By Neville Agnew and Martha Demas
Who has not walked barefoot on a beach of crisp sand and, bemused, examined the trail of footprints, paused, then looked back to see the tide wiping them away? So ephemeral are the traces of our passing.
Yet, astonishingly, the tracks of extinct animals have survived for aeons under unusual circumstances of preservation, recording a fleeting instance millions of years ago. Preservation of such traces occurs under conditions of deep burial whereby the sand or mud into which the prints were impressed is changed into stone, later to be exposed by erosion.
When, in 1978, fossil footprints of an extinct human ancestor were discovered during a palaeontological expedition led by Dr. Mary Leakey, scientific and public attention was immense. The prints, partly exposed through erosion, were found at the site of Laetoli, to the south of the famed Olduvai Gorge in Tanzania, where Louis and Mary Leakey did their pioneering work researching human evolution.
The footprints at Laetoli, dated at around 3.6 million years, resolved one of the major issues of contention in palaeoanthropology (the study of early mankind), a field characterized by fierce rivalries of discovery and interpretation. At Olduvai, Laetoli, and other sites in Africa and beyond, the search for evidence regarding human development has focused on the discovery of fossilized bones. But while fossils have been the primary means of understanding our past, they cannot yield all the answers to the great debates that have beset the study of human evolution. One debate has been over the development of the brain in relation to our ancestors' ability to walk upright. Since Darwin's time it was thought that once upright posture and bipedalism had developed, the hands were then free to evolve manipulative skills. Stone toolmaking, it was supposed, was the critical factor in the emergence of early man. This view, however, was not universally accepted. Some believed that the brain, not erect posture, led the way. Although functional analysis of hominid bones from Africa pointed to early bipedalism, the fossils themselves could not provide the definitive answer.
The Laetoli trackway settled the issue. Excavated by Mary Leakey and her team in 1978 and 1979, the trackway consists of some 70 footprints in two parallel trails about 30 meters long, preserved in hardened volcanic ash. The best-preserved footprints are unmistakably human in appearance and yield evidence of soft tissue anatomy that fossil bones cannot provide. It is significant that the earliest stone tools known are about 2.6 million years old, made nearly a million years after the footprints at Laetoli. The Laetoli hominids were therefore fully bipedal well before the advent of toolmaking—an event considered to define the beginning of culture—and the traces they left behind provide evidence that the feet led the way in the evolution of the modern human brain.
The Conservation Problem
The footprints at Laetoli, recorded by the Leakey team using various techniques including molding, casting, and photogrammetry, were reburied in 1979 as a means of preservation. After the trackway's reburial, the site revegetated. Although its condition was not known, nor was it visited frequently because of its remoteness, it was feared that the trackway might be deteriorating because of the impact of root growth, especially from acacia trees.
Following a request to the Institute by the Director of the Tanzanian Antiquities Unit, Dr. Simon Waane, for assistance in conserving the site, a GCI-Tanzanian team undertook a preliminary investigation in mid-1992. The team opened a three-by-three meter trench, which confirmed fears that root growth had caused damage, though the full extent could not be determined (this will only be known when the tracks are fully reexcavated in subsequent conservation campaigns). Where root growth had not affected the tracks, preservation was excellent, validating the Leakey team's decision to rebury the site and confirming a practice increasingly adopted by archaeologists to conserve excavated sites.
In 1993 field-testing was undertaken by the project team, and last year trees were poisoned, the site mapped, and measures taken to prevent erosion. In addition, the original cast of the trackway, stored at Olduvai since 1979, was remolded and new casts made with the assistance of staff from the National Museum of Tanzania. Since the trackway itself is now too fragile to be remolded, a new master cast will provide the most accurate replica possible. The new casts will also guide reexcavation in the field, and be used for museological displays in Tanzania and elsewhere.
An eight-week campaign is planned for July and August of 1995, during the dry season, when it is possible to reach the site by four-wheel-drive vehicle. A team of specialists from the Institute and Tanzania will reexcavate half of the trackway, record its condition stereophotographically, extract dead roots, stabilize the surface, and rebury it using synthetic geotextile materials layered into the overburden of sand and soil. These fabrics provide protection against root penetration yet allow the trackway surface to "breathe"—that is, to maintain moisture equilibrium between the subsurface of the trackway and the atmosphere. For a short period during the fieldwork, the section of the site undergoing conservation will be open to palaeoanthropologists for further study. Because of its fragility, the site can only be exposed for a very limited period. A similar campaign in 1996 will complete the project, after which a monitoring and maintenance program will be implemented by the Tanzanian authorities to ensure the long-term survival of the tracks.
Many opinions have been voiced as to the best method to save the tracks, and the strategy of reburial has been debated at length. Other ideas have been proposals to uplift the entire trackway (or only the individual footprints) and move it to a site under cover, such as the National Museum in Dar es Salaam, or to build a shelter over the site and open it to the public. The former assumes that the tracks have scientific value only and thus overlooks their cultural significance. The latter is impractical, at least for the moment, because of the site's remoteness and difficulty of access and the lack of infrastructure for displaying, staffing, and securing the site.
The Laetoli footprints are the most ancient traces yet found of humanity's ancestors. To move the site in toto or, worse, to remove only the prints, would be contrary to the widely accepted ethic of conservation in context. The prints were impressed in volcanic ash in that location 3.6 million years ago, in sight of the Sadiman volcano 20 kilometers away, whose subsequent ash falls buried them under 30 meters of deposit. Over the aeons the landscape eroded, until now, less than a few feet of soil protects the fragile surface. Powerful arguments can be mustered for every effort to save the site in its original context. The Tanzanian authorities themselves are committed to this approach.
It is indisputable that burial is an effective preservation measure, if other requirements such as vegetation control through maintenance are also met. Only if these criteria are not achievable should other options be considered. While lifting the tracks is doubtless technically possible, it would be enormously costly, require much research, and risk damage or loss. For these reasons, the decision to rebury the site has been made, and if future conditions allow the site to be opened to visitors, it will have been saved.
The footprints at Laetoli represent an immense distance in time. While we are used to bandying about terms like a million years, we cannot really comprehend them on a human scale. We are comfortable with one or two thousand years. They are within the frame of recorded history, spanning the last hundred human generations. The Laetoli tracks are of another dimension, taking us back perhaps more than one hundred and eighty thousand generations.
The question has been asked why the Getty Conservation Institute, whose work is preservation of the cultural heritage, should be involved in saving a fossil site, even one of immense significance in the study of evolution. The answer to this question will be clear to those who have trod the beach and pondered their own trail of footprints, for there can be scarcely anything so evocative as the Laetoli trail marking humanity's long, wondrous, and mysterious journey. As a nexus between cultural heritage and science, so often uncomprehended in today's world of big science, the footprints are a poignant reminder of our ancient origins. Let Mary Leakey have the last word in talking of one of the hominids who made the trail: "At one point, and you need not be an expert tracker to discern this, she stops, pauses, turns to the left to glance at some possible threat or irregularity, and then continues to the north. This motion, so intensely human, transcends time. Three million six hundred thousand years ago, a remote ancestor—just as you or I—experienced a moment of doubt."
Neville Agnew is Associate Director for Programs at the GCI.
Martha Demas is Acting Director of the GCI's Special Projects.