International Congress in honour of Dr. Mary Douglas Leakey's Outstanding Contribution to Palaeoanthropology, Arusha, Tanzania

A complete skeleton of Brachiosaurus excavated in Tanzania (now standing in the Natural Science Museum of the former East Berlin) represents one of the largest land animals that ever lived. Estimates have put its weight at 80 tonnes (as much as twenty mature elephants), and with its eye level 12.6 metres above the ground it was tall enough to look over a four storey building.

Geographers in Afric Maps

With savage Pictures fill their Gaps

And o'er uninhabitable Downs

Place Elephants for want of Towns

The upright bipedal gait of humans is a unique and highly inefficient mode of locomotion, but the anatomy of modern apes, with 60 per cent of their body weight carried on the hindlegs, indicates that the common ancestor of apes and humans was pre-adapted to bipedalism. Environmental circumstances in Africa provide an explanation of why and how the fully upright stance and bipedal gait evolved in humans.

Laetoli lies roughly 500 kilometres to the south of the Tugen Hills. Late-twentieth-century walking enthusiasts could cover the distance comfortably in twenty-five days, and the journey certainly would not have deterred their ancestors, 4 million years before. Indeed, the safari must always have been enticing: south along the Western wall of the Rift Valley; perhaps taking a slow route through the high forest, where there is fruit and honey in season, or moving more speedily through wooded grasslands borderingthe foothills below. At intervals, perennial streams gush dependably from the Rift wall. There would have been (and are still) predators to be avoided, of course, but also their prey to be scavenged. The route rises up and over the Mau escarpment, where there is an option of following the forested course of the Uaso Nyiro River to Lake Natron, or turning towards the Loita Hills and the cool grasslands above the lake basin. Volcanoes dominated the landscape to the east of Olduvai; southward, herds of antelope and zebra congregated on the plain.

Andrew Hill made the journey by road in September 1976. At Laetoli, Mary Leakey and her co-workers were bringing the season of investigations to a close. The work that year had been inspired by the discovery of hominid fossils (among them a mandible subsequently described as the type specimen of Australopithecus afarensisf during an exploratory visit made during the Christmas holiday of 1974.

A host of fossils had been found, including animals ranging in size from shrew to elephant, tortoises, a clutch of beautifully preserved eggs matching those of the modern guinea fowl, and tiny leaves identical to those on acacias in the woodlands today. The Laetoli fossil beds had been dated to between 3'59 million and-3-77 million years old - just the period during which the bipedal ancestors of humanity were consolidating their presence in the Rift Valley - but hominid finds were scarce at Laetoli: a few fragments of jaw and some isolated teeth were found in T.97S, and some pieces of a juvenile skeleton in 1.976. After a promising start, it seemed the potential of the Laetoli deposits was not to be fulfilled.

That was the state of affairs when Andrew Hill went for a stroll one evening with David Western, a wildlife ecologist also visiting the Laetoli sites. Their walk took them across a dry river bed in which an expanse of fine-grained volcanic tuff was exposed. Elephants had recently passed that way too, and had left a number of their cannonball-sized droppings scattered about the river bed. In equatorial Africa, a sun-dried ball of elephant dung appeals to the same instincts that snowballs awaken in northern latitudes. People fling them at one another and, unsurprisingly, wildlife ecologists tend to be more adept than most. Dr Hill fell as he turned to avoid a particularly well-aimed missile from Dr Western. While on his knees, pleading for a brief cessation of hostilities, he noticed a curious spattering of tiny indentations in the surface of the grey tuff. These were later identified as raindrop prints but, having attracted Hill's attention, they led him to examine the surface more closely. Amid the puzzling indentations he recognized an unmistakable series of animal tracks'

People had crossed that indented tuff surface hundreds of times during the course of the previous two seasons, but always on the way to somewhere else, with a clear picture in mind of the fossils they were looking for. By chance, an airborne ball of elephant dung introduced a fresh point of view, instantly focusing the investigators' attention on the totally different fossil information that lay at their feet - fully visible, but hidden until then by the blinkers of preconceived notion.

Dr Hill's lucky fall redirected the thrust of the Laetoli investigations. Fossil bones were relegated to a level of secondary interest and during the final weeks of the 1076 season the identification of fossil footprints became the primary endeavour.

Hundreds of prints were found, representing more than twenty different animals, ranging in size from cat and hare to elephant, rhinoceros, and giraffe. Guinea-fowl prints were numerous, so too were the prints of small antelopes, hyenas, pigs, baboons, and hipparion, the ancestral three-toed horse. During the 1977 and 1978 seasons, seven distinct sites were located and mapped. Where desirable, overlying soils were removed. Mammal and bird prints occurred everywhere, wonderfully preserved in the fine-grained volcanic ash. Most wonderful of all was the trail, nearly fifty metres long, left by three hominids walking northward from the woodlands down to the plains.

The trail records a unique moment in time and its preservation is little short of miraculous. About 3.6 million years ago, a series of light ash eruptions from a nearby volcano coincided with a series of rain showers, probably at the onset of the rainy season. The ash filled depressions in the landscape, and the rain transformed them into mud pans. Animals crossed the pans while they were still wet, and their tracks were preserved as the ash dried hard as cement. The next shower of ash laid a protective covering over the tracks. A succession of ash and rain showers created at least six distinct surfaces on which prints are preserved; in total they are fifteen centimetres thick.

Sadiman and Lemgarut, the volcanoes whose ash created the Laetoli fossil beds, are no longer active, but the Laetoli landscape is otherwise not very different today from that which its inhabitants knew over 3 million years ago. The highland foothills are covered in dense acacia thornbush, and the upper slopes are swathed in grass that turns from green to golden as the dry season advances. Westward, the plain extends to a distant horizon, the broad undulating expanse broken here and there by huge steep-sided outcrops of granite and gneiss that rise from the grassland-like islands (indeed, in geological terminology they are known as inselbergs - island mountains): Naibardad, Naabi, and Moru, where there is always water. In shallow valleys, strands of woodland mark the watercourses along which the seasonal rains drain away to Olduvai Gorge, about forty kilometres from Laetoli. Elephants come down from the highlands; giraffes cross the plain, their legs blurred in the shimmering heat haze; lions lie concealed in the dun-coloured grass,