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Ife head (probably made 15th century). Bronze statue; from Nigeria

So far in this history of the world through "things" we've encountered all kinds of objects, all eloquent, but not all particularly valuable, or attractive. But today's object is in any view a great work of art. It's a head, cast in brass. It's quite clearly the portrait of a person, but we don't know who. It's without question by a very great artist, but we don't know who. And it must have been made for a ceremony, but we don't know what.

What is certain is that the head is African, it's royal, and it epitomises the great medieval civilisations of West Africa of about seven hundred years ago. It was one of a group of heads discovered in 1938 in the grounds of a palace in Ife, Nigeria, and they astonished the world with their beauty. They were immediately recognised as supreme documents of a culture that had left no written record, and they embody the history of an African kingdom that was one of the most advanced and urbanised of its day. The sculptures of Ife exploded European notions of the history of art, and they forced Europeans to rethink Africa's place in the cultural history of the world. Today they play a key part in how Africans read their own narrative.

"We still don't know much about the African past. What we know right now is a fraction of what is yet to be discovered." (Babatunde Lawal)

"Personally I look at it and I am struck by its tranquillity, its upward gaze. It's not just the tranquillity of power, it's the tranquillity of being in an inner sanctum almost." (Ben Okri)

I'm in the Africa Gallery of the British Museum, looking at the Ife head. Or rather, he is looking at me. His head's a little smaller than life-size, and made of brass, which has now darkened with age. The shape of the face is an elegant oval, covered with finely incised vertical lines, but it's a facial scarring so perfectly symmetrical that it contains rather than disturbs the features. He wears a crown - a high beaded diadem with a striking vertical plume projecting from the top, and that's still got quite a lot of the original red paint. This is an object with extraordinary presence. The alert gaze, the high curve of the cheek, the lips parted as though about to speak - all these are captured with absolute confidence. To grasp the structure of a face like this is possible only after long training and meticulous observation. There's no doubt that this represents a real person. But this is reality not just rendered but transformed. The details of the face have been generalised and abstracted to give an impression of repose. Standing face-to-face with this brass sculpture, I know that I'm in the presence of a ruler imbued with the high serenity of power. When Ben Okri, the Nigerian-born novelist, looks at the Ife head, he sees not only a ruler, but a society and a civilisation:

"Personally it has the effect on me that certain sculptures of the Buddha have. The presence of tranquillity in a work of art speaks of a great internal civilisation. Because you can't have the tranquillity without reflection, you can't have the tranquillity without having asked the great questions about your place in the universe, and having answered those questions to some degree of satisfaction. And that for me is what civilisation is."

The idea of black African civilisation on this level was quite simply unimaginable to a European a hundred years ago. In 1910, when the German anthropologist Leo Frobenius found the very first brass head in a shrine outside the city of Ife, he was so overwhelmed by its technical and aesthetic assurance that he immediately associated it with the greatest art that he knew - the classical sculptures of ancient Greece. But what possible connection could there have been between ancient Greece and Nigeria? There's no record of contact in the literature or in the archaeology. For Frobenius there was an obvious and exhilarating solution to the conundrum. The lost island of Atlantis must have sunk off the coast of Nigeria, and the Greek survivors stepped ashore to make this astonishing sculpture.

It's easy to mock Frobenius, but at the beginning of the twentieth century Europeans had very limited knowledge of the traditions of African art. For painters like Picasso, Nolde or Matisse, African art was dionysiac, exuberant and frenetic, visceral and emotional. But the restrained, rational Apollonian sculptures of Ife clearly came from an orderly world of technological sophistication, sacred power and courtly hierarchy - a world in every way comparable with the historic societies of Europe and Asia. And, as with all great artistic traditions, the sculptures of Ife present a particular view of what it means to be human. Here's Babatunde Lawal, Professor of Art History at Virginia Commonwealth University:

"Frobenius around 1910 assumed that maybe the survivors of the Greek lost Atlantis might have made these heads. And he predicted that if a full figure were to be found, the figure would reflect the typical Greek proportions - say the head constituting about one seventh of the whole body. But when a full figure was eventually discovered at Ife, the head was just about a quarter of the body, complying with the typical proportion characterising much of African art - the emphasis on the head because it is the crown of the body, the seat of the soul, the site of identity, perception and communication."

So it's perhaps not surprising that nearly all of the Ife metal sculptures that we know - and there are only about 30 - are heads. In 1938, an astonishing group of 13, including the one now at the British Museum, was dug up in the precincts of a royal palace at Ife. The quality of the brass casting was superb. And there could be no doubt now that this was a totally African tradition. The 'Illustrated London News' of 8 April 1939 published the find. And in an extraordinary article, the writer, still using the racist language of the 1930s, recognises that what he calls the negro tradition - a word then associated with slavery and primitivism - must, with the Ife sculptures, now take its place in the canon of world art. "Negro" could never again be used in quite the same way:

"One does not have to be a connoisseur or an expert to appreciate the beauty of their modelling, their virility, their reposeful realism, their dignity and their simplicity. No Greek or Roman sculpture of the best periods, not Cellini, not Houdon, ever produced anything that made a more immediate appeal to the senses, or is more immediately satisfying to European ideas of proportion."

It's hard to exaggerate what a profound reversal of prejudice and hierarchy this represented. Along with Greece and Rome, Florence and Paris, now stood Nigeria. If you want an example of how "things" can change thought, then the impact of the Ife heads in 1939 are, I think, as good as you'll find.

Current research suggests that the heads that we know were all made over quite a short stretch of time, possibly in the middle of the fifteenth century. At that point Ife had for centuries been a leading political, economic and spiritual centre. It was a world of forest farming, dominated by cities that developed in the lands west of the Niger River. And it was river systems that connected Ife to the regional trade networks of West Africa, and to the great routes that carried ivory and gold across the Sahara to the Mediterranean coast. In return, came metals that would make the Ife heads. The world of the Mediterranean had provided not the artists, as Frobenius supposed, merely the raw materials.

The forest cities were presided over by their senior ruler, the Ooni of Ife. The ooni's role was not merely political, he also had a great range of spiritual and ritual duties. And the city of Ife has always been the leading religious centre of the Yoruba people. Still today there is an ooni who has high ceremonial status and moral authority, and whose headgear still echoes that of our sculpted head of about seven hundred years ago.

Our head is almost certainly the portrait of an ooni, but it's not at all obvious how such a portrait would have been used. It was clearly not meant to stand on its own, so it might well have been mounted on a wooden body, and there's what looks like a nail hole at the neck that could have been used to attach it. It's been suggested that it might have been carried in processions, or that in certain ceremonies it could have stood in for an absent or even for a dead ooni.

Around the mouth there are a series of small holes. Again, we can't be quite certain what these are for, but they were possibly used to attach a beaded veil that would hide the mouth and the lower part of the face. And we know that the Ooni today still covers his face completely on some ritual occasions - a powerful marker of his distinct status as a person apart, not like other human beings. And astonishingly our sculpture suggests this dual nature - an ooni who is a man and also something more. Here's Ben Okri again:

"This is not just sculpting as representation. I think it's really important to understand that. This is not just 'this is what a certain person or a certain king looked like' - this is more than that. This is kingship in its ritual aspect, this is kingship in its relationship with divinity, this is kingship in relationship with the centrality of the myths of a tribe and of a people. This is kingship as an embodiment of the mysterious power of a people."

But there's a sense in which the Ife sculptures have also become embodiments of a whole continent, of a modern Africa confident in its ancient cultural traditions. Here's Babatunde Lawal again:

"And then of course today many Africans, and Nigerians in particular, are proud of their past, a past that was denigrated as being crude, primitive in the past. And then to realise that their ancestors were not as backward, as it were, as they were portrayed - it was a double source of joy to them. This discovery unfurled a new kind of nationalism in them, and they started walking tall, feeling proud of their past. And then contemporary artists now seek inspiration from this past, to energise their quest for identity in the global village that our world has become."

The discovery of the art of Ife is, I think, a supreme example of a widespread cultural phenomenon: that as we discover our past, so we discover ourselves. And more: to become what we want to be, we have to decide what we were. Like individuals, nations and states define and redefine themselves by revisiting their histories.

In the next programme we're going to be in China, where the interpretation of history has always been a central part of the political debate. We'll be at the same date as the Ife head, and with an equally high-status object... but we'll be looking not at ritual bronze, but at sacred porcelain.