Masters of the Planet (8 page)

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.

The presence of these large molar teeth means that the tooth-rows of
A. afarensis
are long, as those of apes are. But largely because the canine teeth are small, the dental arcades are slightly curved in outline and lack
the
aggressively parallel arrangement exhibited by the apes. The dentition of
A. afarensis
is thus rather like the cranium itself in possessing features reminiscent both of apes and of later hominids.

A recent sophisticated study of the wear produced by chewing on some
A. afarensis
molar teeth suggested that, while members of this species probably preferentially sought out soft fleshy fruits to eat when obtainable, where such foods were unavailable they would have gone after tough, brittle foodstuffs like nuts, seeds, roots, and the underground runners of grasses. This would have made them considerably more omnivorous than today's apes are, and it would be consistent with the typically quite heavy wear that we see overall on
A. afarensis
teeth. A diet of this kind suggests a pretty generalist adaptation to a habitat that spanned the spectrum from closed forest to open woodlands.

The Hadar desert badlands have produced an incredible quantity of hominid fossils over the years, and many localities there have yielded fossils of
Australopithecus afarensis.
Undoubtedly the most extraordinary of these localities is a spot known as AL 333 at which, in 1975 and later, researchers unearthed a trove of some 240 fossils representing the remains of 17 hominid individuals—and, most unusually, fossils of very little else. How these bones came to be buried where they are is a puzzle. They are all broken up, which is consistent with their having been transported from somewhere else by water. But why were they concentrated in one place? They weren't accumulated by a scavenging agent such as hyenas (which are famous for transporting hominid cadavers to their dens) because, although broken, they show no signs of gnawing—and a sluggish river channel is, in any case, hardly the spot you'd expect to find a hyena den. So there is a bit of a mystery here; and it's important that it be solved eventually, because despite the fact that the fine-grained sediments in which the bones were enclosed are typical of those laid down in a slow-moving river, one suggestion is that these are the remains of an entire unfortunate social group that was swept up in a single catastrophic event—maybe a flash-flood—that happened at some time between 3.18 and 3.22 million years ago. And if all of the individuals—nine adults, three adolescents, and five juveniles—actually belonged to one social unit, then all of them must have belonged to the same species.

This
is not otherwise a sure bet because, although all the comparable fossil parts from Site 333 look basically the same, the size range among them is huge. Still, despite all the uncertainty surrounding how the fossils came to be jumbled together in one spot, the current majority opinion is still that all of the Hadar hominids—including Lucy, who is as small as the smallest of the 333 specimens—belonged to the single species
A. afarensis,
which must consequently have varied greatly in size. The most plausible explanation for the large size range among members of the same species is that males were very much larger than females, comparable to what we see among gorillas today, and not at all like chimpanzees and bonobos, in which sexes differ much less in size.

LAETOLI

Around the time the first Hadar discoveries were being made, another group of paleontologists was hard at work at the site of Laetoli, a thousand miles to the south. Laid down in the Tanzanian portion of the Rift Valley near the well-known site of Olduvai Gorge, the geological layers at Laetoli are slightly older than those at Hadar, running from about 3.5 to 3.8 million years old. Between 1974 and 1979 the broken jaws and teeth of three hominid individuals were collected at various Laetoli localities, but the site is most famous for the numerous animal trackways discovered there beginning in 1976. These include footprint trails left by hominids who, some 3.6 million years ago, had walked across a layer of wet cement-like volcanic ash that subsequently hardened. This was an extraordinary find. We can be confident that Lucy walked upright; but we must always remember that this is not something we can observe directly in the bones. Rather, we have to infer it from Lucy's anatomical structure. A footprint, however, is different, in that it is truly fossilized behavior. And the trackways at Laetoli are as eloquent of bipedality as it's possible to get. At one site an arrow-straight double trail of prints some 80 feet long, more or less like those anyone might leave walking along a wet beach, attests clearly to a purposeful bipedal gait. What is unusual is that the Laetoli environment at the time these prints were made was quite open; the hominids were slogging across a flat plain largely devoid of trees, and they must have felt pretty vulnerable as they
did
so. But they were heading directly for the Olduvai Basin, only a few miles away, which at that time would have offered all the hospitable resources of a forest surrounding a shallow lake.

The footprints themselves are clear evidence of bipedality: there is no indication that the hominids steadied themselves using their forelimbs, and the way in which weight was transmitted from one end of each print to the other seems to reflect the way we walk—which is to say, it went from the heel, along the side of the foot and across the ball, with a final thrust concentrated on the big toe. This was not the lurching gait of a bipedal bonobo. The feet that made the prints were structured essentially like ours, with longitudinal and transverse arches and a short big toe set in line with the others. The short distances between successive footfalls suggests that even the bigger individual was of fairly diminutive stature, although it seems the pair was not moving very fast—hardly surprising, given the slushy surface across which they were making their way.

While there is nothing to cast doubt on the bipedality of these 3.6-million-year-old hominids, there has been some debate about the exact gait they employed. Did they, for example, fully extend the knee with each step? Or did they retain some vestige of the bent-kneed gait that today's apes use when moving upright, and which, at some remove in time, the hominid ancestor must also have employed? A recent experimental study, using human subjects moving both straight- and bent-kneed, has confirmed that if you don't fully extend your knee, the impressions your toes make in wet sand are deeper than those made by your heel. And the Laetoli prints clearly show heel and toe depressions that are about the same depth, arguing for a straightened knee. Clearly, in these footprints we have evidence of a serious biped.

The scientists who carried out the experimental work suggest that adopting upright locomotion on the ground allowed the Laetoli hominids to increase their ranging distances without expending extra energy, during a period when the forest was diminishing. Indeed, it's very unlikely that any hominid could have made a decent living in the rather barren ancient environment adjacent to the trackways, making it all the more plausible that the prints in the wet ash had caught them in the act of aiming straight for the forests that lined the nearby Olduvai Basin.

Just
who those bipeds were is another matter. Not far from the footprint tracks at Laetoli are rocks of about the same age that yielded the handful of hominid fossils already mentioned. In an unusual collaboration, the scientists who initially studied the Hadar and Laetoli specimens eventually decided that they were all from the same new sort of hominid. This new species,
Australopithecus afarensis,
was named for the Afar region of Ethiopia in which Hadar is situated, and from which most of the fossils in question came. But under standard zoological procedure, every new species has to be based on a “holotype,” a single specimen to which every other individual assigned to that species has to be compared. And to emphasize their conviction of unity, the scientists chose a lower jaw from Laetoli as the holotype of
A. afarensis.
Not everybody found this appropriate, though, as some scientists felt that they could discern evidence for more than one species of hominid just at Hadar, let alone at the Ethiopian and Tanzanian sites together.

At present there is a state of uneasy truce, with most paleoanthropologists willing to accept at least provisionally that the known bones and teeth, at least, can be assigned to the same species. But the association of
A. afarensis
and the footprints is much more actively debated. Perhaps a majority of paleoanthropologists is willing to believe that individuals of
A. afarensis
made the Laetoli trails; but at least a substantial minority thinks that the fossil foot bones from Hadar indicate a foot far too long and primitive to have produced the strikingly modern Tanzanian footprints. If the majority is right, then we will have to accept that the Lucy's arboreal adaptations, and her broad pelvic proportions, were indeed compatible with remarkably humanlike bipedality. But the jury is still out; and all we know for sure right now is that
somebody
was out there strolling upright through the Tanzanian Rift 3.6 million years ago.

DIKIKA

Only a few years ago, the word “Dikika” was in almost nobody's vocabulary, but now it's one of the hottest buzzwords in paleoanthropology. During the glory days at Hadar in the 1970s, and again in the 1990s, everybody was too busy to look south across the Awash River to the more or less equivalent deposits of Dikika. But when the investigation of those
rocks
finally began at the start of this century, they proved to have a dramatic story to tell. First, some scrappy bits of tooth and jaw attributed to
Australopithecus afarensis
showed up; but these were soon overshadowed by the discovery of the crouched partial skeleton of a three-year-old juvenile. Thought to be female, the skeleton was soon baptized with the informal name “Selam” (“peace”). So well preserved was Selam that it seemed the infant was snatched from its group by floodwaters and almost immediately buried whole in soft mud, some 3.3 million years ago. And this poignant Pliocene misfortune proved a bonanza for the paleontologists, who found that the Selam fossil preserved elements that were not included, or were more poorly preserved, in the extensive
A. afarensis
collections from north of the Awash. Among these elements are a hyoid—the bony portion of the Adam's apple—that resembles that of an ape rather than a human, and a complete scapula (shoulder blade) that is unexpectedly reminiscent in overall shape of a gorilla's. Selam has the ankle of a biped, but the carrying angle between her femur and tibia is not marked, confirming that this is a feature that has a strong component of behavior in its development—carrying angles do not properly develop in modern humans who spend their entire lives in wheelchairs.