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They are the world's oldest human tracks, a set of footprints pressed into volcanic ash that have lain perfectly preserved for more than 3.5 million years.
Made by a group of ancient apemen, the prints represent one of the most important sites in human evolutionary studies, for they show that our ancestors had already stopped walking on four legs and had become upright members of the primate world.
But now the Laetoli steps in northern Tanzania are in danger of destruction. The footprints, although reburied 10 years ago and covered by a special protective coating, are suffering storm erosion, while trees and plants begin to grow through the historic outlines.
The Laetoli steps were discovered in 1976 by scientists led by the late Mary Leakey, mother of conservationist Richard Leakey. They found a couple of prints that had been exposed by the wind and then uncovered a trail that led across an expanse of volcanic ash, like footprints left behind by holidaymakers walking on a wet beach.
The researchers could make out the arch of each foot, the big toe - even the heel. The prints had clearly been made by creatures who had long adapted to walking on two legs. Yet tests showed the prints had been made about 3.6 million years ago.
At that time, the area was populated by short, small-brained species of apeman, known as Australopithicus afarensis, an ancestor of modern human beings. Most scientists believe these were the creators of the Laetoli footprints, individuals who may have been escaping an eruption of the nearby Sadiman volcano. By studying the prints, scientists conclude that a smaller individual - presumed by Leakey to be a female - stopped in her tracks and glanced at some threat or sound to her left. "This motion, so intensely human, transcends time," Leakey wrote in National Geographic.
"Three million, six hundred thousand years ago, a remote ancestor - just as you or I - experienced a moment of doubt." It is this window on human behaviour that makes Laetoli so important, say scientists.
But a study presented at an international conference last month warns that unless urgent action is taken, the Laetoli steps - "the rarest, oldest and most important evidence" documenting humans' ability to walk on two legs - will be lost to civilisation.
"The protective blanket over the prints is already breaking up," said Dr Charles Musiba of the University of Colorado, Denver. "Unless something is done within the next five years, the site is going to suffer serious, irreparable damage."
He added: "The footprints are currently buried for their own protection - which means we can no longer study them, and that is crazy. We could use scanners and other modern tools to learn all sorts of things about the people who made these prints. We need to expose them but protect them as well. Building a museum over them is the perfect solution."
Palaeontologists agree that action is needed, but claim that constructing a building over the steps in remote Laetoli is impossible and would only lead to further degradation. "No matter how good the intentions, any attempt to preserve them in place is doomed to failure," said one of the steps' discoverers, Tim White of the University of Berkeley, California. "Laetoli is remote, inaccessible, and would require infrastructure currently not available or foreseeable to preserve these prints in place."
Professor Terry Harrison of New York University said: "The local people around Laetoli, the Masai, do not appreciate having structures built on their land. They tend to smash things up. These are pastoral people who do not have a sense of property and can be destructive. You would need to guard the museum constantly and carefully."
Harrison and White believe the whole sequence of steps, about 23m in length, should be cut from the local hillside, transported to Dar es Salaam, Tanzania's capital, and installed in a museum. The technology for such an endeavour has precedents. In 1968, engineers relocated the Egyptian temple of Abu Simbel when it was threatened by the creation of the Aswan Dam.
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