Masters of the Planet (7 page)

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 angle” 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.