Andrew Hill

For several decades Laetoli had just missed as a hominid fossil site. Louis Leakey had a try there in 1935, but came up emptyhanded. He did not know that a tooth he had sent to the British Museum labeled as a baboon's was a hominid canine. Not only was it the first adult australopithecine tooth ever found, but it was the first of any kind since the discovery of the Taung Baby. Nevertheless, it lay unnoticed in the Museum collection until 1979, when it was spotted and properly identified by White.

Leakey, meanwhile, not realizing that he had had in his hand the oldest hominid fossil then known, packed up and moved to Olduvai. He was followed at Laetoli in 1938-1939 by a German named Kohl-Larsen, who recovered a bit of an upper jawbone with a couple of premolars in it, and a well-preserved alveolus - or socket - for a canine tooth.

The trouble with those early Laetoli finds was that they were far too old and far too primitive for anyone then to dream that they were not apes or monkeys; the imagination of the 1930s was simply not elastic enough to accommodate them, even though that same imagination was saying to itself, 'Look deeper into time for older ancestors.' This is an odd, schizophrenic view that still persists today.

By 1974, when Mary Leakey decided to have a go at Laetoli, her mind at least was ready to recognize and accept very old specimens of Homo. When one of the Leakey-trained Kenyan field experts, Karnoya Kimeu, took it upon himself to cut a road in to the deposits through the thicket and came out with a hominid, Mary Leakey moved in with a team of her own. In the next couple of years she or her workers found forty-two teeth, some of them associated with bits of jawbone. One in particular, LH-4 (Laetoli Hominid 4), was a fine specimen, a mandible with nine teeth in place.

But what sets Laetoli apart from every other site in the world is some footprints that have been found there, certainly one of the most extraordinary cases of preservation and discovery in all of paleoanthropology.

Laetoli has a nearby volcano, Sadiman, that is extinct today. Not quite four million years ago it was active. One day it spat out a cloud of carbonitite ash. This stuff has a consistency not unlike that of very fine beach sand, and it powdered down over the surrounding landscape in a layer that reached a thickness of about half an inch before the eruption stopped. This fall of superfine cinders must have been extremely unpleasant for the local animals and birds while it was coming down, but there is no evidence that it did more than make them uncomfortable, because they stayed in the area. That first puff of ash - probably not lasting more than a day - was followed by a rain. The ash became wet and, almost like a newly laid cement sidewalk, began taking clear impressions of everything that walked across it: elephants, giraffes, antelopes, hares, rhinos, pigs. There were also terrestrial birds like guinea fowl and ostriches, and even the small tracks of millipedes.

In the hot sun of Laetoli the wet ash layer quickly dried and hardened, preserving the footprints that crisscrossed it. Then, before it could rain again, Sadiman spoke a second time. Another cloud of ash drifted down, covering the first and sealing in the footprints. This happened a number of times over a period estimated to have been no longer than a month, producing a single volcanic tuff about eight inches thick. .But because of the periodic puffing of Sadiman and the periodic hardening of the ash that fell, the tuff is actually composed of between a dozen and two dozen distinct thin layers. Some of these layers have been exposed recently by erosion, and are visible here and there at Laetoli in the form of a gray substrate wherever the mat of coarse turf above them has been carried off.

One afternoon in 1976, some of the more boisterous members of Mary Leakey's field team were amusing themselves by throwing hunks of dried elephant dung at each other. This may seem a peculiar pastime, but recreational resources are limited on paleontological digs, and there are times when young spirits need to blow off steam. One who felt this urge was Andrew Hill, a paleontologist from the National Museum of Kenya, who, while ducking flying dung and looking for ammunition to fire back, found himself standing in a dry stream bed on some exposed ash layers. One of these had some unusual dents in it. When Hill paused to examine them, he concluded they probably were animal footprints. That diagnosis was confirmed when a larger area was surveyed and other prints found. But no serious effort was made to follow up this extraordinary discovery until the following year, 1977, when a number of large elephant tracks were found by Mary Leakey's son Philip and a co-worker, Peter Jones, and alongside them some tracks that looked suspiciously like human footprints.

The world heard about the footprints later that year when Mary Leakey came to the United States to report on them in a series of press conferences and interviews. To many it seemed almost inconceivable that anything so ephemeral as a footprint should have been preserved for so long. But Mary was positive about the hominid ones. She went on to describe the latter as having been made by a creature that was an imperfect walker; the prints indicated that it had shuffled. She also reported the probable presence at Laetoli of knuckle-walking apes and the existence of a water hole around which the animals and birds appeared to have clustered. She even saw some evidence of panic in the tracks, suggesting that the animals had been fleeing the eruption.

Those revelations by Mary Leakey electrified everybody who heard them. She resolved to devote much of the next season's effort at Laetoli to footprints, and asked the American footprint expert Louise Robbins to join her team. White went to the Laetoli site for the first time that year, and found three other young scientists there: Peter Jones, Paul Abell and Richard Hay. These men had some doubts about Mary's interpretation of the footprints. White questioned the presence of knuckle-walking apes; he had examined those prints and said that they had been made by large extinct baboons that walked flat-footed. Jones said there had been no panicky exodus from the area, because birds, which could have flown away easily and quickly, continued to walk about in the ash it was crisscrossed with their tracks. Hay could find no evidence of a water hole.

These disagreements made for a good many nights of heated argument in camp, during which the supposedly human footprints had their ups and downs. No one could agree on them. Then Paul Abell, prospecting alone one day, found a broken impression - but a much clearer one - that he said he was quite confident was a hominid print. White and Jones made some Polaroid shots of it and came back with a strong impression that Abell was right. They recommended that excavation in the area be started immediately. But Louise Robbins, the footprint expert, examined it and declared that it was the print of a bovid (a hoofed animal). She told Mary Leakey that further investigation would be a waste of time. The men objected.

By then Mary Leakey had become thoroughly exasperated by all the arguing that had been going on. She announced that there would be no excavation. Jones, now convinced that it was a hominid, continued to plead with her for permission to make an excavation. A very small one, he said, was all he asked. Mary was adamant. Louise Robbins, the authority, had spoken; there was too much incomplete excavation at the site already. If there was going to be any digging, let it be done by somebody who had nothing better to do. She pointed to Ndibo, the maintenance man, the man in camp with the least archeological training.

Ndibo, however, proved equal to the task. He returned to camp the next day and reported not one, but two footprints. One was very large. He held his hands up, about a foot apart.

'Those Africans are always exaggerating,' said Mary. But she did go out to have a look, and there they were. White was permitted to start an excavation.

The direction of the prints indicated that their maker had been walking north under some sections of turf that had not yet been eroded. Because of the dense tangle of roots at the bottom of the turf, the task of exposing a clear ash surface without destroying it - not to mention the exact ash out of a dozen or more thin layers of it turned out to be extraordinarily difficult. But Tim is an extraordinarily patient and determined man. He found another print, and then another. He proceeded to protect the prints by hardening them with a preservative, which he poured into them in very small amounts, letting the material dry and then strengthening it by adding more. Working with agonising slowness, he inched his way farther and farther into the turf and discovered that the trail consisted of the tracks of two hominids.

Now he had the riveted attention of the entire camp. Others joined the work and ultimately were able to reveal more than fifty prints covering a distance of seventy-seven feet. Louise Robbins, her interest in the footprints suddenly rekindled, issued another opinion: indeed there were two hominids; they were probably walking together; one (with slightly larger prints) was a male; the other, possibly pregnant she said, was a female; on the evidence of the prints, this type of hominid had been an erect walker for at least a million years.

These are entertaining speculations. There is no way of telling what sex the makers of the footprints were, if one was pregnant, or how long their ancestors had walked erect. The hard truth is that 3.7 million years ago erect hominids of indeterminate sex did walk through fresh-fallen ash at Laetoli and leave an imperishable record of their passing. After seventy-seven feet their trails disappeared under the overlying ash; the particular layer that marked it has been washed away. Tim's work on the footprints stopped at that point, which also marked the end of the season. But he felt strongly that the trail could be picked up again a little farther along and that it would yield more prints if proper excavation were carried out. Work in that direction was done in 1979 by Ron Clarke, and the trail picked up again.

Tim was not a party to this further work. His arguments with Louise Robbins over interpretation of the footprints have made him as unwelcome now at Laetoli as he is at Lake Turkana - a pity, because in each instance he was only trying to help the proprietors.

Tim's concern today is that as more prints are found, they be handled with the utmost care. They are supremely fragile, and the slightest mistake in excavating them can destroy them completely. Some have already been damaged. They are not like fossils, those rocklike models of durability. They are only spaces, mere shapes in a relatively soft and frangible matrix. If that matrix is nudged incorrectly, it will crumble - and the footprints will be gone.

But, by a wildly improbable linkage of random events, they are there. Sadiman had to blow out a particular kind of ash. Rain had to fall on it almost immediately. Hominids had to follow on the heels of the rain. The sun had to come out promptly and harden their footprints. Then another blast from Sadiman had to cover and preserve them before another obliterating shower came along.

All this had to happen over a period of only a few days. And the volcano had to synchronise its activity with that of the seasons. If its bursts had not come just when they did - at the beginning of the rains - the footprints would not have been preserved. A month or two earlier, during the dry season, the ash would not have had the consistency to take a sharp imprint. It would have been a hopelessly blurry one, a mere dent, like the one a passerby today makes in the dry sand on the upper margin of a beach. If it had come later, at the height of the rainy season, it is overwhelmingly likely that there would have' been too much rain; the footprints would have been washed away before they could have been baked hard by the sun. Indeed, there had to be just what the beginning of a rainy season produces: sporadic showers interspersed with intervals of hot sun.

All things considered, the preservation and recovery of the Laetoli footprints are nothing short of a miracle. They confirm without a shadow of a doubt what Lucy confirmed at Hadar: that hominids were fully erect walkers at three million B.C. and earlier. At Hadar the evidence is in the fossils, in the shape of leg and foot bones. But at Laetoli, where the fossil remains - some extremely scrappy and enigmatic postcranial bits, jaw parts, and some teeth - are of very poor quality, there is no way without the footprints of deducing how those hominids got around.

'Make no mistake about it.' says Tim. 'Thev are like modern footprints. If one were left in the sand of a California beach today, and a four-year-old were asked what it was, he would instantly say that somebody had walked there. He wouldn't be able to tell it from a hundred other prints on the beach, nor would you. The external morphology is the same. There is a well-shaped modern heel with a strong arch and a good ball of the foot in front of it. The big toe is straight in line. It doesn't stick out to the side like an ape toe, or like the big toe in so many drawings you see of australopithecines in books.

I don't mean to say that there may not have been some slight differences in the foot bones; that's to be expected. But to all intents and purposes, those Laetoli hominids walked like you and me, and not in a shuffling run, as so many people have claimed for so long. Owen Lovejoy deduced all that from studying the Hadar bones. Now the footprints prove him right. I think they rank with the most wonderful and illuminating discoveries in decades. Although it didn't end too happily for me, I'm still grateful that. I was lucky enough to have participated in the work on them.'

Twenty miles east of Laetoli, a volcano known as Sadiman erupted 3.65 million years ago. As the ashes settled to the ground, there was a series of rain showers.

Following one of these showers, a group of hominids walked in the Laetoli area. The ground was still wet and mushy, so they left their footprints behind them. When the sun came out and baked the ashes, these footprints were preserved, as hard as a fossil. Over the years, the winds brought in new sediments, and layers of earth covered many of these footprints. Some, however, were exposed by rains again recently. The first footprints to be found belonged to hares and to birds similar to guinea fowl.

While the KBS controversy was brewing in the mid-1970s, Mary Leakey devoted herself to exploring the Laetoli region. Much of her work at Olduvai had been completed, and she moved to this older site nearby in hopes of finding older stone tools. At the time, the oldest known stone tools were from the Omo, dated at 2.2 million years old. But Mary and her team had found older hominid fossils at the Laetoli sites, and the big question was, did these hominids make tools? As Mary has said, "In archaeology you almost never find what you set out to find."

The way the first footprints were found is as remarkable as it is funny. In September 1976, Mary welcomed a group of friends who had come down to Tanzania for a quick visit. The group included Kay Behrensmeyer, Andrew Hill, a paleontologist who worked for the Nairobi Museum, and Jonah Western [think she means David Western], an ecologist who worked at Kenya's Amboseli National Park. Mary was showing them the sites one day, and as they walked back to camp in the evening. Hill and Western began to toss elephant dung at each other. Elephant dung, when dried, is not offensive at all; it doesn't smell; it's just like a big cake of dried grass. Western buried a big piece at Hill, who ducked and fell on the ground.

Hill noticed some interesting-looking imprints in a flat gray surface. The first little dents he saw were later identified as raindrop prints. But it made him look around, and he found footprints of hares, birds and rhinos. Since then literally tens of thousands of footprints have been found in what became known as Site A, ranging from the tiny tracks left by insects to the massive depressions left by elephants.

Two years later a footprint was found that looked like the mark of a human heel. Excavations began, and it turned outthat the path of footprints went for over 20 yards. It looked as if two different hominids had walked there. Mary Leakey thinks they didn't walk at the same time. The path of one individual was very clear and distinct, while the other set of footprints was blurred and not as sharp. It appeared that they may have been impressed upon two different layers of ash which fell at different times.

Mary Leakey describes the Laetoli footprints as "perhaps the most remarkable finds I have made in my whole career." Because the footprints were so humanlike, she felt they could only have been left by one of our direct ancestors.

What do these footprints tell us about our ancestors? That they walked upright 3.65 million years ago. The shape of the foot is very similar to our own. Because no tools were found in this same level, these hominids must have walked upright before they used tools.

In addition to the footprints, Mary's team found part of a child's skeleton and several fossil remains of adults�two lower jaws, part of an upper jaw and a number of teeth. The best specimen of the lower jaw was called LH 4, for Laetoli Hominid number four. It would become a huge bone of contention.

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,