Tentative hominid family tree, sketching in some possible relationships among species and showing how multiple hominid species have typically coexisted— until the appearance of Homo sapiens. Diagram by Jennifer Steffey, ©Ian Tattersall.
This mystery isn’t going away any time soon. Meanwhile, though, Ardi’s discoverers were at pains to emphasize that their fossil’s forelimb did not resemble that of either African ape—something that nobody really expected anyway. What is a lot more remarkable is that the rest of Ardi’s postcranial skeleton doesn’t resemble anything else we know, either. The Ardi pelvis is badly crushed, and had to be restored to its original form using a lot of subjective judgment. As reconstructed, the iliac blades of the pelvis (the elements that flare sideways at the back) are shorter from top to bottom than they are in apes, and are thus marginally more humanlike. What’s more, there is a large ridge or “spine” at the front of the pelvis. This structure is associated both with a strong ligament that is helpful in maintaining balance during upright walking, and with a well-developed muscle that helps extend the leg. In humans the ridge is thus quite large, while in the quadrupedal apes it is much smaller. The Ardi team thinks that the short ilia and large spine in their fossil suggest some capacity for upright walking. But in view of the fact that our old late Miocene friend Oreopithecus showed these features too, it may be more plausible to associate them with habitual upright posture in the trees than with walking on the ground.
Looking at Ardi’s foot reinforces this impression. This is emphatically not what we have come to think of as a hominid foot, where the big toe projects forward in line with the other toes. Rather, it is the long, curving foot of a tree-climber, with a divergent great toe adept at grasping branches. So again, we see a structure in Ardi that is not particularly reminiscent of that of any modern ape; but neither is this foot at all well suited for walking on the ground.
So how did Ardi locomote? Right now, that’s hard to judge. With a foot ill-fitted for life on the ground, this big-bodied grasping climber weighed so much that its life in the trees would have been hugely restricted had it moved around only on the tops of branches large enough to support its weight. Today the heavy orangutan deals with a similar weight problem by being a “four-handed climber” that frequently suspends itself from clusters of small branches; but the Ardi team categorically denies that its subject shows any anatomical tendencies to a suspensory way of life.
Ardi, then, is a mysterious beast. It has no close living parallels in the structure of its body skeleton, and its cranial construction is at least a little ambiguous. If it is a hominid, it is certainly not directly in the line of later hominids; for not only is it anatomically bizarre but, as we’ll see in a moment, there is a much better candidate for the role of hominid progenitor from only a bit later in time. So, if Ardi is a hominid, we have to see it—recent as it may be compared to Sahelanthropus—as a late representative of an early branch off the hominid tree. And if that’s correct, this strange creature helps, right at the beginning, to set the pattern of remarkable diversity among hominids that was to continue right up to the appearance of our own species. We are alone in the world today; but until very recently there have typically been lots of hominid species around, as the figure on page 12 shows.
WHY BE BIPEDAL?
Ardi forcefully reminds us that the climatically changing world of the Pliocene set the stage for extensive evolutionary experimentation among hominoids, including the exploration of more terrestrial life-styles. Whatever pressured these creatures to move away from the trees was evidently powerful; for it should never be forgotten that leaving the trees for at least a partly terrestrial life was no small thing. It was, in fact, a huge leap in the dark. In a forest habitat an adept climber, particularly a biggish one such as Ardi, would have been menaced by few predators, at least as an adult. Its food supply would have fluctuated seasonally, but in a relatively predictable way; and its basic life-style was underwritten by many tens of millions of years of primate evolution. In contrast, the expanding areas of forest edge, woodland, and grassland would have teemed with ferocious killers such as lions and sabertooths; and at the same time an entirely new foraging strategy would have been required to obtain the unfamiliar resources these habitats offered. For any primate to move into these novel environments meant entering a fundamentally unfamiliar and difficult ecological zone, and for the first hominids it was certainly a huge gamble—albeit one that eventually paid off in spades.
All primates are four-limbed creatures, and why one of them should have taken up erect bipedality on the ground has been incessantly debated. The advantages of this way of getting around are not hugely obvious, while the initial disadvantages—most obviously, the sacrifice of speed in an environment abounding in fleet-footed predators—are manifest. So there really is a big puzzle here. Echoing the default approach to recognizing the earliest hominids, paleoanthropologists have usually framed the “why bipedality?” question in terms of a “key benefit” conferred by this unusual form of locomotion—either in the form of some advantage bestowed by the locomotor style itself, or of some spinoff benefit. Speculations as to what this particular benefit might have been are rife, not least because bipedality opened a host of unique opportunities to hominids.
The ways in which humans have capitalized on those opportunities have caught the attention of paleoanthropologists since the very earliest days of their science. As far back as the mid-nineteenth century, Charles Darwin associated hominid bipedality with the freeing of the hands to modify objects and make tools: a proposal later enlarged by adding the ability to carry things, including food, over long distances. Sadly for the original conjecture at least, it is now known that hominids were bipedal long before they began to make tools.
The array of other speculated advantages to moving upright on the ground is little short of breathtaking in its diversity. At one extreme it has been considered a matter of energetics, and scientists have expended huge efforts to discover how much energy hominoids on the ground use while moving quadrupedally and on two legs. Predictably, the answer is not simple. It all depends on how fast you are going, or on whether you’re walking or running, on how rough the terrain is, and on precisely how you’re built and move your limbs. In terms of energy used per unit of distance, it’s clear that modern humans are more efficient walkers than they are runners; and it has been calculated that on average human running costs are higher than for the average quadruped, while walking costs are lower. So as long as they moved slowly, and avoided the notice of those predators, maybe early hominids saved energy by tottering around on two legs.
But although some researchers have concluded that human bipedal walking is significantly more energy efficient than the locomotion of a quadrupedally ambling chimpanzee, others have been altogether unimpressed by the energy efficiency of modern humans in general. And for an even less efficient bipedal early hominid, costs would have been higher all around than they are for us. This debate will certainly continue, but right now it looks really unlikely that early hominids chose upright walking because it was a more economical way of getting from here to there over open ground.
If you’re looking for a physiological explanation for uprightness, a more plausible one is provided by the regulation of body temperature. Mammals in general need to maintain a reasonably constant body temperature, and the brain in particular is sensitive to overheating. Only a small spike in the brain’s temperature can mean irreversible damage. Primates are tropical animals, but they have no special mechanisms for cooling the brain, so the only way for them to achieve this away from the shade of the trees is to keep the entire body cool. If a quadruped stands up out there in the open, the area of its body exposed directly to the hot vertical midday rays of the sun is reduced; and this minimizes the absorption of heat, an important consideration in any animal’s temperature budget. In addition, most of the body surface is raised away from the hot ground, maximizing exposure to cooling breezes. This is impor
tant for us, because in hot climates humans depend on losing excess heat by the evaporation of sweat. This is a powerful reason, by the way, for believing that the adoption of an upright stance might (at some point) also have been associated with the reduction of evaporation-impeding body hair that is such a remarkable feature of us “naked apes” today.
Some of the consequences of being a biped vs. a quadruped in the unshaded tropical savanna. Compared to the quadrupedal ape, the upright human reduces the area of his body receiving heat from the sun and from the ground, while maximizing the skin area able to radiate body heat. The bulk of the body is also raised off the ground, thus benefiting from the cooling effects of the wind. Illustration by Diana Salles.
These elements all add up to a great story, and they may well have somehow been important individually in the early human drama. But as an explanation for the adoption of bipedality, this beautiful theory is, alas, slain by an inconvenient fact: early hominid fossils are generally associated with forested or at least wooded conditions—indicating that bipedality was adopted well before the shelter of the trees was entirely abandoned.
The same observation, by the way, also disposes of the once-popular notion that hominids originally stood upright to see farther over savanna grasses, and were thus enabled to spot predators in the offing more effectively. When you go to the Serengeti Plains today—for most of us, the quintessential vision of Africa—you cannot help but be overawed by the huge open expanses of grassland, and by those panoramas that seem to continue into infinity under the cotton-clouded blue skies. But back in the Pliocene, habitats were typically more closed, and Serengeti-style savannas were pretty much a thing of the distant future. In view of this, some paleoanthropologists have suggested that standing up allows one to reach higher to pick low-hanging fruit from savanna trees—as open-country-living chimps have been observed to do—and have cited this as a possible incentive for early hominids to move around upright. But then, because quadrupedal chimpanzees can do this too, it’s obvious that you don’t have to be a full-time biped to take advantage of the facultative ability to stand up.
Still, possible benefits of uprightness don’t stop with physiology and being effectively taller (also a possible deterrent to predators). Walking upright has been correlated with certain forms of social behavior. One recent suggestion, harking back in some respects to Darwin’s original observation, implicates monogamy. By this reckoning, bipedal early hominid males were able to range far and wide for food that they were then able to carry back to their mates, who were tethered to local areas by the burden of their mutual offspring (though it’s also been argued that bipedality made it easier to carry infants around). Male bipedality allowed for genital displays to keep the females attracted; while at the same time, by hiding their genitalia between the thighs, female bipedality concealed ovulation so that males needed to be continuously attentive to their mates, reinforcing their fidelity. Well, maybe; but among monogamous primates the two sexes are typically similar in body size, while there is good reason to believe that early hominid females were significantly smaller than males.
The list of potential key advantages (and of objections to them) could go on; but to elongate it here would be to miss the point, for the most important thing to bear in mind when you’re wondering why hominids first stood upright is that, once you have adopted bipedality, all of its potential advantages are there—and all of its disadvantages too. So perhaps we should abandon the idea of key benefits, and return to the underlying question of why any early hominid would ever have stood up in response to the undoubted challenges of living on the ground—whatever those challenges may have been. And the only plausible answer to this question is that the first hominids to spend any significant amount of time on the ground were already most comfortable standing and moving upright. It’s clear that the ancestral hominid would never have adopted this difficult terrestrial posture, with all its attendant problems of balance and weight transmission, unless it was simply the natural thing for it to do. Yes, those cute meerkats you see on TV “stand” upright when scanning for predators, but if they see one, they rapidly drop to all fours and scamper away; and this goes for monkeys and living apes as well. No committed quadruped would ever have walked upright against its instincts purely because of some potential benefit that modern researchers might think up.
Almost certainly, then, the progenitors of our family felt most comfortable lurching around vulnerably on two legs because they were already posturally upright. Presumably they descended from a hominoid ape lineage that habitually held the trunk erect when moving around in the trees—just as the remotely related Pierolapithecus and Oreopithecus had evidently done as well. This posture would certainly have made sense for creatures that were pretty heavy for tree-dwellers: such animals would have benefited disproportionately from the ability to suspend themselves efficiently by the arms in the small peripheral branches of the trees, where most of the fruit is. Today’s African apes are knuckle-walkers because their ancestors were basically arboreal quadrupeds: too committed anatomically to a horizontal stance for their descendants to move upright for any distance on the forest floor, or when venturing beyond the trees. For the early hominids, the reality must have been the opposite: that moving quadrupedally on the ground felt awkward. This is certainly the case for the sifakas of Madagascar, long-legged primates that cling and leap vertically in the trees, and bound bipedally on their rare excursions to the ground
As large-bodied climbers, then, it makes a lot of sense that the hominid precursors should have held their trunks upright when moving and foraging around in the trees. Suspensory orangutans, which tend to hold their bodies erect in the trees, are actually pretty good bipeds on the ground, so perhaps it’s legitimate to imagine our remote ancestors, at least in their body form, as “orangutans-but-more-so.” Whatever the case, though, the transition from tree-dweller to part-time terrestrial biped must have been difficult, since a climber will find a grasping foot a hindrance on the ground. Most likely, the hominid ancestor lost that type of foot posthaste once it ventured to the ground. But exactly how and in what precise context the in-line terrestrial foot was acquired remains tantalizingly obscure. This deficit in our knowledge is hugely unfortunate because, given that everything that happened later was dependent on the fateful transition from the trees to the forest floor, it presents us with one of the most fundamental mysteries in all of paleoanthropology.
BIPEDAL APES
Not much more than a decade ago, the earliest known hominid fossils belonged to the genus Australopithecus (“southern ape”). The first member of this genus was discovered at a South African site in 1924, and numerous others have since been published from localities both there and in eastern Africa (with one central-west African outlier in Chad). But until 1995 all of these “australopiths” dated from between about two and less than four million years ago. Then a new species of Australopithecus, A. anamensis, was reported from a couple of sites near the shores of Lake Turkana, a large body of water in arid northern Kenya. The species name given to this form came from the local word for lake (anam); and the sediments in which the fossils were found dated from 3.9 and 4.2 million years ago. This extends the range of Australopithecus well back in time; indeed, marginally into the “earliest hominid” range of the forms we’ve just been discussing.
Knowing just how old the fossil-bearing rocks in the Lake Turkana basin are is facilitated by very active volcanism in the region over the past several million years. This is because volcanic rocks contain minerals that incorporate unstable (radioactive) forms of various elements, and these decay to stable states at known and steady rates. When the volcanic rocks—which come in the form of both lava flows and layers of ash-fall that interrupt and interleave the layer cake of accumulating sediments—start to cool after being deposited atop the sediment pile, they do not contain any of the stable products of decay. As a result, any such products that you measure in them must have formed by decay, in a span
of time that you can calculate from the known decay rate. Hence you know the age of the volcanic layer, and any fossil-containing sediments lying just above or below it will be (hopefully just a little) younger or older, respectively. Of course, things are rarely quite as uncomplicated as this thumbnail sketch suggests—geological faulting, for example, can tilt, deform, and misalign sedimentary sequences—but over the last half century geochronologists have become quite adept at producing accurate dates, as they have at knowing when the data just aren’t good enough to be relied upon. But take note that most of the dates measured in years you’ll read about in this book, including all of the early ones, are on rocks, rather than on fossils themselves.
Still, the dating of the Kenyan Australopithecus anamensis fossils (and of others some 4.12 million years old from neighboring Ethiopia) is pretty well established. And unlike Toumaï and Ardi, which raise a host of questions despite being represented by more complete specimens, these fossils bear a reassuring similarity to their presumed descendants in the genus Australopithecus. What is more, Australopithecus anamensis is the earliest hominid we know of that had, beyond a shadow of doubt, acquired important specializations for upright bipedality.
Most of the known fossils of this species are teeth and bits of jaws, but there are some postcranial bones as well, and a particularly vital clue is provided by a broken tibia (lower leg bone). The distal (ankle) end of this bone is especially interesting: it has a large joint surface oriented so as to suggest that the weight of the body was passed directly downward to the ankle joint from the knee, rather than at an angle, as in the apes. This is important because, while apes are capable of moving bipedally, they do not walk upright exactly as we do. Their femora descend directly from the hip joint to the knee in a straight line that is continued downward by the tibia. This is natural in a quadruped, which needs to support itself more or less as a table is supported at each corner by its four legs. But when that same quadruped rears up and walks on two feet, all the rules of balance change. Those two feet are wide apart, meaning that during forward motion each foot has to pivot around the other, describing a wide circle, just as the moving point of a pair of compasses does around the stationary point. This is not only ungainly, but it’s hugely wasteful of energy, and apes soon tire when walking upright for any distance. In modern humans, in contrast, each femur angles sharply inward from the hip joint, so that the shaft of the bone forms a “carrying angle” with the vertical tibia below. As a result our knees pass close together when we walk and our feet move in a straight line ahead of us, so that our body weight is not rocked inefficiently from side to side with every step.
Masters of the Planet Page 4
