Masters of the Planet (6 page)

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.

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.