became scarce. So we modern humans are now very efficient in converting any excess food we eat and don't immediately need the energy, so it gets turned into fat. And we are very miserly in using up that fat when we exercise.

That's why it is so much easier to put on weight than to take it off once we have gained it. It is also why, in the last forty years, when so much more food has become available and affordable, the whole population has become much fatter.

The average young woman is now a size sixteen instead of size ten or twelve. Count the number of men and women in any street who are slim, without a midriff bulge. You will be shocked. Look at crowds in an old newsreel, and compare them with today's, and you get the measure of how we have changed.

People used to think that being a bit overweight wasn't bad - it could even be an advantage if you fell ill, they said. There was a presumption that fat people could resist illness better than thin people, because they could rely on their food stores for longer.

It wasn't, and isn't, true. Being overweight leads to diabetes, high blood pressure, and a higher risk of dying early from heart attacks, strokes, cancer and the complications of diabetes. It is also very uncomfortable to be so overweight - try carrying a big bag of bricks around with you all the time if you want to know what it feels like.

Which is why researchers are looking for easy ways to help people to lose weight. If they get it right, they will hit the biggest commercial jackpot in history.

Have they got anywhere in their quest? I've just been looking at the latest review of possible new pills to curb appetite and lose weight. Several that were publicised as possibles - such as leptin, a substance that caused mice to stop eating too much, just don't work in humans.

You will be hearing more about a hormone called peptide YY, which causes people to reduce their food intake by a third, but may not keep working for long enough to be useful.

Another hopeful is called pancreatic polypeptide: there may be plans to put it into chewing gum, as you absorb it through the mouth, not the stomach. Chewing that may help fat people not to chew food.

I remain sceptical. The drives to eat and to be lazy are so powerfully ingrained in us that it is very difficult for people to resist them.

We can't go back to life as it was before modern society changed us all forever. As we face up to our responsibility for our own health, the first step is to understand why we become so overweight. Taking the next step - to exercise more and eat less and more healthily - is the big one. If you are overweight, can you do it? Because you certainly can't leave all the work to your medical and nursing teams.