The Sediments of Time

by Meave Leakey

  All the evidence pointed to an aquatic habitat heavily influenced by changes in its water system. All the Turkana Basin fossil sites are associated with an ancient water course. Without the water, there would be no sedimentation—and therefore no fossil record. But something bigger and more interesting seemed to be afoot at Lothagam. A pattern was unfolding and repeating itself across the main aquatic groups. We found archaic species, often making their last appearance in the Nawata, living alongside modern species that were making their debut and becoming more common in the later sediments. Kathy found that all the small-sized fish she recovered from the Nawata Formation were archaic species, whereas these archaic elements disappeared or were rare in the Apak Member; extant genera predominated there instead. The side-necked turtle that was using Lothagam as a nesting ground is another case in point. This curious-looking creature belongs to a lineage of side-necked turtles that is represented today by only one species on Madagascar, and until the Lothagam fossils turned up, nothing at all was known about its ancestry. Turkanemys pattersoni represents the last species we know of from this lineage on the African continent, and it lived alongside the earliest-known fossil of the modern soft-shelled turtle, Cycloderma frenatum. A second extinct soft-shelled turtle is also known only from Lothagam.

  The pattern repeats itself with crocodiles. The only crocodile alive today in Lake Turkana is the Nile crocodile, Crocodylus niloticus, and we found the earliest-known record of this species at Lothagam, which was virtually indistinguishable from its modern descendants. The other modern crocodile that persists today only in Lake Tanganyika, Crocodylus cataphractus, also turns up for the first time at Lothagam. These two crocodiles lived alongside now extinct species, the long-snouted Euthecodon crocodile, the giant, broad-nosed crocodile whose length we estimated using Samira’s ratios, and a new species of gavial, Eogavialis andrewsi. Gavials are found today only in India and Pakistan, and the Lothagam gavial represents a new geographic record for these crocodilians that were previously unknown in East Africa.

  The bountiful hippo fossils also told an intriguing story. Modern hippos belong to two genera, both found only in Africa. Hippopotamus amphibius is the scientific name for the widely occurring sub-Saharan hippo. Choeropsis liberiensis is a forest-dwelling pygmy hippo now confined to the lowland forests of West Africa from Guinea to the Ivory Coast. In the common modern hippo, the eyes, ears, and nose are positioned high on the head, so when the hippo is submerged, they protrude just above the water line. It is this morphology that allows Hippopotamus amphibius to still be able to see, hear, and smell above the water. The most commonly occurring hippo in Lothagam is Archaeopotamus harvardi. This creature shows the beginning of the modern configuration: although positioned close to the top of the skull, its ear, eye, and nasal sockets are not as high as in modern hippos. A second much rarer and smaller hippo of the same genus, Archaeopota­mus lothagamensis, also lived during the Nawata interval. The ancestral Archaeopotamus lineage eventually became extinct and was gradually replaced by a hippo that has long been thought to be the earliest known species of the modern genus: Hippopotamus protamphibius, which was first found at Lothagam in the Apak Member.

  A massive change was clearly underway that caused a wave of extinction and a generation of new species. The turnover was not limited to a particular group of animals but repeats itself again and again. Why was this? What did such a change signify? We needed to systematically study all the groups of animals we were finding to see how widely the pattern was recurring and figure out what was driving it. The teeth from the Archaeopotamus harvardi specimens seemed to indicate that its diet might have been changing over time because more recent specimens had progressively smaller incisors. Might this be a clue? An even more intriguing question then arose—was this change caused by a local event, such as localised tectonic activity, or was some larger, more global change underway five to six million years ago? If this was global, then could it be the same change that led an early ape to habitually walk upright rather than on all fours?

  Working at Lothagam with its logistical and geological complexities, and at a time of personal worries because of Richard’s new political life, was daunting. Yet it was also exhilarating. Our discoveries proved to be critical to understanding the environment in which our early bipedal ancestors lived and when major upheavals in the environment of Turkana created opportunities for new species to evolve. With this new understanding, I could now look again at our maps of fossil exposures and plan our next step in search of the hominins themselves.

  6

  A Brave New World

  From the outset, it was abundantly clear that I needed reinforcements at Lothagam. New fossils were pouring in. We kept running out of plaster of paris to encase the fragile bones for their journey back to Nairobi, and were it not for Richard’s visits to camp when he invariably departed in a considerably heavier aircraft than he arrived in, I honestly don’t know how we would have safely transported them all back. The wealth of knowledge in these bones was astounding, and I needed help from specialists to sift through it all.

  This was a distinction of scale rather than scope: from Richard’s very first expedition in 1968 when he led the Kenyan team during the trinational exploration of the Omo region of Ethiopia, we had always benefited hugely from close collaboration with other scientists. But the possible roads of enquiry from Lothagam seemed almost limitless. Eventually, this translated into a decade of rigorous, collaborative research with experts in a wide array of subjects. I derived great satisfaction from the fruits of this labour—a big fat monograph of our findings inspired by (and, I like to think, fully worthy of) Mary Leakey’s exacting standards in her own monographs of her work at Olduvai Gorge and Laetoli, the site where she found the world’s most famous set of footprints.

  Kathy Stewart first joined us in our second season in 1991 when she and I spent so many memorable hours battling the wind and the flies as we sifted through our bone fragments, and she returned each subsequent year in the field. Eleanor Weston, a PhD student at the time, first came to Lothagam in 1992 to study hippos. From the fossils laid out in the wooden trays under the shade of the mess tent, Kathy and Eleanor noticed the patterns emerging in the fish and hippo fossils early on. Other experts later scrutinised different aspects of the collection back at the lab in Nairobi. What we really wanted to know was: is the change so strikingly underway in the aquatic realm evident among the terrestrial mammals too? If so and if this proved to be a global rather than a local climatic event, it could point to the impetus behind our own evolution into bipedalism.

  John Harris, a soft-spoken, hardworking, and careful scientist with a perpetual nervous energy that I suspect was at least partly fuelled by his fondness for endless cups of strong black coffee, joined us in these studies. He had worked with us from the very beginning—starting at Koobi Fora in 1968, where he later fell in love with my sister, whom he subsequently married. He and Judy are no longer together, but our scientific relationship endured. He worked with us all over the Turkana Basin and knows the fauna intimately, so he was ideally suited to the task. I had to work hard to persuade John to compile the Lothagam monograph with me, however; having helped Mary edit the Laetoli volume, as well as two of the volumes on our research at East Turkana, he knew precisely how much work was involved and had vowed never to do this kind of thing ever again. It was fortunate for me that he changed his mind. John not only studied the fauna and coauthored many of the chapters but his previous editorial experience was also indispensable as my coeditor.

  John and I took on the pigs together, and he tackled his specialty, the bovids, along with some of the other herbivores. I shared the monkeys with Mark Teaford, an expert in the interpretation of primate dental wear and diets, and Carol Ward, who had worked with me previously in studying monkey skeletons. Sure enough, all these mammal groupings seemed to fit into a pattern. Species that were common at earlier Miocene sites took their curtain call at Lothagam. And another telling denomin
ator was also soon apparent: there were key differences in the teeth of the ancient and modern groupings. Through time, individual teeth of many of the herbivores became more complex with additional cusps, higher crowns, and more intricate enamel patterns. These adaptations all increase the resistance of the tooth to wear by abrasion, so they are typically found in mammals that eat grass. This is because grass contains minute silicon capsules called phytoliths. Phytoliths are extremely abrasive and wear teeth down with ruthless efficiency. Once an animal’s teeth are worn out, it can no longer feed. So for grazers, adaptations that increase the longevity of the tooth are very important for the reproductive success and long-term survival of the species.

  Now that we knew what we were looking for, it was obvious. Among the mammals, all the primitive representatives tended to be browsers feeding on leaves: a primitive elephant (Stegotetrabelodon), a hornless rhino (Brachypotherium), primitive browsing antelopes (Tragoportax), and a primitive giraffid (Palaeotragus). Living alongside these ancient creatures were much more modern-looking herbivores—most with teeth suited to eating grass—which appear in the fossil record for the very first time at Lothagam. The “moderns” are all close relatives of mammals familiar today on the East African plains. They included early elephants as well as the ancestors of modern rhinos, giraffes, and pigs.

  The pigs (Suidae) gave a textbook illustration of an evolutionary trend in dentition towards increasingly complex, high-crowned molars. Several lineages of suids replace one another serially through the ages in the Turkana Basin, and pig teeth are so distinctive that they are excellent indicators of the age of newly discovered sites before they can be dated by more precise methods. In the various species within each lineage, the length and height of the third molars increase significantly with a corresponding reduction in the size of the premolars. At Lothagam, changes in pig teeth were particularly clear in the different species of Nyanzachoerus. The earlier species, Nyanzachoerus syrticus, is the most common in the Nawata Formation. Through time, its third molars become increasingly more complex, adding extra cusps to the back of the teeth and increasing the height of the crowns. In the later Apak Member, the common pig is Nyanzachoerus australis. This species has even longer, more complex, and higher-crowned third molars than N. syrticus.

  Bryan Patterson found many mandibles of N. syrticus, both male and female. His team also found a complete and beautifully preserved male skull with enormous knobs on either side of its head and strong ridges along the top of the skull. The female skull remained elusive, however, and I was always very curious about whether the female skull would resemble the ornate male one. One day, the field crew told me that I should look at a large pig skull sitting on the exposures. Other than the complete hominin skull Richard and I found on our camel survey in 1969, this was the only time I ever saw a complete skull of a large mammal totally exposed on the surface with little damage to the bone. A female N. syrticus skull was lying on the rocky outcrop as if someone had recently put it there for us to find. And it lacked the striking decoration of the males.

  These two key insights from Lothagam—that ancient animal species were being replaced by the ancestors of modern ones around five million years ago and that in many mammals this meant a shift from browsing to grazing, and thus the evolution of taller and more complex teeth that take longer to wear down—are beautifully reflected in the evolution of elephants. Pascal Tassy, a charming, soft-spoken Frenchman and the best expert to be found on elephant evolution, helped us to untangle the elephant story at Lothagam. Elephants have evolved their own elaborate system of tooth replacement, which uniquely solves the problem of how to prolong the longevity of their teeth against the daily grind of tough fibrous vegetation. Modern elephants have a single molar in each quadrant of the mouth that is composed of a number of parallel plates. After the eruption of the first tiny molar before the elephant is born, these teeth are replaced five times during its lifetime, each replacement tooth being larger than the previous one and with more plates, in a process known as serial replacement. The teeth erupt at the back of the jaw and grow forward as the preceding tooth is worn down and plates break off at the front—so instead of wearing down all their teeth as they grow and age, elephants wear one tooth at a time, which lengthens their chewing lives. The evolution of this ingenious method of tooth replacement can be partially traced at Lothagam—and it evolved twice, an example of parallel evolution down two different families of elephantoids responding to the same adaptive pressures.

  At Lothagam, the evolution of proboscideans towards serial tooth replacement is clearly documented. This solution allowed for a change in diet towards tougher fibrous vegetation to meet the challenges of a changing habitat during the Miocene and ensured their survival.

  The story of elephant evolution begins long before the first sediments were deposited at Lothagam, and it is easiest to conceive in the form of a family tree like the one you might find in the front of old family Bibles. Ancestral proboscideans—the group to which modern elephants belong—evolved fifty-five million years ago as part of the great radiation of mammals that followed the extinction of the dinosaurs. These ancestral proboscidiens begat palaeomastodons, which in turn begat the first gomphotheres in Africa some twenty-four million years ago. Gomphotheres are an ancient group of proboscideans that had elongated jaws with primitive, thickly enameled teeth and a cumbersome arrangement of two tusks in each jaw. They are significant in our story because they are the first proboscideans to show a clear enlargement of the molars and the beginnings of an evolutionary adaptation that resulted in modern elephants.

  Gomphotheres thrived in the Early and Middle Miocene (twenty-­three to ten million years ago). Up until then, the most recent East African fossil (Choerolophodon ngorora) was thought to be from this time. Thus, to my very great astonishment, Pascal Tassy one day showed me a single very primitive and worn lower molar of a “trilophodont” (meaning “three lophs,” the cross plates that make up elephantoid teeth) gomphothere from the Apak Member at Lothagam. This new Apak Member tooth is about five million years younger than the Early and Middle Miocene. It is now the very last known occurrence of a trilophodont gomphothere in East Africa and adds to the evidence of the late survivorship of ancient species in the Turkana Basin.

  But this ancient gomphothere was not the only one of its kind at Lothagam. It was already competing with no less than four other proboscidean species. By far the most common elephantid at Lothagam was the primitive-looking Stegotetrabelodon. This curious creature had cumbersome tusks both in its upper and lower jaw like the gomphotheres, but its thickly enameled teeth seem to show the beginning of serial tooth replacement. The second group are the resilient deinotheres. These rather uncommon elephant-sized proboscideans had tusks in their lower jaws instead of their upper jaws and did not have serial replacement of their much simpler teeth. They are the exception in the Late Miocene elephant world and remained browsers throughout their evolution. But the real evolutionary challengers were the other two groups that had already evolved—in parallel—more efficient grazing teeth through serial replacement.

  The first of these is a proboscidean called Anancus. This creature had teeth with very thick enamel and fewer tooth lophs than modern elephants but is otherwise very similar to them: the mandible is similarly shaped, and the teeth serially replace over the animal’s life. The fact that Anancus, like modern elephants, also evolved a means of living longer by wearing down its teeth one at a time at the end of the Miocene shows how much grittier and tougher the diet of elephants at the time had become.

  But the biggest discovery was yet to come. One day, I was lucky enough to find the earliest known specimen of an ancestor of modern elephants—known as Palaeoloxodon. I climbed a small peak towards the western side of Lothagam, where I came across the lower jaw of a young elephant partially exposed on the surface. I could see that it had been eroding from the sediments for some time as there were hundreds of broken fragments lying nearby. I was surprised t
hat this jaw had teeth closely resembling those of a modern elephant. It had big molars with many plates in a short jaw showing obvious serial replacement. But it was also clearly from the Upper Nawata sediments. Over the following days, I carefully reconstructed as many of the fragments as I could before excavating the hidden parts. This mandible would turn out to be hugely significant because it showed that the modern group of elephants evolved in the Late Miocene more than 5.5 million years ago.

  Lothagam has the youngest gomphothere, an ancient form evolving serial tooth replacement (Stegotetrabelodon), and two new lineages—Anancus and the oldest true elephant (Palaeoloxodon)—that had independently evolved teeth that allowed them to graze effectively. Amazingly, the precursor to the modern African elephant, Loxodonta, with its distinctive “lozenge” shape of the lophs on its teeth, makes its first appearance in the Apak Member—making Lothagam the site with the greatest diversity of known proboscideans. With the exception of the ecologically conservative and rare deinotheres, the diverse species of proboscideans at Lothagam had all found the same way to maintain their large bodies on a diet of abrasive grass. Quite independently, they evolved hard-wearing and serially replacing teeth. Once again, we were seeing the disappearance and replacement of primitive species by the earliest species of modern genera as well as the temporal evolution of increasingly high-crowned molars, thinner enamel, and complex occlusal enamel patterns that dramatically increase the cutting surface of each tooth.

  But why were we seeing such a significant ecological shift with clear, widespread adaptations to grass eating instead of browsing? What happened to herald the end for so many Miocene mammals and usher in a brand-new era? And could the early evolution of our hominin ancestors be driven by the same adaptive pressures?