Masters of the Planet

by Ian Tattersall

  In the half century preceding World War II, paleoanthropology was the province largely of human anatomists, scientists whose training was in the minutiae of human physical variation. They were not forced to confront the riotous diversity of species in nature that other natural historians had to contend with. One byproduct of this insular history was that few paleoanthropologists of the period had much if any training either in evolutionary process, or in the procedures and requirements that should underpin the naming of new species. This led to the liberal description of new hominid genera and species, almost as if each new fossil that showed up needed to be baptized with its own genus and species name, much as Western humans individually receive family and given names. By the time World War II rolled around there were at least 15 hominid genus names commonly in use, and countless species—all for a fossil record that was then of modest size.

  In the long run this was bound to be a pretty untenable situation. And it was particularly untenable at a time when a movement that became known as the Evolutionary Synthesis was taking firm hold in most areas of evolutionary biology. The Synthesis was a meeting of previously disparate minds in the fields of genetics, systematics and paleontology, each of which had previously itself harbored multiple versions of the evolutionary process. On the one hand, the Synthesis emphasized the importance of variation within populations and species of living creatures, and on the other, it preached the basic continuities of the evolutionary process. The Synthesis allowed for the splitting of evolutionary lineages of organisms (without which we would never have achieved the luxuriant diversity of Nature that we see today). But at the same time it stressed that evolutionary change was the result of slow alterations in the frequencies of genes within established lineages, under the guiding hand of natural selection. Species were hence viewed principally as arbitrary segments of ever-modifying lineages: as transitory units, highly variable at any one point in time. And they were thus expected to slowly evolve themselves out of existence. So compelling was this gradualist message that, between the late 1920s and the mid-1940s, the Synthesis became the central paradigm of evolutionary biology in the English-speaking world. Virtually the last holdout, as a result of its peculiar history, was paleoanthropology. But not for long.

  Perhaps the most influential architect of the Evolutionary Synthesis was the geneticist Theodosius Dobzhansky, who was declaring as early as 1944 that based on the fossil evidence there had never been more than one (highly variable) hominid species at any one point in time. At an influential conference held at Long Island’s Cold Spring Harbor Laboratory in 1950, Dobzhansky was joined by his ornithologist colleague Ernst Mayr, who took this proposition even farther. Mayr argued that culture broadened the human ecological niche to such a degree that, even in principle, there could only ever have been one human species at a time. And it’s worth remembering at this point that Mayr’s notion is intuitively a very attractive proposition to members of a storytelling species that also happens to be the only hominid in the world today. It is somehow inherently appealing to us to believe that uncovering the story of human evolution should involve projecting this one species back into the past: to think that humanity has, like the hero of some ancient epic poem, struggled single-mindedly from primitiveness to its present peak of perfection.

  Although he’d probably never seen a hominid fossil in his life, Mayr then took all of the many genera that were cluttering the hominid fossil record, and reduced them to one: the genus Homo. What’s more, he reduced the species involved to a mere three. These formed a single succession: Homo transvaalensis (the australopiths) gave rise to the middle stage we call Homo erectus, which ultimately transformed into Homo sapiens (which included the Neanderthals). Mayr’s Cold Spring Harbor declaration hit paleoanthropology like a bombshell. Before long, even he was forced by ongoing discoveries of robust australopiths to admit that there had indeed been at least one side branch from the mainstream of hominid evolution. But Mayr’s reductionist view of the human fossil record still held paleoanthropology in thrall for the next several decades. Perhaps because paleoanthropologists had never really paid much attention to evolutionary theory before, paleoanthropology found itself suddenly dominated by the Synthesis. Indeed, so thoroughly was the field traumatized by Mayr’s remonstrations that, all through the 1950s and well into the 1960s, many paleoanthropologists hardly dared to use zoological names at all, preferring to refer to individual fossils by the names of the sites they came from. That way, they couldn’t be accused by their colleagues of being biologically naïve.

  Once the trauma had worn off to the point that they felt comfortable with zoological names again, paleoanthropologists lapsed into a style of taxonomic inclusiveness. The attitude seemed to be, if we have to use zoological names, let’s use as few as possible. And even though the burgeoning fossil record has since made it impossible to ignore the fact that there was indeed a lot of hominid diversity out there, most paleoanthropologists still hew to the rather minimalist mindset that so thoroughly dominated in the days when most of today’s leading practitioners were trained. Of course, paleoanthropologists aren’t fools, and nobody denies any more that the human evolutionary tree looks more like a forking bush than a slender sunflower. Even more significantly, a lot of different hominid species are now widely accepted, as the illustration in chapter 2 shows. Nonetheless, despite wide recognition that there is a great deal more to the evolutionary process than simple lineage modification under natural selection, the gradualist mindset still lingers in a residual reluctance among paleoanthropologists to recognize more branches in that tree than absolutely necessary. Perhaps once this reluctance has faded a bit farther we will be able to look more realistically at the diversity of the “early Homo” fossils, and to detect where within that assemblage the roots of our genus actually lie.

  We haven’t yet got very far with this process, but an important first step was made in 1999 by the English paleoanthropologists Bernard Wood and Mark Collard. These scientists looked at the criteria their colleagues had used to place various very early hominid fossils into the genus Homo, and they rapidly determined that those criteria were deficient. Instead of starting with the mass of fossils that had been allocated to our genus, and rationalizing why they should all be classified in Homo as Louis Leakey and his successors had done, Wood and Collard began at the other end—with the defining species, Homo sapiens—and worked outward from there. Starting from scratch in this way, they concluded that any member of a morphologically coherent genus Homo had to conform to a set of criteria (including body size and form, reduced jaws and teeth, and lengthened developmental schedule) that excluded all the australopiths. What’s more, these criteria also excluded all of the fossils variously allocated to Homo habilis, Homo rudolfensis, and “early Homo.”

  Unfortunately, Wood and Collard also recommended that the expelled fossils should be rehoused within Australopithecus, something that made this genus even untidier than before. But that situation was alleviated to some extent when Meave Leakey and colleagues suggested a couple of years later that 1470 (and, by extension, Homo rudolfensis as a whole) should be assigned to their new genus Kenyanthropus, based on facial similarities to that equally imponderable skull from west Turkana. To create not only a new species but an entire new genus in this way was a brave move on the part of these scientists, especially given that their type specimen was far from wonderfully preserved; but it was a very necessary step, and with any luck it heralds a more realistic approach to hominid taxonomy by future researchers. Meanwhile, the expulsion from Homo of the rag-bag of fossils we’ve just been looking at makes for a much tidier notion of our own genus—although it still embraces a long time and quite a wide variety of morphologies.

  FULL-TIME BIPEDS

  When, in 1894, Eugene Dubois described the ancient hominid Pithecanthropus (now Homo) erectus from the site of Trinil, in Java, he knew it was old: part of a fossil fauna that contained not only many species but also many genera of animals that
are now extinct. As for the hominid itself, he only had a couple of teeth, a skullcap, and some thigh bones that looked remarkably human to go on. Indeed, paleoanthropologists still debate whether the leg bones and the much more primitive-looking skullcap are properly associated. The skullcap was long and low, and had contained a brain of about 950 cc in volume. Its shape reminded many of the braincases of the later Neanderthals, the only other extinct hominids known at the time, though it was much smaller. The Neanderthals had, on average, brains as big as or maybe even bigger than ours (which have a mean value of around 1350 cc). In contrast, the skullcap was strikingly different from that of modern humans, with a strong brow ridge overhanging the (missing) eye sockets at the front, and a distinct angulation at the back. The leg bones, though, were very humanlike and firmly indicative of upright posture, which is why Dubois named this species as he did.

  Advances in dating have allowed us to determine that the Trinil fossil is between about a million and 700 thousand years old, and subsequent discoveries elsewhere in Java have revealed that Trinil Homo erectus is part of an endemic hominid group (which included the famous Peking Man) that flourished in its eastern Asian redoubt from perhaps as much as 1.8 million years ago until as recently as 40 thousand. Although there is a good bit of variation among the fossil specimens concerned, it seems reasonable to embrace them all within Homo erectus. All share regional characters that clearly differentiate them from African and European hominids in the same time range.

  Nonetheless, Ernst Mayr had insisted that Homo erectus was simply the middle stage in the evolving lineage that led from the australopiths to Homo sapiens, and many paleoanthropologists continue to agree with him. As a result, a motley assortment of fossils in the 1.9- to 0.4-million-year range has subsequently been attributed to this species, largely based on their “intermediate” age, rather than on what they actually look like. And while by now it’s generally agreed that there are no Homo erectus fossils known from Europe, many scientists still like to refer to a group of fossils they call “early African Homo erectus.” This is, however, to push the notion of Homo erectus beyond reasonable biological limits; and a preferable name for the early African forms is Homo ergaster (“work man,” in rather Engels-like acknowledgment of the stone tools it made), a name bestowed on a 1.5-million-year-old mandible from East Turkana in 1975. If the truth be told, even the fossils sequestered away in Homo ergaster make up a pretty varied bunch. But its members do seem to belong to a fairly coherent larger group. Until the details are sorted out, the species Homo ergaster provides a fairly convincing umbrella for all of them.

  THE TURKANA BOY

  Until the mid-1980s, the iconic Homo ergaster specimen was a cranium known as KNM-ER 3733, which was discovered in 1.8-million-year-old sediments to the east of Lake Turkana in 1975. Ancient as it is, it does not in the least resemble anything we know from earlier times. Although its face sits boldly at the front of a weakly inflated cranial vault, it does not strongly project like that of an ape. It would, however, have possessed a somewhat protruding nose, and this is a striking departure from the flattened mid-face that characterizes the living apes and was also present in the australopiths. Its cranial vault had contained a brain of about 850 cc, well in excess of the brain size estimated for 1470, and not far shy of the much younger specimen from Trinil. Altogether, here for the first time was a hominid skull that anticipated what was to come, rather than harking back to the past. This was something that deserved consideration as a member of the genus Homo. The 3733 cranium lacked all teeth but one, but in conjunction with other partial skulls and teeth from East Turkana it established that by 1.8 million years ago some east African hominids had reached an entirely new plateau—what many paleoanthropologists would call a new “grade”—that is also exemplified by Homo erectus.

  Quite how distinctive that new grade was did not become fully evident until 1984, when fieldwork on the west side of Lake Turkana turned up most of the skeleton of an adolescent male known technically as KNM-WT 15000, but more widely familiar as the “Turkana Boy.” Before this discovery, various hominid postcranial bones had been recovered to the east of the lake, but apart from one partial skeleton that was riddled with pathologies, all such finds had been of separate elements—and there was no way to know for sure with what kind of hominid they were associated. Here, though, was the almost entire skeleton of an unfortunate individual who had died prematurely, face down in lakeside swampy mud, some 1.6 million years ago. Fortunately for us, his remains had been covered by soft protective sediments before they could attract the attention of scavengers. The result was a bonanza to paleoanthropologists. Because now, for the first time, fossils were available to them that showed just how a single Homo ergaster individual had actually been built.

  Skeleton of KNM-WT 15000, the “Turkana Boy,” from Nariokotome, northern Kenya. About 1.6 million years old, this spectacular specimen is the only reasonably complete skeleton we have of a Homo ergaster from East Africa, and though its brain was only of modest size it shows basically modern body proportions. Drawing by Don McGranaghan.

  It is a minor inconvenience that the Turkana Boy died before achieving maturity, complicating the task of reconstructing just what an adult Homo ergaster would have been like in life. Modern human children grow and mature very slowly compared to young apes (and australopiths), and they undergo an “adolescent growth spurt” beginning around the developmental stage at which the Turkana Boy died. It is reckoned that the Boy had stood about five feet three inches tall when he perished, and that if he had been poised to develop on a modern human schedule he would have stood about six feet one inch tall on achieving maturity. Tall, slim, and weighing maybe 150 pounds, in life he would have been a far cry from his small-bodied and stocky bipedal ape predecessors.

  But there is also a major scientific advantage to his immaturity: we are able to see that the Boy had not developed as we do. Although his teeth had erupted and his bones had knit to about the degree you see in a modern 12-year-old, the painstaking process of counting the growth increments in his teeth under powerful microscopy indicated that he had actually lived for only about eight years. Evidently, his developmental schedule had been fast; and, although it was already modified in our direction, it had resembled that of apes more closely than it did that of modern humans. This in turn implies that, when he died, the Boy had already completed most of his growth. As a result, it’s looking improbable that, even if he had lived a lot longer, he would ever have come close to hitting the six-foot mark.

  Still, most modern humans don’t hit that mark either; and development aside, what is most remarkable about the Boy is that his skeleton presents a striking contrast to that of Lucy and other bipedal apes. The Boy was tall, with long legs that contributed importantly to basic body proportions that are close to our own. Some echoes of the past remain; but in all the essentials we see a creature not too dissimilar from ourselves, at least below the neck. Here, at last, is a hominid adapted to striding out across the open savanna, far from the shelter of trees. Gone are the “have your cake and eat it” ambiguities of the bipedal ape skeleton. The Boy’s body is that of an obligate biped, rather than simply a facultative one: it is the body of a creature that was committed to upright bipedality as a way of life, rather than one that simply had this way of getting around as an option.

  To put the situation another way, the Boy and others like him had adopted the savanna as their home. By 1.6 million years ago, grasslands had already become widespread in Africa, although those open, Serengeti-style savannas where the view goes on forever were still several hundred thousand years in the future. The environments through which Homo ergaster moved still largely resembled the mosaic of the past, with larger or smaller patches of grassland interspersed with clumped or scattered trees, true forest in hollows and along watercourses, and swamplands along lake margins such as the one on which the Turkana Boy died. But the new body form certainly reflects—or permitted, or even mandated—a novel w
ay of exploiting the environment, with an emphasis on the resources available in the more open areas.

  The definitive abandonment of the trees is reflected throughout the Turkana Boy’s skeleton. For example, the extravagantly broad pelvis of Lucy had disappeared, apparently in concert with a lengthening of the leg. For while Lucy had needed a wide pelvis whose horizontal rotation could counteract excessive dropping of her center of gravity as she swung each leg forward, the Boy’s long legs provided an alternate means of achieving the same thing. Compared to ours the Boy’s arms were longish, but they were a far cry from those of apes. At the upper end they fit into a shoulder socket that faced out, like yours and mine, rather than up like that of an ape. But it also faced a bit more forward than ours does, and this has prompted speculation that the Boy’s throwing capacities would have been limited. Sadly, the skeleton preserves few hand or foot bones; but the fact that a hominid like the Turkana Boy was striding around the Turkana Basin some 1.5 million years ago is confirmed by the recent discovery of large hominid footprints on the lake’s eastern shore. These prints demonstrate both a long stride and a basically modern foot anatomy.

  RADICAL CHANGE

  The newly evolved body form represents a giant step along the road to becoming fully human; and, however exactly it was achieved, it is entirely unanticipated in the fossil record we have to hand. For, as I’ve already intimated, there is nothing in that record that we can regard as a convincing intermediate between any australopith or “early Homo,” and the Turkana Boy. Based on the available evidence, then, the Boy does not fit at all comfortably into the expectation, derived from the Synthesis, that innovations should have appeared gradually in the hominid lineage. This discordance is hardly unprecedented: back in the mid-nineteenth century even Charles Darwin and his (otherwise) doughty defender Thomas Henry Huxley were already in deep disagreement over whether or not “Nature makes jumps.” Darwin focused on slow, incremental change, while Huxley was worried by the many discontinuities he saw in the fossil record—and in nature in general—that were inconsistent with this pattern. Darwin’s favored mechanism of natural selection provided a persuasive mechanism for gradual change, but Huxley’s reservations were based on compelling evidence. Fortunately, recent advances in molecular genetics are finally helping us to understand what must actually have happened in the origin of human body form, as well as in a host of other apparent natural discontinuities.