Masters of the Planet (5 page)

Modern apes (especially males) have very large, pointed upper canine teeth which hone against the front premolars below. In contrast, modern humans have very reduced upper and lower canines that barely project beyond the other teeth, if at all. In determining if a fossil is that of an ape or a hominid, one thing paleoanthropologists look for is evidence of canine reduction. Here we see a side view of the teeth of a presumed male
Ardipithecus ramidus
(in the center), compared to a male chimpanzee (above) and a human male (who also, like many of us, lacks wisdom teeth and has an overbite).
Ardipithecus
shows an intermediate condition in which both the upper and lower canines are both reduced, but remain pointy and slightly projecting. Other hominids of the “very early” group show a broadly similar conformation. Illustration by Jennifer Steffey.

But what a difference below the neck! The arm and hand bones of Ardi are those of a highly arboreal animal that was well adapted for climbing in trees. Given what we already knew about later hominids, which retain climbing features in the upper body, this came as no surprise. Perhaps more remarkably, though, these bones do not show any of the “knuckle-walking” features seen in the forearms and hands of the chimpanzees and gorillas usually reckoned to be our closest living relatives. Both extant African apes are essentially arboreal creatures (except for adult male gorillas, which are just too heavy to clamber around in most trees). When on the ground, they occasionally rear up and walk short distances on their hind limbs, making displays or even carrying objects; but all apes are basically quadrupeds while on the forest floor— and the long, slender fingers that they depend upon for grasping tree limbs would get in the way during movement on the ground, except for one thing. So when walking on all fours, both chimpanzees and gorillas curl their fingers up into a fist, bearing the weight of the front of their bodies on the outside of the first knuckles. In this way they reduce the effective length of their arms relative to their legs, and this permits more comfortable four-legged walking while also getting those vulnerable long fingers out of harm's way. This unusual accommodation to compressive weight-bearing, by extremities that are basically adapted to the tensile strains of arboreal life, is clearly reflected in the structure of the apes' hands and wrists.

But of course, apes are apes and humans are humans. So why is the absence of any hint of knuckle-walking in Ardi a worry? After all, we
Homo sapiens
show no structural signs of being descended from a knuckle-walking ancestor. The question arises because the molecular systematists who compare the structure of human and ape DNA agree in concluding that
humans
are more closely related to chimpanzees than they are to gorillas, sharing more DNA similarities. They are even prepared to hazard estimates of when gorillas split from the human/chimpanzee group, and when humans split from chimpanzees, based on the assumption of a more or less regular rate of change in the DNA molecule over time.

Such molecular age-of-split estimates usually tend to look a little low to paleontologists: most are in the range of 5 to 7 million years ago for humans and chimpanzees, with the gorillas peeling off a couple of
million
years earlier. But whatever the exact times of divergence, this all means that if the common ancestor of the knuckle-walking chimpanzees and gorillas also walked that way, then so must the chimpanzee-human ancestor. In which case, knuckle-walking must have been lost in the human lineage after the chimpanzee-human split—and you might expect to find some telltale signs of a knuckle-walking past in the wrist and hand of an alleged early human ancestor such as Ardi. The absence of any such signs in Ardi makes you wonder a bit either about Ardi itself, or about our current received wisdom concerning relationships among humans and their closest living relatives.

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 important 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.