Masters of the Planet (19 page)

Such resistance is particularly important when we look at the shape of the Turkana Boy's brain. Unlike most bones of the body, which are pre-formed in cartilage that is gradually converted to hard bone as the individual grows, the bone of the skull vault forms in membranes that are carried outward by the expanding brain inside. Most of the increase in the size of our brains relative to those of apes is accounted for by expansion of the cerebral cortex, the outer layer of the brain; and since large-brained hominids are cramming a lot of extra cortex into a relatively small space, this expansion has caused the cortex to become deeply folded and wrinkled over the course of human evolution, providing a larger surface area. The key thing here is that the major wrinkles outline what have traditionally been identified as major functional areas of the brain; and because of the intimate developmental relationship between the bone and the outside of the brain, the inside of the skull vault provides a record of these important demarcations. The brain itself doesn't preserve, but since it fits so closely into the inside of the braincase that contains it, an impression (or “endocast”) of the inside of a fossil skull such as the Boy's can accurately represent what the organ it contained had looked like externally. There is, of course, a limit to what you can actually tell from this information, because how
the
brain is wired internally is critical to how it works; but nonetheless the external details can be informative.

One of the things that caught researchers' eyes early in studies of the Turkana Boy's endocast was that it prominently outlines a small region, called “Broca's area,” that lies on the left frontal lobe of the cortex. The eponymous Paul Broca was a nineteenth-century French physician who noticed that patients with injuries to this particular area of the brain typically had difficulty speaking, even though they still readily comprehended speech. Clearly, here was a part of the brain (actually, two parts, since neuroanatomists now subdivide it on grounds of cellular structure) that was somehow involved in the production of speech, and it was among the first identifiable external brain areas to be implicated in a specific function. This was an important step in the recognition that specific areas of the brain—different clusters and types of neurons—are responsive in particular tasks. We don't think or respond to stimuli in a holistic way, with our whole brains. In a way this is disappointing, because it means that paleontologists can't take absolute or even relative brain sizes as proxies for anything very specific. But it makes everything much more interesting.

Perhaps the most significant advances since Broca's time in understanding how the brain works have been made possible by the development of techniques for imaging the activity going on in living brains while their possessors undertake various mental tasks. An important result of such real-time investigations has been the realization that most functions, including speech, are more widely distributed in the physical brain than simple superficial mapping would suggest by itself. Nonetheless, the identification of Broca's area in the Turkana Boy led to speculation that the Boy could have talked. But nothing is quite that simple, and Broca's area is now also known to be involved in a whole slew of memory and executive functions unrelated to language. Clearly, having one of the many features whose proper function is necessary for speech production cannot be taken as
prima facie
evidence of speech itself; and anyway, the possession of structures that might be related to the latent ability to produce speech is a long way from implying that these hominids had language as we know it.

Another
aspect of the Boy's anatomy also argued strongly that they didn't possess linguistic skills. The spinal column not only supports the upper body but also conducts the spinal cord down from the brain, to control and receive information from the rest of the body via the network of nerves branching from it. The width of the canal through which the cord travels is pretty constant in most primates, including hominids; but in modern
Homo sapiens
(and, to be fair, in Neanderthals as well) it is unusually broad in the thoracic region, where the lungs sit. This additional breadth accommodates an increased volume of nervous tissue supplying the muscles of the thorax and abdominal wall, and it is suggested that the extra nerves are devoted to an enhanced control of breathing—a fine control that is, among other things, necessary to produce the subtle modulation of the sounds we use in speech. Interestingly, the Turkana Boy is an average primate in this regard. And it has accordingly been suggested that, regardless of the properties of his brain, he did not possess the peripheral ability to produce speech.

It has also been controversially proposed that there may have been something pathological in the narrow space for the Boy's spinal cord. That's as may be; but there are plenty of independent reasons to believe that however he and his kind communicated—and they undoubtedly had a sophisticated form of communication—they did not share information using language as it is familiar to us. For a start, modern articulate language is the ultimate symbolic activity, and we find nothing in the archaeological record associated with the
Homo ergaster
/
Homo erectus
group, at any point in its long tenure, that suggests any symbolic mental manipulation of the information received from the outside world. Indeed, sketchy though it is, the archaeological record left behind by these early members of the genus
Homo
is remarkable for the conspicuous
lack
of any such evidence. Perhaps most strikingly, the Turkana Boy and his kin made stone tools that were identical in concept to those that had been made at Gona almost a million years before. On the technological level, nothing significant had changed that we can detect over that whole vast period of time. The appearance of a radically new physical type had not resulted in—or from—any kind of technological innovation; and we have little material evidence to confirm that
Homo ergaster
had a
substantively
different lifestyle from its predecessors, even though the anatomical indicators prompt us to speculate that there must have been changes.

While it may seem counterintuitive that a new (and larger-brained) hominid should not have brought a new technology with it, this disconnect actually reflects a pattern already established among hominids: the very first toolmakers had, after all, been bipedal apes, not members of the genus
Homo.
This pattern set a template for future developments in that we cannot associate any later introduction of a new technology with the appearance of a new species of
Homo.
And this makes a lot of sense when you think about it, because ultimately a technology has to be invented by an individual—who has to belong to a pre-existing species. Innovations of all kinds must originate
within
species, if only because there is nowhere else for them to happen.

SIX

LIFE ON THE SAVANNA

The extraordinary skeleton of the Turkana Boy gives us a remarkable insight into his species
Homo ergaster:
a hominid that grew up fast but physically was like nothing we know from earlier in time, and a creature that was clearly at ease away from the ancestral forest. This radically different environmental setting made enormous new demands upon the young species, but it clearly did not initially respond by making technological adjustments: as far as we can tell, the very first
Homo ergaster
continued making the same kinds of tools that its more anatomically archaic predecessors had made. And in the absence of substantial evidence of technological change, we have to fall back on physical and other indirect indicators if we want to understand what was new in the life of
Homo ergaster.
But these indicators are highly suggestive, even though we are hard put to draw specific conclusions.

Although the Turkana Boy had a slender build, he was no weakling. In mechanical terms the shafts of the long bones of the limbs are basically hollow cylinders; and although the material of which they are made is hard and strong, it is not static. Instead, it remodels throughout life to resist the stresses placed on the limbs; and the varying thicknesses of the shaft walls reflect how high those stresses were in life, and how they were distributed. This is why fencers and tennis players have stouter
bones
in their dominant arms than in their passive ones, while astronauts' bones thin out after too much time in microgravity. One major respect in which the Boy's limb bones differed from ours is that, as in other early hominids, their shaft walls were much more robust than you see in humans today. This could indicate that in life the Boy was already immensely strong, and that he had also maintained a much higher level of activity than is typical of modern humans. Of course, our contemporary sedentary lifestyle is a very recent phenomenon; but even our ancient hunting-and-gathering
Homo sapiens
forerunners had relatively thin-walled long bones. Overall, since the Boy's time the thickness of the bone in the shafts of the limb elements has plummeted, implying that bodily strength has become a significantly less important factor in the hominid way of life.

The Boy's environment was not an easy one, and at least to begin with he and his fellows were out there on the still tree-studded African savanna without a significantly improved toolkit. There is every reason to believe that—as relatively poor climbers—they could not and did not depend on the trees for shelter to the extent their bipedal ape predecessors had. And in the more open areas they would have favored, there roved an array of predators as fearsome as those still lurking on the forest fringes. Mainly, but far from entirely, these were big cats of a much greater variety than is found in Africa today, and they were all ready to pounce on any unwary mammal they encountered. By our standards
Homo ergaster
individuals were strong; but they were nonetheless relatively defenseless, lacking big jaws and slashing canine teeth. How did they respond to this hazardous new environment? And how did they exploit it? There is no shortage of ideas; and although there is little evidence to substantiate any of them, a circumstantial case can be built.

One coherent scenario involves the notion that, with its modestly increased brain size,
Homo ergaster
needed a higher-quality diet than the varied but still plant-based one on which its predecessors had subsisted. This is because, although the benefits of a bigger brain seem self-evident to us
Homo sapiens,
the costs are at least equally evident. As I've already briefly noted, in metabolic terms the brain is among the most “expensive” tissues of the body. For, while the mass between our ears only accounts for some two percent of our body weight, it actually consumes
something
between 20 and 25 percent of all the energy we take in. This has major implications for the body's overall economy, including that of the digestive system. The broad abdomens of the Turkana Boy's australopith ancestors had almost certainly contained huge digestive systems, a feature that stands in stark contrast to modern humans. One of our most striking characteristics—almost as striking as our large brains—is that we have remarkably small internal organs for our body size. This was also true for the relatively narrow-hipped
Homo ergaster,
and it has important implications for the diet of the Turkana Boy and his fellows. For the internal organs are almost as “expensive” in energetic terms as the brain; and it has been powerfully argued that gut reduction in human evolution has not only been a necessary trade-off for brain expansion, but that at the same time it also exacerbated the need for a high-quality diet. Thus, although at the time of
Homo ergaster
hominids were only at the very beginning of their period of dizzying brain expansion, their reduced guts alone may have mandated dependence on high-yield foods.

Diorama in the American Museum of Natural History, showing two
Homo ergaster
in northern Kenya, some 1.8 million years ago. It is left to the viewer to decide whether the hominids had scavenged or hunted the impala they are shown butchering. Figures by John Holmes. Photo: Denis Finnin.

So
where did the extra energy come from in the big-bodied and modestly brained
Homo ergaster?
One obvious answer is that these early hominids had turned their attention to the highest-quality diet available to them: animal proteins and fats. This resource was, after all, roaming the African savannas in enormous quantities: mammals of all sizes abounded in the newly adopted environment. At the same time, though, these tasty beasts also attracted a hugely diverse fauna of specialized carnivores, far more numerous than their counterparts occupying the continent today. In going after savanna grazers the hominids would have had not only to compete for their food with these professional predators, but they would also have had to protect themselves from them.