Masters of the Planet

by Ian Tattersall

  The lower leg bone of Australopithecus anamensis, which was also reinforced at its knee end like ours, shows that this early hominid had acquired at least the basic prerequisites of efficient bipedal motion. In the upper limb, one wrist bone from Kenya also suggests that this structure was stiffer than that of apes, and more closely resembled the wrist of later hominids. In contrast, while the teeth of A. anamensis show general similarities to those of more recent Australopithecus, particularly in their thick enamel, in the large, broad premolars and molars, and in the lack of a premolar honing mechanism, in some respects they hark back to earlier times. Thus the incisor teeth are large, even if they are not quite in the league of the fruit-eating apes; the front lower premolar is pointy; and the tooth-rows are long and parallel. The lower jawline also retreats sharply from top to bottom, as in apes. Still, all in all, A. anamensis makes a pretty plausible primitive antecedent form for later Australopithecus, and it convincingly sidelines the only slightly earlier Ardipithecus ramidus as a potential direct ancestor of later hominids.

  Associated fossils suggest that Australopithecus anamensis typically lived in forest-to-bushland habitats near water, reinforcing the view that the initial adoption of bipedality was not achieved as part of a process of accommodation to encroaching grasslands. Indeed, even after early hominids had developed some important elements of the requisite anatomy, they did not preferentially seek open environments. To complete or at least to augment the picture, a finger bone from Ethiopia is elongated and strongly curved, signifying a powerful grasping hand; and it is believed that A. anamensis included agile climbing as part of its behavioral repertoire.

  When you place all this in the context of the larger environmental picture, it makes a lot of sense. At maybe 110 to 120 pounds, the average A. anamensis probably weighed a little bit more than a typical Ardipithecus; but while this is large for a tree-dweller, certainly enough to significantly reduce fears of predation in the arboreal milieu, these primates would have made a tasty morsel for the fearsome predators that prowled the woodlands. So it’s probable not only that A. anamensis would have sought much of its sustenance in the trees, but that its members would have routinely sought shelter in the branches at night, the time when they were at their most vulnerable.

  Although australopiths from later in time are clearly members of the hominid family, paleoanthropologists often like to describe them as “bipedal apes.” One reason for this is that australopiths combined humanlike specializations in the pelvis and legs for upright walking with apelike proportions of the skull. They had large, projecting faces hafted in front of tiny braincases, exactly the reverse of what you find in our own skulls, which have tiny faces tucked under the front of the huge globular vault containing the large brain. Another reason is that these early hominids retained characteristics of the forelimbs and torso that would have greatly aided in clambering around in trees. Bipedal they were; but in other respects they were far more apelike than human. Nothing we know of the earlier Australopithecus anamensis impedes applying the same description to this form, which is easily fitted into the beginning of the hominid story. Indeed, some authorities point to evidence suggesting that A. anamensis was gradually transformed into the successor species Australopithecus afarensis, which we will meet in a moment. They are braver than I am; but it is certainly fair to say that in A. anamensis we appear to have a worthy precursor for the best-known of all the australopiths, A. afarensis.

  The skeleton of the human leg has accommodated to bipedality in many ways. One of the most important of these is the “carrying angle” between the shafts of the bones comprising the upper and lower leg. The thighbone slants inwards at an angle toward the knee, but the weight of the body is then borne straight downward through the shin bone and ankle to the foot. Because of this geometry, the feet pass close together during walking and running, without requiring the body’s center of gravity to move from side to side as weight is shifted from one foot to the other. There is also no sideways component to weight transmission through the ankle. The apes are quadrupeds, and lack these diagnostic modifications for bipedality. In this diagram a modern human left leg skeleton (on the right) is contrasted with its counterpart in a gorilla. The angle at the knee in both primates is emphasized by the intersection of the bold lines. Note also the very different proportions of the pelvis, and the relative length of the legs. Bear in mind that the gorilla is shown in a posture for which it is not specialized (but that the line of weight transmission is basically straight down from the hip joint and somewhat across the ankle), and that the drawings are not to scale: the human leg is in reality much longer than the gorilla’s. Drawing by Jennifer Steffey.

  TWO

  THE RISE OF THE BIPEDAL APES

  One minor irony of paleoanthropology is that the major components of the human fossil record were discovered in an order exactly inverse to their geological ages. Our recent relatives the Neanderthals initially came to light back in the mid-nineteenth century, when antiquarianism was still the domain of amateurs; the older species Homo erectus showed up a half century later, as the result of the very first deliberate search for ancient hominids in the tropical zone; and the yet more ancient australopiths only became decently documented another half century after that, more or less announcing the dawn of the modern age of paleoanthropology. As a result of this history, the Holy Grail of paleoanthropologists has become the extension of the hominid record into the past.

  It’s interesting to speculate how differently we might interpret hominid evolutionary history today had the older fossils been discovered first; but while there is no way to know exactly how our views would have differed in that event, what is beyond doubt is that the order of discovery of our fossil relatives has deeply influenced their interpretation. Still, the core of this book is a chronological account of the long and astonishing process whereby our ancient ancestor, an unusual but not particularly extraordinary primate variation, became transformed into the amazing and unprecedented creature that Homo sapiens is today. And, since trying to interpolate the history of discovery and ideas in paleoanthropology would inevitably have interrupted the flow of the story, I have tried to avoid it wherever possible. But it should never be forgotten that everything we believe today is conditioned in some important way by what we thought yesterday; and some current controversies are caused, or at least stoked, by a reluctance to abandon received ideas that may well have outlived their usefulness. In such cases there will be no getting around a bit of explanation of how we got to our current perspective; and the australopiths are no exception.

  THE LUCY SHOW

  In keeping with the pattern of paleoanthropological discovery I’ve just outlined, the geologically oldest hominid species known before the recent spate of “earliest hominid” finds was also the most lately discovered. This is the aforementioned Australopithecus afarensis, and its most famous representative is the fabled “Lucy.” Lucy was discovered at Hadar, northeastern Ethiopia, in 1974, and she consists of a relatively complete (about 40 percent) skeleton of a tiny hominid individual, usually considered female by dint of her small size. She lived some 3.18 million years ago in a region that is today arid desert, one of the most hostile areas in which humans currently live, but which was much friendlier to hominids back then. The pile of sediments in the Hadar region contains rocks and fossils deposited between about 2.9 and 3.4 million years ago in the valley of the broad, meandering ancestor of today’s Awash River. Careful studies of fossils and ancient soils here show clearly that over this period there was some climatic fluctuation, both from drier to wetter and from cooler to warmer. But the area remained one of grassy woodlands overall, with denser forest near the river itself. Sometimes there was more bushland, sometimes less, but trees never grew too far away, and the structure of Lucy’s body reflects this.

  Lucy’s discovery was presaged by the discovery the year before, also at Hadar, of both parts of a hominid knee joint that clearly showed the telltale “carrying an
gle” between the femur above and the tibia below. Whoever this knee joint had belonged to, there was no question that the knees had passed close to each other during walking, and that the feet had swung straight ahead with each stride. At the time, this was the earliest known evidence of a bipedal hominid by several hundred thousand years. So imagine the excitement and anticipation when the paleontologists went into the field at Hadar the next year, and the too-good-to-be-true feeling when the entire skeleton of a similar individual was unearthed.

  Paleontologists don’t usually expect to find whole, or even partial, fossil skeletons of land-dwelling vertebrates—too much can happen between the moment when an individual dies on the landscape and whenever, if ever, what is left of it becomes buried by sediments. Only a tiny fraction of remains buried in this way are ever again exposed at the Earth’s surface by erosion, and then picked up by human collectors before wind and weather have obliterated them. So a tolerably complete skeleton from this incredibly remote period in time was an almost unimaginable piece of luck. In the 1970s, even partial hominid skeletons were virtually unknown before the rather recent era of our close relatives the Neanderthals, who for the first time had hit on the idea of protectively burying their dead. Small wonder, then, that Lucy turned out not to have a complete knee. But the top and bottom elements of the knee were preserved on different sides, and they showed the same features as the 1974 knee joint. Lucy had walked upright.

  The “Lucy” skeleton, NME AL 188, from Hadar, in Ethiopia. When it was found in 1974 this was the most complete early hominid skeleton ever recovered, and it inaugurated an era of spectacular paleoanthropological discoveries in Ethiopia. Drawing by Diana Salles.

  And that wasn’t all. In life Lucy had stood not a whole lot more than three feet tall, and had weighed perhaps 60 pounds. (Australopithecus afarensis males would have stood up to a foot taller, and would have weighed considerably more.) If you happened by some miracle to meet the diminutive Lucy, you would hardly have recognized her as a particularly close member of the family. But a lot more than her knees attests to her bipedality. The structure that has attracted the most attention in this respect is her pelvis, of which enough remains to make a good reconstruction of the whole. Living apes have narrow pelvises with tall, slender, forwardly sloping iliac blades. The three gluteal muscles that attach behind them are concerned principally with extension of the leg and support of the back during sitting. The tall ilia of apes also raise the lower attachment of strong muscles that go up and across the back all the way to the upper arm and are important in powerful climbing. The modern human pelvis, in contrast, is completely reproportioned. Our pelvises have shortened and become more curved, with more backwardly rotated ilia that efficiently distribute the stresses generated by upright posture, and that cup the abdominal contents lying above. The broad iliac blades also shift the two “lesser” gluteal muscles sideways, enabling them to stabilize the pelvis and upper body during bipedal walking while at the same time being overshadowed, in size at least, by the formerly fairly insignificant gluteus maximus muscle. This has become the biggest muscle in our bodies, and it serves the new purpose of stopping the trunk from tipping forward at each foot strike.

  Human and ape pelvises are thus significantly different in form, each closely expressing a particular way of getting around. Given that she lived much closer in time to the ape-human ancestor than we do, you might expect to find that Lucy’s pelvis had a shape somewhere in between that of an ape and a modern human—perhaps something similar to the reconstructed pelvis of Ardipithecus. Amazingly, though, it isn’t this way in the least: the Australopithecus afarensis pelvis is the very antithesis of the high, narrow pelvis of an ape. Like ours, Lucy’s hipbones are really short from top to bottom, revealing that the musculature they bore had been reorganized in very much the way that ours has. But her iliac blades are even broader than ours are, showing a dramatic sideways flare. Early interpretations of this unusual anatomy led to the notion that Lucy was a sort of “super-biped,” whose pelvis-stabilizing muscles had even better mechanical advantage in bipedal movement than ours do. This breadth and presumed advantage would have been yet further exaggerated by the structure of the ball-and-socket hip joints, in which the head of the femur (the “ball”), which fits into a socket at the side of the pelvis, is connected to the bone’s shaft by a “neck” much longer than its equivalent in ourselves.

  It always seemed a bit odd that an ancestral biped should have been better adapted than its presumed descendant to the unique upright locomotor style of hominids. But this strange situation can be explained in terms of the dual function of the pelvis, which provides the birth outlet in addition to gut support and muscle attachment areas. Modern humans face a substantial obstacle in getting a newborn’s huge round head through the birth canal, which is why obstetrical problems are relatively frequent in our species. When you look down on Lucy’s flaring pelvis, its outline is that of an elongated oval—and the birth canal inside it is oval as well.

  Since hominid brains were very small back in Lucy’s day, it is thought that such anatomical modification of the outlet in the interests of locomotor efficiency would have posed no problems for females during the passage of the infant through the birth canal (though a rotation of the baby on the way out might have been necessary). However, it turns out that having a wide birth canal itself has biomechanical consequences, since it affects the spacing of the hip joints. When a biped walks, its pelvis rotates horizontally as each foot swings forward, and this effect is exaggerated the farther apart the hips are, bringing with it a whole slew of biomechanical disadvantages. One reason why human females tend to run more slowly than males is the greater average width of their hips.

  While numerous features of the pelvis attest beyond doubt that Lucy was a biped, others reveal that she was not bipedal in quite the same way we are. A similar conclusion emerges from looking at her leg bones, in which that telltale carrying angle at the knee, and a quite convincingly bipedal (though quite mobile) ankle joint, are combined with the remarkably short length of the limbs themselves. Compared to her torso and forelimbs, Lucy had pretty short legs—indeed, her legs were as short as a bonobo’s. These proportions were hardly ideal for a strider, but they were a distinct advantage in climbing—and the later lengthening of the hominid leg is widely recognized as a clear sign of a greater commitment to the ground than we see in Lucy. And for biomechanical reasons, such lengthening also permitted some narrowing of the later hominid pelvis relative to Lucy’s.

  What’s more, while Lucy herself possesses only a couple of preserved foot bones, parts of the foot attributed to other individuals of her species indicate that her feet would have been quite long, and her toes a bit curved (although the mid-foot may have been relatively advanced). This was certainly not a committed branch-grasping foot like the ones we see in the modern apes and Ardipithecus, with their long, curved digits and widely divergent big toes; but it is a foot that would have been substantially more capable in the trees than ours are. The bones of Lucy’s upper limb continue the arboreal theme, although relative to the rest of her body her arms were shorter than those of bonobos. Her rib cage, however, tapered sharply upward from its broad base, so that the somewhat upwardly oriented shoulder joints were quite closely spaced. Both of these attributes would have been pretty useful in the trees. And while the Lucy skeleton is a bit short on hand bones as well as of those of the foot, hand elements from other Australopithecus afarensis individuals found at Hadar are much shorter than those of apes, but still show some apelike features of the wrist bones in combination with finger bones that are quite curved. They also show markings for strong flexor tendons, signifying a strong grasping ability. Taking everything into account, a picture emerges of Australopithecus afarensis as a creature less fully adapted to bipedality than we are, but much more capable than us in the trees.

  This is a configuration unlike anything else we know, except among other australopiths. Certainly as far as their
locomotor and habitat preferences go, it would be misleading to think of Lucy and her companions either as an advanced form of ape, or as a primitive form of human. Her species, and its similarly proportioned relatives, had hit upon a unique solution to the challenges of living and moving in the new environments presented to them by climate change and the fragmentation of the forests.

  But Lucy and her like are certainly not frequently (if inaccurately) described as “bipedal apes” purely because of their odd combination of bodily features. In the structure of their skulls they show a similarly unprecedented amalgam of characteristics. Lucy herself has only a lower jaw and some tiny fragments of the cranium. But as two 3-million-year-old crania also discovered at Hadar eloquently attest, the general skull proportions of Australopithecus afarensis are broadly apelike, in the sense that they combine a small braincase (which had contained a brain not much bigger than that of an ape of similar body size) with a large and forwardly projecting face. That face, however, has very robust jaws that house teeth distinctly different from those of any ape. In the upper jaw, the central incisors of A. afarensis are large and flanked by considerably smaller teeth, much as you see in the African apes; but immediately behind the incisors the aspect of the tooth-row changes. As among the “very early hominid” contenders described in chapter 1, the canine teeth are reduced in size, even if they are not exactly dainty. The honing mechanism against the lower front premolar is also essentially gone, although traces of it remain. The rear premolar teeth are broad, and the molars behind them are flattish and quite large relative to the jaw, setting the pattern of “postcanine megadonty” (big chewing teeth) that was to characterize early hominids for some time to come.