The Sediments of Time

by Meave Leakey

  According to the now-fabled legend, some visiting scientists were engaged in some rather unscientific slinging of elephant dung in an impromptu battle. Ducking down in a flat gully to replenish his arsenal of the large fibrous balls, palaeontologist Andrew Hill noticed numerous prints of birds and other animals preserved in an exposed layer of fossilised volcanic ash. The game forgotten, everyone began to look for evidence of prints. Having found hominin fossils in the area, Mary immediately put the whole team to work on the task, hoping to find evidence of human prints. The ash layer was exposed in eighteen different sites, and on the largest of these, Mary and geologist Richard Hay calculated that approximately 18,400 tracks crisscrossed the surface. They eventually identified twenty different species that ranged from an insect to an elephant. Even without hominin prints, the other tracks, combined with the fossil remains of many of the animals at Laetoli, represented a mass of new information.

  It was a long shot, but hominin prints were eventually found in 1978, surpassing Mary’s greatest hopes. After many weeks of excavation over two field expeditions, a trail of more than twenty-seven metres (eighty feet) was exposed. Two individuals walked across the ashen landscape side by side, with a third smaller one trailing closely behind and carefully placing each foot in one of the prints left by the individual in front. A serendipitous sequence of events preserved this mesmerizing snapshot. The simmering Sadiman, a volcano that flanks the Serengeti plains, first gently showered a layer of ash on the ground. Before the trio set off across the ash-covered surface, a light rain fell. Due to the unusual chemical composition of the ash, the added moisture gave the layer a consistency akin to that of wet concrete. Like concrete, this layer hardened as it dried, preserving the indentations (there are even slight impressions of raindrops recorded). Then a subsequent ashfall from the volcano, followed by more light rain, blanketed and sealed the prints, effectively safekeeping them for Mary’s team to uncover some 3.6 million years later. This trail vividly conjures an evocative, uncannily humanlike image—my own children would often amuse themselves the same way when Richard and I took them for a walk along the sandy lakeshore at Koobi Fora after a hard day’s work. The Laetoli footprints, set in stone in exquisite and graphic detail, put firmly to rest any lingering doubts that early hominins walked on two legs. Fossils, however complete, can only be interpreted. Here, the behaviour was actually recorded—and no one could argue with that. Bipedalism had been pushed back in time from Lucy’s age of 3.2 million years to 3.6 million years ago.

  In the first few years at Hadar, Don Johanson would come through Nairobi with his fossils on his way back to America and would stay with us in our home. For security reasons, the Nairobi National Museum technicians made casts of his specimens before he flew to America with the originals. While he waited, we would all eagerly compare his finds with our own from Koobi Fora and those from Tanzania, and spend many enjoyable hours discussing what they might mean. One hot topic was the question of just how many species all these fossils represented.

  Not only were the Laetoli bones thought to be some half a million years older than those from Hadar, but within the entire sample, there was considerable variation in the size and shape of some of the bones. Was this variation within a normal range for a sexually dimorphic species that had much larger males with some prominent defining features that were different from their female counterparts? Or were there two entirely different types of creatures?

  Mary invited Tim White to study the Laetoli material. Tim, now acclaimed as a meticulous scientist, was a student at the University of Michigan. Fast gaining a reputation that has endured for being intolerant of bad science, Tim was already outspoken and frank in conveying what he believed, although he was charming and a gentleman in less formal situations. In 1977, he and Don Johanson teamed up to study the Hadar and Laetoli fossils. They set about assessing just how different the individuals were, comparing them to a chimpanzee and a human, and taking key measurements on all the specimens to compare how much they differed from one another. Johanson and Taieb’s initial paper in Nature describing the Hadar finds from 1973 and 1974 had concluded that two or three species were represented at the site. But the detailed analysis by White and Johanson suggested that this was not so. Based on an exceptionally large sample of this human ancestor (which now, after more than forty years of fieldwork, numbers more than one hundred individuals) and after much thought and careful study, the two scientists concluded that the entire sample represented a single species. In 1978, they named this species A. afarensis (southern ape-man from the Afar). They attributed the large variation to a high degree of sexual dimorphism, with males being considerably larger than females.

  Johanson and White did not only conclude that this was a single species—they went further. Contending that this was the only species living at the time, and the earliest known biped, they also asserted that A. afarensis was the common ancestor of all later hominins. They boldly drew a family tree that depicted A. afarensis on the main trunk, with all other known hominins branching out from this stem species. Although the type specimen for A. afarensis hailed from Laetoli, Lucy came to represent this single ancestor, having attained a sort of celebrity status among fossils for her completeness and her endearing nickname. She would reign unchallenged for twenty-five years in this position until our discoveries from Kanapoi because there was little or no evidence to prove that she was not alone.

  However, there were divergent opinions as to the correct interpretation of these Hadar and Laetoli fossils. Mary, who had been the one to invite Tim White to study the Laetoli specimens, was dismayed by the two scientists’ choice to merge the hominins from both sites, and name the species by the Afar site but select the holotype (the specimen that best describes the attributes of the species) from Laetoli. This choice—to derive a name from one site and choose a holotype from another site—was unprecedented and taxonomically unacceptable to many. A great many miles and, it was believed at the time, nearly three-quarters of a million years separated the two sites (more accurate dates would later prove that the Hadar sediments were older than previously thought, narrowing the age gap between the two sites). Arguably, since Lucy was the most complete specimen known, she was a logical choice for the holotype as she allowed for an exceptionally detailed description. Johanson’s reasoning for countering this argument was that Lucy would be inappropriate for a holotype because she was not an average representative of the sample because she one of the smallest of the bunch. In contrast, LH-4 (Laetoli Hominin 4), the mandible they chose, was closer in features to a mathematically derived “average individual.”

  But Mary vehemently disagreed with Johanson and White’s decision to assign all the fossils to the australopithecine genus and a single species because she believed that her fossils should have been assigned to the genus Homo. She disassociated herself from the paper they submitted, and her previously close relationship with Tim White became strained. Meanwhile, Richard’s own relationship with the pair was also breaking down because Richard refused to accept the single-ancestor theory, saying only that Lucy may or may not turn out to be an ancestor. This was falsely interpreted by Don as a failure to acknowledge the enormous importance of his spectacular find. But Richard refused to be drawn into the fray, insisting that only the discovery of more fossils would provide the answers as to what Lucy really represented. The media made much of the disagreements, painting in highly colourful, personal terms what was, at least to us, a professional difference of opinion.

  The controversy about whether these specimens should be attributed to the genus Australopithecus has since largely been settled, although how to best interpret the huge amount of variation in the sample remains controversial. A partial 3.6-million-year-old skeleton discovered in 2005 at Woranso-Mille and nicknamed Kadanuumuu (“big man” in the Afar language) has stretched the range of variation in afarensis even further to encompass individuals who have gorilla and humanlike shoulder blades and may be more than two feet tal
ler than one another. But, independent of these arguments, I have always disagreed with Johanson and White’s other central assertion, namely that A. afarensis is a common ancestor for all later hominins. When seen in context of other mammalian species and the generally accepted evolutionary theory, Lucy should not have been alone. It just didn’t make sense.

  After the appearance of a major new adaptive feature such as bipedality, a radiation of species would be expected to have followed with a plethora of different hominins evolving to fill the new feeding niches that bipedality opened up. The evidence from Lothagam had pointed overwhelmingly to huge habitat change—a strong impetus for just such a dramatic evolutionary stride. In this context, 3.2 million years is relatively recent—if our ancestors split from the ape lineage approximately six million years ago, then why was there no radiation of hominins for three million years after this initial divergence that corresponded to the change in climate? Across the animal kingdom, no other newly emerged lineage with a comparable record of three million years of unilinear evolution after the appearance of a significant novel trait is known.

  All this was very much at the forefront of my mind as I looked at the Kanapoi material and revisited the casts of bones from Hadar and Laetoli. Although larger, our tibia was similar in shape to Lucy’s and almost identical to the larger tibiae from Hadar. The Kanapoi tibia presented incontrovertible evidence of full bipedality more than four million years ago, pushing the date back in time more than half a million years from when the Laetoli trio stepped out across the ash-covered plain. Bipedality is the hallmark human trait that set hominins on a separate trajectory from apes. No doubt about it, Kamoya’s tibia was the most significant find we’d had in a number of years.

  But little else could be told from this isolated piece of tibia and Wambua’s old, worn maxilla. The new ancestor remained an enigma. What did our mystery hominin eat? The teeth were simply too worn to tell us much about its diet. And what did it look like? Although we knew it was definitely bipedal, how dexterous were its hands? Was it A. afarensis or one of Lucy’s ancestors? Perhaps it was an ancestor of an altogether different lineage, which would demonstrate the diversity at that time that I had long suspected but not been able to prove? We desperately needed more fossils to tell us about this creature, and the team set about finding them with even more determination than ever.

  * * *

  1994 WAS A GOOD YEAR for us. The great drought that had parched the landscape and cloaked it in dust when we worked at Lothagam was replaced that year at Kanapoi by a beautiful feather-light sprinkling of lush green that slowly turned to yellow gold as the rains faded. On some days, the heavens opened, flooding the river where we were camped and grounding us. The normally hard exposures softened like putty, and walking across them would risk snapping fragile fossils as our feet sank in the mud. With the overcast sky hanging low and pewter, we used these days to transfer our field catalogue of fossils to the computer. The crew caught up on laundry and their correspondence home, and enjoyed a well-earned rest while I waited impatiently for the ground to dry out enough to safely walk on the exposures again.

  Even better for us, our hominin drought also ended. Soon after Wambua found his maxilla, Nzube found three beautiful unworn and isolated hominin teeth: an incisor, a premolar, and a huge canine. They were in pristine condition and must have come from a really large individual. This latest find further infused the team with competitiveness, and the mood in camp was very upbeat in spite of the fact that each of these finds added another bout of seemingly endless sieving. This particular sieve was especially rewarding and produced all the missing teeth. The result was a hardly worn, beautifully preserved, and astonishingly complete lower dentition.

  Not everything turned out to be what we hoped, however. On July 28, which happens to be my birthday, Wambua found what initially looked like a marvelous birthday present, a potential hominin skull. But something about it wasn’t quite right. As I excavated this fragile specimen, I thought at one moment that it was a hominin, and the next that it was not. Finally, we lifted it out and found that it was even more strange-looking underneath. I had no idea what it could be, and Kamoya promptly nicknamed it the bahasha (“envelope” in Kiswahili) for its flattened appearance. The specimen had fallen out of the bank in some relatively recent rain, and as we sieved, we retrieved a number of fragments of tooth roots scattered among the stones and sand in the gulley, all with very fresh breaks. When we finally managed to extract it, we immediately saw that we had been misled. Our odd birthday find was an enormous knob from the skull of an ancient pig, which was used for display to attract a mate!

  Kamoya returned one day to the sieve site of his tibia and came back to camp in the late afternoon pleased as punch with the distal (lower) end of the bone. This second discovery made the find doubly important and raised high hopes of finding the middle section. To our enormous disappointment, we never did find this piece despite extensive sieving.

  Samuel Ngui, who had helped Kathy Stewart search for fish at Lothagam, also found his very first hominin at Kanapoi that year. Like Nzube, Ngui specialised in finding monkeys, which are often impossibly small and even harder to find than hominins. He spotted two matching pieces of a mandible from the left and right sides with the crowns of the two teeth in each fragment broken off. The following year, after six endless weeks of sieving this site, we had an almost complete beautiful lower jaw with all its teeth.

  * * *

  LATE IN THE 1994 field season, on September 10, Richard and some friends, including the photographer Bob Campbell who’d been with Louise that last field season in Lothagam, joined me in camp for a long weekend. Richard now came to the field very infrequently as he found the extreme heat and uneven terrain very difficult. Not only was getting around a chore, but without legs, his body could not stay cool in the high desert temperatures because of its decreased surface area. But he couldn’t have picked a better weekend to visit.

  Kamoya and I took our guests on a tour of the exposures, stopping first at Nzube’s sieve site so I could get Richard’s advice on how much farther to go. He agreed with my assessment: we could finish up for the season, but it would be worth our while to extend to the flanking slope the following year. We then proceeded to an elephant skull that we had just finished excavating. Richard had promised to oversee the delicate task of plastering and removing this in one piece. If not done correctly, the weight of such a large specimen can result in its falling apart, so I was much relieved to have his advice. Amid much banter and talk of the old days, we finished plastering the top half of the elephant and had turned it successfully on its side when I urged Nzube to join the sieving team. But he did not get very far. “Meave, come and see what I’ve found!” he cried with a smug, delighted grin. “You are not going to believe your eyes,” he added with great certainty. Although his expression usually gave him away, Nzube was usually much more taciturn and noncommittal in the way he spoke about his discoveries. Needing no further encouragement, I rushed after him, leaving a rather frustrated Richard sitting on the ground beside the elephant.

  It was indeed incredible. A beautiful mandible was concealed among the pebbles. All that could be seen on the surface was an almost complete set of teeth glittering like a string of jewels in the soft morning light. Nearby lay a temporal bone from the skull that included the external opening for the ear and the bone surface that articulates with the lower jaw. Three incisors were missing from the mandible, and the third molars were not exposed. But the skull fragment indicated that more might be there if we were lucky. We knelt and stared at it in disbelief. There is nothing quite like the thrill of being one of the very first to set eyes on an ancient link to our past, and this one was just exquisite. Unlike the maxilla Wambua had found, these teeth were immaculate and relatively unworn so we would be able to learn much from them. Bob snapped away busily with his cameras while I rushed back to tell Richard.

  We retreated to camp during the heat and glaring light of midda
y, elated by the tremendous discovery. In the afternoon, we returned to the site, and Richard excavated the mandible while Kamoya and I began to move the larger stones in preparation for sieving. For one blissful afternoon, with Richard by my side and the rest of the old gang working in companionable silence and the thrill of an important discovery humming in the air, it felt just like old times.

  * * *

  THIS STOLEN AFTERNOON was all the more precious because the new bent of Richard’s interests did not sit well with the government and I continued to worry for his safety. While he was in hospital in Nottingham, jealous colleagues took advantage of his absence by accusing him of mismanagement and corruption at KWS, just as they had done during his kidney transplant. Once again, an enquiry was held, and no evidence whatsoever of corruption was found. Amid mounting political pressure and interference upon his return, it became increasingly impossible for him to do the job as he wanted, and he resigned in early 1994. He was briefly reinstated, but a few months later, he was forced to resign again. His experience at KWS had led him to believe that while strong institutions could be built in Kenya they would only endure with a solid and more mature political foundation. He then threw himself wholeheartedly into politics, founding a new political party in May 1995 called Safina (“ark” in Kiswahili). But if Richard and his ardent supporters believed they could rescue Kenyan politics and sail off into a democratic future, the current government believed otherwise. The party members were thwarted at every turn, their licenses denied and their members detained. A year after we sat peacefully excavating Nzube’s hominin would find me back at Kanapoi sitting anxiously by a crackling radio listening to Richard warning me that I might hear of a “little trouble” he’d had in Nakuru.