A tall man sits with his chin propped up on his knees; a woman puts her back into grinding - just two superb pieces among the 2-3000-year-old Nigerian terracotta sculptures which inundated the black market of the antique art world in the 1990s. Thousands lie with private collectors across the globe, their original provenance unknown and their thermoluminescence dates a secret.

This plundering of Africa is greater now than in colonial times: antiquities are an easy form of money laundering within the wider pattern of capital flight from Africa. The major casualties of this trade have been the Nok and Kwatakwashi (or Sokoto) terracotta figurines of middle-belt and northern Nigeria.

In 1983, at Dan Baure, south-west of Kano, I was shown fragments of deliberately smashed terracotta figurines. "Images of the human form are contrary to Islamic belief," explained my guide. "People thought these were idols." Tenderly, I picked up part of a beautifully sculptured face, then an almost intact belly with umbilical hernia: there was little else that could be saved for the new Gidan Makama Museum in Kano.

In 1993 I discovered antiquities mining at Kwatakwashi Hill, once the major centre for Magiro or Kakan Tsafi ("grandfather of all fetish"), controlling ancestor worship over the middle Niger region. Pretending to be a buyer, I photographed recently dug terracotta heads, then asked why the traders were selling off their own culture - a volte-face that created panic and sent some traders running. "Hankali! (Careful!)" I called, "I can do nothing!" When the commotion subsided, their chief explained that they did not know what these mysterious figurines were. In return, I forwarded the western cultural conservation message, bitterly aware of its naivety, for commercial pressures had already created two teams, each a thousand strong, systematically digging up Nigeria.

Traders obtained legal mining permits to "break the ground", but instead of seeking cassiterite, they told local people to look for pottery sherds, circular stone foundations or a triangle of cooking pot stones and then to dig for the horizontal stone slabs, which covered terracotta grave goods. This quasi-legal activity laid bare two ancient cultures pre-dating the Hausa kingdoms, but secrecy still shrouds the early artists of this "Birthplace of African Art", for both the superb Nok and Kwatakwashi terracottas have been robbed of their primary cultural contexts by these uncontrolled excavations.

These early artists spread to the adjacent Bantu Cradle Area in eastern Nigeria, so when an arid phase precipitated the Bantu expansion 2000 years ago, their art went with the migrants, whose shifting cultivation practices continued to move them eastwards and southwards over a third of Africa. This is by far the most serious tragedy ever inflicted on Africa's art, for it affects its very roots. As for those who dug their ancestors' graves and sold their irreplaceable birthright to aliens overseas, they have not "cleansed a nation of its idols"; rather, they now face the angry spirits they have disturbed.