Anthropologists regard the footprints, made by man's early ancestors walking across a landscape sprinkled with damp volcanic ash, as one of the most important discoveries of human evolution. They were made by at least three hominids, two walking side by side and a third trailing behind and treading in the footprints of the largest individual walking ahead.
Analysis showed that the gait of the hominids was much the same as that of modern humans, a clear departure from the ''knuckle-walking' of apes. The prints also show that hominids at this time did not have a gap between the big toe and the other toes, a characteristic of apes.
The footprints are proof that bipedalism evolved before the development of the large brain of modern humans. The footprints were most likely made by Australopithecus afarensis. These early human ancestors were only 5ft tall and, although they walked upright, they had small brain cases and relatively long arms. Some anthropologists think they spent at least some time in trees, foraging for food on the ground during the day.
Mary Leakey, the distinguished archaeologist, discovered the tracks at Laetoli in 1978. The prints were analysed before being covered up in 1979 with polythene, sand and rocks to protect them.
However, scientists who recently visited the site were horrified by the neglect. Termites had eaten the polythene, torrential rain had washed away much of the sand, and acacia trees were growing over the tracks, raising fears that their roots had begun to break up the brittle volcanic ash.
The Tanzanian government, which is responsible for maintaining the site, commissioned the American Getty Conservation Institute to review the situation. Their confidential report, due to be presented to Tanzanian officials this month, is understood to conclude that half of the 70 footprints have been destroyed or damaged.
A source close to the investigation said: 'Money is not really the problem - it's neglect. It's 13 years of letting trees grow over the site.'
Desmond Clark, emeritus professor of anthropology at the University of California, who was among the team that visited the site this summer, said: 'If the footprints are going to be saved something has to be done very quickly.'
Professor Michael Day, of the Natural History Museum, said: 'The Tanzanian government should be encouraged to build a site museum, properly fenced and looked after. But someone would have to fund it.'