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Olduvai stone chopping tool (made 1.8 million years ago) found in Olduvai Gorge, Tanzania, East Africa.

Perhaps the best thing of all about being Director of the British Museum, and one that still gives me the most enormous thrill is that, now and then, I'm allowed to take some of the objects out of the cases and hold them.

And today I'm being allowed to hold something absolutely astonishing. I've got to admit that if any of us saw this just lying on the ground, we'd probably walk past it, but in fact it's the oldest object in the British Museum, and it was made nearly two million years ago, in Africa. It looks like a large, chipped grey cobble. The naturalist and broadcaster Sir David Attenborough is, along with me, the latest hominid to handle it:

"Holding this, I can feel what it was like to be out on the African savannahs, needing to cut flesh for example, needing to cut into a carcass, in order to get a meal."

This is one of the first things that humans ever consciously made. And holding it puts me directly in touch with them. In this history of the world that I'm trying to tell through 'things', this chipped stone from Africa - from modern Tanzania - is where it all begins.

One of the points of any museum is to allow you to travel through time, but our understanding of just how much time there is for us to travel through has expanded dramatically since the British Museum opened its doors in 1759. At that point, most of the visitors would probably have agreed that the world had begun in 4004 BC, to be precise at the very beginning of Sunday 23 October. This astonishingly exact date had been calculated by the mathematically-minded clergyman Archbishop Ussher, who preached just down the road in Lincoln's Inn. Ussher had carefully trawled the Bible totting up the lifespan of everyone descended from Adam and Eve to reach his date but, as you and I know, we don't now celebrate 23 October as Start the World Day and that's because, in the last couple of centuries, archaeologists, geologists and museum curators have steadily been pushing back the chronology of human history - back from Archbishop Ussher's six thousand years to an almost unimaginable two million. So if the scientists have been suggesting that Adam and Eve no longer stood at the beginning of human time in the Garden of Eden in 4004 BC, who did? And where? There are many theories, but no conclusive answers and certainly no conclusive date until 1931, when a young archaeologist called Louis Leakey set off on a British Museum-sponsored expedition bound for Africa.

Leakey's goal was Olduvai Gorge, a deep crack in the flat savannah of northern Tanzania, not far from the border with Kenya. It's part of the East African Rift Valley, a massive tear in the earth's surface, thousands of miles long. It was here that time had been frozen within exposed layers of geology where, as Leakey examined the rocks shaped by the sun, the wind and the rain on the savannahs, he reached a layer where the rocks were also shaped by something else - human hands. They were found next to bones, and it was clear that these stones had been shaped into butchering tools to strip meat and break into the bones of animals killed on the savannah. The layer where the tools were found was roughly two million years old. This was archaeological dynamite.

Leakey's excavations produced the oldest humanly made things anywhere in the world, and they demonstrated that not only human beings, but human culture, had begun in Africa. Our stone chopping tool was one of the ones that Leakey found.

"Picking it up, your first reaction is it's very heavy, and if it's heavy of course it gives power behind your blow. The second is that it fits without any compromise into the palm of the hand, and in a position where there is a sharp edge running from my forefinger to my wrist. So I have in my hand now a sharp knife. And what is more, it's got a bulge on it so I can get a firm grip on the edge which has been chipped specially, which is sharp ... I could perfectly effectively cut meat with this. That's the sensation I have that links me with the man who actually laboriously chipped it once, twice, three times, four times, five times on one side. One, two, three ... three times at the other ... so eight specific actions by him, knocking it with another stone, to take off a flake, and to leave this almost straight line, which is a sharp edge." (David Attenborough)

In the British Museum, we've recently made a new chopping tool using the same techniques as would have been used in Olduvai Gorge. If I now hold that new one in my hand, it becomes very clear how well you can use it to strike meat off an animal. We don't have any African wildebeests to hand, so I can try it using a bit of roast chicken. But these chopping tools are marvellous and quick at getting the meat off the bone and then, with one sharp blow, I could break the bone and we'd be able to get to the marrow. But you can of course use a tool like this also to strip bark off trees or roots, so that you could eat them as well. This is, in fact, a very very versatile kitchen implement. The early humans who used chopping tools like this were probably not hunters themselves, but they were brilliant opportunists - they waited until lions, leopards or other beasts had killed their prey and then they moved in with their chopping tools, secured the meat and the marrow, and hit the protein jackpot.

Marrow fat doesn't sound tremendously appetising, but it is hugely nutritious - fuel not just for physical strength but also for a large brain. The brain is an extremely power- hungry mechanism. Although it accounts for only 2 per cent of our body weight, it consumes 20 per cent of our entire energy intake, and it requires constant nourishment. Our ancestors of nearly two million years ago, secured their future by being really rather sneaky. When stronger, faster fiercer predators were at rest out of the heat, 'they' were able to look for food. Using tools like this one to obtain bone marrow, the most nutritious part of a carcass, they set in train an ancient virtuous circle. This food for body and mind, meant that larger-brained individuals would survive to breed larger-brained children, capable in their turn of making ever more complex tools, and you and I are just the latest iteration of this continuing process.

Lots of animals use objects, particularly of course apes, but what sets us apart from them at this moment in our evolution is that, unlike them, we make tools before we need them. And once we have used them we keep them to use again. It's the beginning of the tool box.

The human brain then carries on evolving steadily over thousands of years. And what's really interesting, is that our brain starts to become asymmetrical as it gets to grips with a whole range of different functions - logic, language, the co-ordinated movement needed for tool-making, imagination and creative thought - quite unlike the ape's brain, which remains smaller and symmetrical. So what we're looking at in this chopping tool is the moment at which we became distinctly smarter and with an impulse not just to make things, but to imagine how we could make things 'better'.

"This object sits at the base of a process which has become almost obsessive amongst human beings. This object is something created from a natural substance for a particular purpose, and in a particular way, with a notion in the maker's mind of what he needed it for. Is it more complex than was needed to actually serve the function which he used it for? Do you know, I think you could almost say it is. Did he really need to do one, two, three, four, five chips on one side and four on the other? Could he have got away with two? I think he might have done so. I think the man or woman who held this, made it just for that particular job and perhaps got some satisfaction from knowing that it was going to do it very effectively, very economically and very neatly. In time, you'd say he'd done it beautifully but, maybe not yet ... the start of a journey." (David Attenborough)

Without those extra chips on the edge of the chopping tool, this whole series would be impossible, because those chips tell us that right from the beginning, we - unlike other animals - have wanted to make things more complicated than they need to be. You see, objects carry powerful messages about their makers, and the chopping tool is the beginning of a relationship between humans and the things they create, which is both a love affair and a dependency.

From this point on, we can't survive without the things we make and, in this sense, it is making things that makes us human. Leakey's discoveries in the warm earth of the Rift Valley did more than push humans back in time, they made it clear that all of us descend from those African ancestors, that every one of us is part of a huge African diaspora - we all have Africa in our DNA and all our culture began in the same place. Wangari Maathai is a Kenyan environmentalist and a Nobel Peace Prize winner:

"So far it seems like the information we have tells us that we came from somewhere within this part of the world in eastern Africa. And that of course for many people must be surprising - because I think we are so used to being divided along ethnic lines, along racial lines, and we look all the time for reasons to be different from each other - it must be surprising to some of us to realise that what differentiates us is usually very superficial, like the colour of your skin or the colour of your eyes or the texture of your hair, but essentially that we are all from the same stem, the same origin. So, I think that as we continue to understand ourselves and to appreciate each other, and especially when we get to understand, for example, that we all come from the same origin - we will shed a lot of the prejudices that we have harboured in the past."

Listening to the news on the radio, it's easy to imagine the world is divided into rival tribes and competing civilisations. So it's good, it's essential in fact, to be reminded that the idea of our common humanity is not just an enlightenment dream, but a genetic and a cultural reality. It's something we'll see again and again in this series.

Our next object is the tool that people took with them when they first left Africa and began to spread around the world - it's been called the Swiss Army knife of the Stone Age ... it's the hand axe.