The Sediments of Time

by Meave Leakey

  These are the two defining characteristics—bipedalism and small canines—that can be argued about as defining a hominin versus an ape. Yet, like the ring species of herring gulls I mentioned in the last chapter, the line between the early differentiating ape and hominin lineages must surely be an arbitrary one. Nevertheless, the scientists who discovered these key specimens emphasize the hominin attributes, perhaps thinking that this would somehow be more interesting. Yet an ape at the threshold of this momentous split would tell us what the ancestral condition actually was. How egocentric of us humans to assume that while we have undergone sweeping evolutionary changes the apes have muddled along without changing much at all!

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  AMONG THE CONTENDERS of would-be earliest hominins is Ardi­pithecus kadabba, an older species presumed to be an ancestor of Ar. ramidus. This was found by the same Yohannes Haile-Selassie who recently published the cranium of A. anamensis from Ethiopia. What is most interesting about Ar. kadabba, dated to between 5.8 and 5.6 million years, is that it has a partially honing canine that self-sharpens against a thickly enameled lower premolar. Nevertheless, the cross section and shape of the upper canine is closer to that of a hominin than an ape. It would seem to be a transitional species, but there are frustratingly few bones and individuals represented in the sample.

  A similar-aged fossil, called Orrorin tugenensis, was found by Brigitte Senut and Martin Pickford in the Tugen Hills near Lake Baringo, Kenya. The tragedy of this potentially fascinating find, nicknamed Millennium Man, is that few scientists have had the opportunity to study or even set eyes on the fossils. They are said to be locked away in a bank vault in contravention of Kenya’s antiquity laws. Nevertheless, from the published descriptions, Orrorin would appear to have had fairly primitive teeth. It has small molars, and its front teeth are similar to those of a modern female chimpanzee’s. However, its enamel is as thick as it is in hominins—perhaps another example of transitionary characteristics. A further interesting aspect of Millennium Man is its partial femur. From the shape of this femur, Senut and Pickford initially concluded that it walked bipedally. More recently, two other scientists have had the rare opportunity to study the femur. They found that it resembles that of australopithecines, pointing to the likelihood of Millennium Man being bipedal. Ar. kadabba also has a partial femur and a single toe bone, the shape of which led Haile-Selassie to reach the same conclusion.

  The most riveting of these early discoveries that have been published is from Chad, where the French palaeontologist Michel Brunet found an even earlier, beautifully preserved, and remarkably complete skull and mandible from two different individuals of another half-ape, half-manlike creature. This find is all the more spectacular for the extreme conditions that Michel and his team had to operate in. The blasting hot winds and searing temperatures of Chad’s Djurab Desert make Turkana seem like a positively mild working environment in comparison. Brunet came to Lamu with his then-wife Agnès to spend Christmas with us shortly after his sensational find in 2001. They brought with them a cast replica of the skull and a laptop loaded with digital images of their fieldwork. For several evenings, we were regaled with astonishing stories about their camp life. Brunet had to fly all his water (along with his wine!) to his remote camp, relying heavily on the French army to resupply him. Sediments laid down along the lush, well-vegetated shores of ancient Lake Chad now lie below a moving sea of sand dunes, which are propelled over the underlying bedrock by the force of the wind. Brunet’s sites are ephemeral, here one year and gone the next when they are reburied beneath a mountain of sand—decreasing the chances of finding such a stunning fossil a thousandfold.

  Because there are no datable layers in the sandy desert, the only way Brunet could date his fossil skull and mandible was to infer a date based on the fossils of the fauna that lived both in Chad and somewhere else where their age is known. This puts his fossils reliably between six and seven million years old, pushing the evidence for the date of the split between humans and apes further back. Lothagam was one of the sites where Brunet found similar fauna—so we were looking at the right age ourselves. Since then, Brunet and his team have applied a new method for dating to the material from Chad—cosmogenic nuclide dating—and the age they obtained (between 6.8 and 7.2 million years) is the same as they had predicted based on the fauna. Brunet’s find is still more unique for its location far to the west of the Great Rift Valley. It serves as an important reminder that our ancestors were spread over large parts of Africa.

  Since the initial discovery, Brunet’s team has also found additional pieces of lower jaws and several isolated teeth. The well-preserved skull has several large cracks that have caused some distortion, but CT (computer tomography) scanning technology has enabled the research team to reconstruct its original form. The results are astonishing. It had a tiny brain—the smallest capacity of any adult hominin, yet it didn’t look anything like an ape. Most noticeably, it did not have the long snout that early australopithecines share with apes. The canines were rather small, and they were nonhoning, a surprisingly “modern” character for a hominin that age, particularly when considering that it is much older than Ar. kadabba, which has partially honing canines. And the forward position of the foramen magnum and the orientation of the eye sockets also follow the requirements for bipedality and differ from that of apes. So Brunet concluded his fossils belonged to a biped and a hominin. That said, the incompleteness of the other specimens he found meant that comparisons could only be made of the teeth. These are sufficiently different that Brunet named both a new genus and a new species for his fossils, which he called Sahelanthropus tchadensis.

  It is a real stretch, even with the fragmentary nature of Orrorin tugenensis and Ar. kadabba, to force these three—from widely different geographic areas and different features—into a single evolving lineage. It involves bunching thick and thin enamel and honing and nonhoning canines, for starters. It is far more likely that this is the evidence of early hominin diversity we have all been looking for. Any one of these, or an as yet undiscovered creature, could have been the stem species that evolved into the australopithecine line. While a tantalizing picture of a chronologically, geographically, and morphologically diverse set of potential ancestors for Australopithecus anamensis begins to form, the question of who its descendants were also remains unanswered.

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  THE TREND that we begin to see in the anamensis-afarensis line could be expected to continue in later species since the climate continues to dry over the next few million years. The most obvious of these is one that has to be related to diet—in response to the increasingly abrasive and tougher food found in open-country vegetation, the molars and premolars steadily increase in size, the enamel becomes thicker, and the incisors and canines reduce in size. Correspondingly, the skull morphology changes to accommodate the attachment of bigger muscles for more energetic and prolonged chewing as well as for biting with greater force. Since the point of having a long snout is to be able to bare the impressive canines by opening the mouth as wide as possible, the constraints on the size of the snout disappear if you no longer have large canines to display. If your diet involves a prodigious amount of forceful chewing of tough foods, a shorter snout provides a better platform for large molar teeth and strong muscles. As we would expect, the snout gets shorter in more recent australopithecines with less need of a display function.

  This means the beginning of thicker enameled and larger chewing teeth (which scientists poetically call “megadonts”) and bigger and better surfaces to attach muscles to on the skull ought to become more pronounced through time. That other hallmark of the hominin line, manual dexterity, which had already begun in anamensis, could also be expected to become more manifest in progressively younger hominins. These trends are indeed apparent in some later hominins—but other than manual dexterity, they are notably absent in our own Homo line. Did we really arise from the australopithecines? The jury is still very much hung on where the aust
ralopithecine lineage leads. But before delving into the twinned problem of their evolutionary destiny and the origins of our own genus, we need to consider the important insights from the hominins that existed after A. anamensis.

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  THERE IS A GRATIFYINGLY large sample of another well-known australopithecine. This species includes the very first hominin fossils ever found in Africa. Serendipity led to this first discovery from limestone cave deposits in the Transvaal. A student of Raymond Dart’s had previously brought him a skull of a fossil baboon blasted from the Buxton Limeworks near Taung, which had piqued his curiosity, and Dart had requested any further fossils the owner of the quarry might find. Nestled in the jumble of limestone rocks and fossils that were subsequently delivered, Dart recognised the cast of a perfectly preserved brain and also found the skull that had enclosed the brain, and he would spend many months chipping away at the limestone matrix encasing it until he was able to reveal an almost perfectly preserved skull and mandible of a young child. Dart, an anatomist by training, was uniquely qualified to recognise the “convolutions and furrows of the brain” complete with the blood vessels of the skull. He had looked at enough human brains to see the unmistakably human attributes of what lay before him, and he was broadminded enough to see the implications. Too many differences separated the Taung Child from the baboon he had expected to find. Most telling of all was its diminutive humanlike milk canines and the position of the foramen magnum, which was far enough forward to suggest bipedal locomotion.

  One problem was that the Taung Child was discovered in 1924, a time when many in the palaeontological establishment accepted as a matter of course the alleged supremacy of Europeans over other races. The fossils met with an extremely frosty reception when Dart hastily penned off a paper to Nature in which he enthusiastically announced his ape-man from Africa, Australopithecus africanus. For the palaeontological community, the very notion that human origins arose in Africa was anathema. Because of the prejudiced insistence that humans must have originated in Europe and the prevailing view that our large brain evolved before other adaptations such as bipedalism, Dart’s tremendous small-brained African discovery was ridiculed and dismissed as merely some kind of juvenile ape.

  Many more finds followed, including an adult female skull, nicknamed Mrs. Ples, found twelve years after the Taung Child by Robert Broom, a maverick Scot who contributed much to the discovery and interpretation of the early South African hominin collections. But it wasn’t until several decades later that Dart’s discovery and interpretation were at last accepted.

  A. africanus is a slightly built creature that in many ways resembles A. afarensis. Along with its small canines, it has megadont cheek teeth that, like A. afarensis, have thick enamel and are enlarged from the primitive condition. It was most certainly a biped, but until now, nothing could be said about how dexterous its hands were. This was about to change—for one of the greatest discoveries ever made will include the very bones we needed. This sample of africanus fossils was retrieved from the Silberberg Grotto, a cavern deep in the Sterkfontein cave system in Krugersdorp. They included a collection of foot bones belonging to an adult individual. But as so often happens in palaeontology, it turned out not to be quite that simple!

  In 1978, Phillip Tobias, an eminent and highly respected palaeoanthropologist from the University of Witwatersrand, asked his field director, Alun Hughes, to look in the older and lower levels that are exposed in the Silberberg Grotto. Many of the fossils recovered from these caves were retrieved from lumps of limestone called breccias, which were blasted out by miners using explosives. They raised the blocks of breccia and rubble from the cave floor and extracted predominantly monkey fossils. These bones were stored in labelled boxes in the university’s anatomy department and a work shed at Sterkfontein. Nearly two decades later, Ronald Clarke was puzzling over the bones at the University of Witwatersrand, where he works in the anatomy department. Ron is an old friend from when he worked at the Nairobi National Museum for Mary and Louis as an extremely skilled preparator, cleaning many of the Olduvai fossils and trying to stick the thousands of pieces of hominin fragments back together. Among the Silberberg fossils, Ron had noticed a surprising absence of antelope bones. He was looking for the missing bovids in the boxes in the Sterkfontein work shed one day when, to his astonishment, he noticed several bones belonging to a foot that was unmistakably hominin. These beautifully preserved and hitherto discarded fossils represented the first known A. africanus foot. Because of its small size, it soon became known as Little Foot.

  In a completely separate coincidence, Ron was later looking for some fragments of a different hominin skull in the safe in the anatomy department when he noticed another box labelled as containing monkeys from the Silberberg Grotto. Seeing this, he immediately took the box with him to have a closer look and was rewarded with more foot bones that surely belonged to Little Foot. A fragment from the lower end of the tibia was also in the box, and this discovery spurred him to scrutinise the box’s contents further. Sure enough, in a nondescript bag labelled BOVID TIBIA, Ron found a much more complete piece of hominin tibia that articulated perfectly with the left talus (the anklebone), one of the original four foot bones.

  Ron next tried a literal shot in the dark. He dispatched two of his men with a torch to the grotto, tibia in hand. The cave walls and lumps of breccias are all studded with numerous protruding pieces of broken-off bone from the miners’ blasting some sixty-five years earlier. After only two days of searching, they were able to match the broken-off surface of the left tibia to a piece of bone embedded in the breccias in the cave. And another break next to it matched up with the piece of the right tibia. The odds were that there could be far more of Little Foot’s skeleton concealed in the breccias, and Ron has recently disclosed that this remarkable specimen now includes almost the entire skeleton, including a full set of hand bones. This has to be the most complete early hominin skeleton ever to be discovered, and its hand will surely shed light on the crucial question of how manual dexterity evolved.

  But major controversies surround Little Foot. One relates to its age, the other to its identity among australopithecines. A big problem with the fantastic fossils emerging from South Africa is that they have all been found in limestone caves. Caves present notorious dating problems because the sedimentary layers get jumbled up as water erosion causes parts of the cave to fall in and disrupt the order of the sequence by mixing older bones with newer ones. Both stratigraphic dating methods and faunal comparisons are subject to a large degree of uncertainty—so large that two different initial interpretations put Little Foot at 1.07 million years old and 3.3 million years old. Since then, Ron and his team have applied the same novel dating technique that Brunet used to date the Sahelanthropus site—and proposed that the fossil dates to 3.67 million years ago. This makes it the same age as Lucy and only slightly younger than anamensis and, most important, significantly older than the Australopithecus africanus fossils found in other parts of the same cave at Sterkfontein that date between three and two million years ago. Most intriguing, Little Foot shows many primitive features that have led Ron Clarke to give it a new species name, Australopithecus prometheus, with possible closer affinities to the anamensis-afarensis fossils from East Africa than later South African remains. These proposals remain contentious, and more research is needed before we can establish with certainty how these South African hominins relate to their East African cousins.

  These dating difficulties have also been the predominant challenge in sorting out how the different hominins from South Africa relate to one another. Other types of australopithecine have been found in other caves in the Transvaal—at Kromdraai, Swartkrans, Drimolen, and Malapa. One of these is relatively unique. While clearly an australopithecine, it shows a number of derived features that buck many of the trends observed in other australopithecines. Its name is Australopithecus sediba, and it was discovered by Lee Berger in the form of six partial skeletons at Malapa.
The fossils, which include adults and infants, are two million years old and offer us insight into the range of adaptive opportunities hominins were facing at that time. Particularly interesting among these is the hand of A. sediba, which with its short fingers and long thumb suggests a precision grip. However, its wristbones, stunningly complete in one of the two adult skeletons, show primitive features suggesting little rotation and flexibility—a true evolutionary mosaic of features.

  The other type of australopithecine has been known since the very early days of Broom and Dart, and is so different that most scientists now believe it should have its own genus. This creature is a true megadont: its molars are massive. Yet the incisors and canines aren’t nearly as impressive—and the male canine is so small that it would barely scare away a mouse! In keeping with the nondescript front teeth, the snoutlike face of earlier australopithecines has also been replaced with a dish-shaped, snub-nosed face—as though somebody forcefully punched the nose in. The reason for this effect is that the architecture of the skull is all about chewing. To power the huge molar teeth, the face has big flaring cheekbones that provide room for seriously big chewing muscles and give the punched-in look. Because the brain is still too small to allow much surface area to attach muscles to, the skull has a very distinctive and prominent crest running along the midline at the top—a sagittal crest. This extends the needed surface to attach the muscles to.