Rival masterpieces: 5th century BC

By one of the strange coincidences of history, the 5th century BC produces the first masterpieces in two incompatible styles of sculpture. Nearly 2500 years later, these styles become bitter rivals in the studios of our own time.

One is the classical realism which will prevail from the Renaissance to the end of the 19th century. The other is the sculpture of Africa, distorting human features and limbs in a dramatically expressive manner. African figures in this long and vibrant tradition inspire Picasso's experiments with Cubism, which launch the mainstream of modern art.

The characteristic sculpture of Africa, which forms the largest part of what is usually considered primitive art, can be seen as early as 500 BC in the Nok culture - named from the village in Nigeria where pottery figures of this kind were first found.

The Nok statuettes are mainly of human subjects. Made of terracotta, they combine strong formal elements with a complete disregard for precise anatomy. Their expressive quality places them unmistakably at the start of the African sculptural tradition.

African terracotta figures: from the 5th century BC

The longest surviving tradition of African sculpture is figures in terracotta. Cast metal is the only other material to withstand the continent's termites (fatal to the carved wood of most African sculpture). But the superb Metal sculptures of Nigeria, beginning in about the 12th century, are of a much later period than the first terracottas.

West Africa, and in particular modern Nigeria, provides the longest and richest sequence of terracotta figures. They date back two and a half millennia to the extraordinary Nok sculptures. By around the 1st centuryfigures of a wonderful severity are being modelled in the Sokoto region of northwest Nigeria.

Terracotta heads and figures have been found in Ife, dating from the 12th to 15th century - the same period as the first cast-Metal sculptures of this region. At Jenne, further north in Mali, archaeologists (followed unfortunately by thieves) have recently unearthed superb terracottas of the same period.

One extraordinary group of terracottas is the exception in this mainly west African story, in that they come from south Africa where they are the earliest known sculptures. They are seven heads, found at Lydenburg in the Transvaal. Modelled in a brutally chunky style, they date from about the 6th century AD.

Powerful terracotta figures in traditional style continue to be made in Africa in the 19th and 20th century, contemporary with the superb carved wooden figures which survive from those two centuries.

Unlike European painting or sculpture, style does not greatly change over the years in African tribal art. So it is a safe assumption that the astonishing imaginative range of African carving familiar to us today was just as evident many centuries ago, though the objects themselves have now crumbled to dust.

Ife and Benin: from the 12th century AD

An unusual tradition within African sculpture is the cast-metal work done from about the 12th century in what is now southern Nigeria. It reaches a peak of perfection among the Yoruba people of Ife. Between the 12th and the 15th century life-size heads and masks, and smaller full-length figures - all of astonishing realism - are cast in brass and sometimes in pure copper (technically much more difficult). These figures have an extraordinary quiet intensity.

This craft, perfected by the Yoruba people, is continued from the 15th century in Benin - still today a great centre of metal casting. The Benin heads, delightful but less powerful in their impact than those of Ife, are commonly known as Benin bronzes.

In fact they are made of brass, melted down from vessels and ornaments arriving on the trade routes (in 1505-7 alone, the Portuguese agent delivers 12,750 brass bracelets to Benin). The arrival of the Portuguese prompts the Benin sculptors to undertake a new style of work - brass plaques with scenes in relief, in which the Portuguese themselves sometimes feature. These plaques are nailed as decoration to the wooden pillars of the royal palace.

African wood carving: 19th - 20th century AD

In Africa, south of the Sahara, wood is the natural material for carving. In the 20th century sculpture in wood is still very much a living tradition. Examples from the 19th century have been preserved in reasonable number, largely by the efforts of collectors. But earlier work has crumbled irretrievably, eaten by ants or rotted by damp.

Even so, the body of art surviving to us in this tradition is immensely rich. It powerfully suggests how much has been lost.

It is difficult to imagine how African tribal sculptors have viewed their own work, but they have certainly not seen it as art in the self-conscious western manner of recent centuries.

Tribal carving is done for a clear and practical purpose. A figure may represent an ancestor, destined to stand in a shrine. A mask may be intended for use by a Shaman just once a year in a special dance. A post may be designed to prop up a chief's verandah or to form part of a palisade round his house. An elaborate chair is likely to be for the chief himself to sit on. All of them will be better if carved in a dramatic or propitious way.

Tribal art and cubism: 20th century AD

Whatever the reason for the range of tribal art, the result is an unrivalled display of the power of the imagination. The basic subject, as in western sculpture, is the human body. But the tribal sculptor is liberated from the straitjacket of realism.

His ingredients may be limited to the parts of the body, but he constantly reassembles them in new dimensions and relationships. From a central axis of eyes, nose, mouth, navel and genital organs, to the peripheral cast list of hair, ears, arms, breasts, legs and buttocks, there is no predicting which of these elements will take the starring roles in any one production. Startling imbalance is restored to balance by the force of strong design.

It is hard to know whether a particular image may be intended to seem sad or terrifying (or neither, or even nothing), for this is a subjective matter on which an outsider may often be mistaken. But in these carvings there is no mistaking the energy and playfulness with which the human body is turned, by confident distortion, into such a gallery of wonderful creatures.

It is not surprising that Picasso, the most playful genius of the 20th century, is inspired by these fragmentations of dull reality to find a new direction of his own in cubism.

This History is as yet incomplete.