In deference to tradition, we will return to the gracile Olduvai hominid and other such fossils in chapter 5, when we look at the evidence for the origin of the genus Homo. Meanwhile, though, the robust australopith from Olduvai became simply the first of many such fossils to be published from sites in Tanzania, Kenya, and Ethiopia. In the 1960s, expeditions to contiguous regions of southern Ethiopia and northern Kenya turned up evidence of fossil hominids in the time frame between about 2.6 and 1.5 million years ago. Many of these were “hyper-robusts.” The earliest of them, in the 2.6- to 2.0-million-year bracket, came from the Omo Basin in southern Ethiopia, and were pretty fragmentary. Still, the jaw fragments were massive; and because they contained the same combination of huge, flat molars and tiny front teeth seen in Nutcracker Man, they were generally assigned to Paranthropus boisei—though one toothless jaw some 2.6 million years old received the name Paranthropus aethiopicus, for the country in which it was found.
Just to the south, on the eastern shores of Lake Turkana in northern Kenya, somewhat younger (1.9- to 1.5-million-year-old) robust australopiths began to show up in the late 1960s. These included one pretty complete, albeit toothless, robust skull that looks rather different from the Olduvai robust specimen, with a much broader and shorter face. Still, its dental proportions would have been basically similar and it, too, was assigned to the species Paranthropus boisei. Interestingly, we now know a frontal bone from East Turkana that looks just like its counterpart in the Olduvai cranium, and different from its Kenyan coeval. So the betting might be that we have more than one kind of robust australopith represented in the Turkana Basin at around 1.9 million years ago. Whatever the case, all of the East African hyper-robusts had similarly huge molar teeth that recent isotopic analyses have suggested were used to process large quantities of low-quality plant foods such as grasses and sedges. Apparently their diet was much more specialized than that of their South African relatives, and they may have been an exception to the rule of australopith omnivory.
One of the most exciting robust australopith finds at East Turkana was made in 1970, with the discovery of a partial skull of an individual who had been much smaller than the owner of the toothless robust skull, but who had belonged to the same species. Here at last was good evidence of sexual dimorphism—marked size differences between males and females—in robust australopiths. This find put an end, once and for all, to any lingering desire to categorize the robusts and graciles as males and females of the same kind of hominid, for it showed that female robusts didn’t look like graciles, but rather like smaller versions of the robust males.
The “Black Skull,” KNM-WT 17000, from Lomekwi, Kenya. Some 2.5 million years old, this is the most complete skull we have of the species Paranthropus aethiopicus, the earliest species of the “robust” australopith lineage.
Work continued in East Turkana through the 1970s, but in the 1980s the focus of attention in the Turkana Basin moved to the western side of the lake, where slightly older fossil-bearing deposits were coming to light. In 1985 a famous specimen dubbed “the Black Skull” turned up. This specimen has many of the cranial characteristics of Paranthropus boisei, but its face is longer and concave in profile, and the braincase shows marked sagittal cresting at the very rear. The consensus rapidly developed that here was a form, ancestral both to P. boisei and to the South African robusts, which took the robust lineage back in time to 2.5 million years ago. Largely as a matter of convenience, it has been given the name of Paranthropus aethiopicus (after that jaw from Omo). At the other end of the time range, a 1.4-million-year-old skull from a place called Konso in southern Ethiopia has been identified as the last-known survivor of Paranthropus boisei—indeed, the last-known australopith of any kind. By that time, members of our genus Homo were all over the landscape; and indeed, advanced stone tools of the kind otherwise associated only with Homo fossils are also known from the Konso deposits.
Despite their strikingly different histories of interpretation, the eastern and southern African records are thus both beginning to give us a picture of the australopiths as a vigorous adaptive radiation, in which a whole variety of different early biped species were actively experimenting with ways to exploit their hominid heritage. Yet as far as we can tell, the basic pattern of adaptation—bodies light enough for agility in the trees, so fairly small, with a broad pelvis and short legs; omnivorous habits and with mobile forelimbs; relatively small-brained yet manually adept—persisted, even after the invention of stone tools announced that these revolutionary yet physically archaic (compared to us) creatures had developed an entirely new way of perceiving and dealing with the world. Theirs was truly a successful physical and behavioral strategy; and although it bridged the gap between the ancestral existence in the forest and the future occupation of open territory, it cannot be described as in any way “transitional” between the two. It was a way of life entirely unto itself, and it lingered on well after the point at which recognizable members of our genus Homo had come on the scene. Still, eventually the australopiths succumbed to those closer human relatives. Evidently, early Homo were invincible competitors, even if they were in competition for only a portion of the resources that the australopiths had exploited.
But even then the picture is not yet complete. In 2001, paleoanthropologists from the Kenya National Museums in Nairobi announced the discovery of a tantalizingly different fossil hominid, from sites to the west of Lake Turkana dating from 3.5 to 3.2 million years ago. The principal specimen is a crushed and cracked cranium that, even allowing for some distortion, is different from any of the other hominids known from the Turkana Basin. The chewing teeth seem to have been thick-enameled but quite small; and the skull itself is notably short-faced, hence the name Kenyanthropus platyops (“flat-faced Kenyan man”) that the finders chose for their new fossil. Sadly, the condition of the specimen does not allow us to say much more about it; but the describers noticed distinct similarities to a skull known from much later deposits at East Turkana. Identified by the unromantic moniker of KNM-ER 1470 (its museum catalog number) this 1.9-million-year-old fossil (with a brain size a bit above the australopith range) had gained some notoriety in the early 1970s as the first skull to be discovered of Homo habilis—thus appearing to prove the reality of this species, since it didn’t look like any known australopith. The unfortunate truth, however, is that this specimen also is so poorly preserved that it is difficult to know what to do with it. We’ll look more closely at this fossil in the next chapter; suffice it to say here that placing it in Kenyanthropus seems for the time being to make eminent sense, if only because it sits very uncomfortably with either Australopithecus or Homo.
Clearly, a lot more was going on in human evolution at the end of the Pliocene than just the gradual refinement of a single central hominid lineage. The number of australopith species that we recognize from this late stage is multiplying, especially with the finding of forms such as Australopithecus sediba which appears to have a number of advanced features, especially in the pelvis, that does not at all have the dramatic sideways flare that we saw in Lucy’s. This is also the time when the first claimed members of the genus Homo begin to appear, and whether or not they deserve this title it is becoming abundantly clear that this was a time of great evolutionary ferment among our precursors. The hominid stage was packed with actors, all pushing and shoving for the limelight; and the only thing we can be sure of is that the australopiths ultimately lost out.
FIVE
STRIDING OUT
People have referred to themselves as “human” since long before anyone had the faintest idea that our species is connected to the rest of the natural world by an extended series of long-vanished intermediate forms. And, at least until the notion came along that all living things are connected by ancestry, there was no compelling reason to define the term “human” precisely. This is why, a century before Charles Darwin published On the Origin of Species, the great Swedish natural historian Carolus Linnaeus was content to bru
sh off Homo sapiens with the remark nosce te ipsum (know thyself). (Linnaeus gave us the system of classifying organisms that we use today, and one of his great innovations was to furnish diagnostic physical characters for every other species he named.) Clearly, Linnaeus and his contemporaries felt that our species is so distinct from all other living creatures as not to require formal delineation. And who could blame them for that? Given what was known of our zoological context in the eighteenth century, defining exactly what humans are simply wasn’t a practical scientific problem, even though it had long entertained philosophers.
Today, though, it’s different. For while we are the only “human” beings currently alive on Earth, we now know we have a whole range of close relatives—much closer than the apes—that are now extinct. What’s more, those fossil relatives become more and more unlike ourselves as we trace them back in time. This naturally raises the matter of when exactly our precursors became “human,” a query that obviously leads one also to ask just what changes that transition may have involved. But while these questions are obvious ones to pose, and have been asked regularly for well over a century, this doesn’t mean they have been answered to everyone’s or even to anyone’s satisfaction. “Human” means different things to different people, and even to the same person in different situations. For example, I am happy to use the term “human evolution” when referring to the entire history of humankind, back to its common ancestor with today’s great apes. In that context, the term “human” is pretty much equivalent to “hominid.” But does this really imply that all hominids are “human”? I, for one, would be very reluctant to use this term to describe any of the bipedal ape species that populate the first few million years of that history; and indeed I only find anything I might want to call “fully human” at the very uppermost tip of the human evolutionary tree. But that’s just an opinion, and there’s plenty of room for legitimate disagreement here—there is certainly no official, or even generally accepted, definition of this elusive word “human.” Remarkably, we have hardly advanced in this respect since, some two and a half centuries ago, Linnaeus’s almost exact contemporary Samuel Johnson defined human in his great English Dictionary as “having the qualities of a man,” and man as “a human being.” Still, even though paleoanthropologists are a famously argumentative breed, it’s probably fair to say that most of us broadly concur that the first creatures we can in some meaningful sense call “humans” are the most ancient representatives in the fossil record of our own genus Homo.
Unfortunately this consensus in principle hardly clarifies matters much in practice. For there is no agreement on what the “qualities of a man” actually are, even in the relatively simple terms that necessarily apply to fossil forms known only from bones and teeth. As a result, there is a lot of confusion about exactly which fossils should be placed in the genus Homo. To understand the current state of play, we need to return to history for a moment. As we saw in chapter 4, back in the 1960s Louis Leakey and his colleagues extended the definition of the genus Homo back beyond Homo erectus by whatever it took to include in it the gracile “handy man” fossils from those 1.8-million-year-old rocks at the bottom of Olduvai Gorge. Although the partial lower jaw that Leakey and his associates deemed the holotype of Homo habilis didn’t look vastly different from its counterparts among the gracile Australopithecus fossils from South Africa, Leakey felt that some fragments of braincase indicated a brain a bit bigger than was typical of the latter (though it was still smallish, at under 700 cc). In addition, the lower jaw was putatively associated with a partial foot of what had clearly been an upright biped, with an in-line great toe and nicely sprung arches. At the time there was nothing remotely comparable to this foot so far back in the hominid fossil record, and its features fit nicely with Leakey’s long-established predilection for the notion that the roots of our genus lay way back in time—just as the crude stone tools found in the same sediments matched perfectly with his attraction to the idea of “Man the Toolmaker.” So it was that the morphological concept of the genus Homo became stretched to include some very ancient morphologies indeed.
It took a few years for paleoanthropologists to become comfortable with the idea of embracing the rather archaic-looking Olduvai hominid within our own genus. But once they came around to the rather bizarre notion that the genus Homo could somehow accommodate a range of morphologies extending all the way from modern Homo sapiens to the ancient Tanzanian fossils, the way was open for them to start including in Homo habilis a motley assortment of specimens from other African sites. The process started in 1972, when the toothless 1.9-million-year-old cranium KNM-ER 1470 from east Turkana was discovered, and shortly thereafter hailed as the best-preserved Homo habilis skull yet. Its allocation to Homo was made largely on the basis of an impressive estimated brain volume of about 800 cc (later reduced to 750 cc); but as noted the specimen is rather poorly preserved, and it is still hard to know quite what kind of hominid it represents. The discovery of 1470 was followed by a flowering of other hominid fossil finds in eastern Africa, and diverse cranial and postcranial specimens, from Olduvai Gorge and East Turkana—and as far away as South Africa—were subsequently shoehorned into Homo habilis. As each of these fossils was engulfed, the plasticity of the genus Homo appeared even greater.
Ironically, even before the extreme untidiness of this assemblage of fossils became too blatant to ignore, our old friend 1470, the very hominid that had convinced most paleoanthropologists that Homo habilis was a reality to be accepted, became the standard-bearer of a new name. In the mid-1980s a Russian paleoanthropologist renamed this fossil Pithecanthropus rudolfensis (oddly using Eugene Dubois’s ancient genus name, rather than the universally accepted Homo). Within a few years, other paleoanthropologists started to pick up on the species Homo rudolfensis; and, in parallel with the ballooning of Homo habilis, this second ancient species of Homo acquired new exemplars in Kenya and even as far afield as Malawi. Some of these fossils are as old as 2.5 million years, but most date from around 2.0 million years ago or a little less, and all are pretty fragmentary.
The partial skull KNM-ER 1470, from East Turkana, Kenya. Some 1.9 million years old, this individual had boasted a brain of about 750 ml in volume, larger than is typical for australopiths; and its discovery convinced many paleoanthropologists that Homo habilis was indeed a real species. Drawing by Don McGranaghan.
The critical time span between two and two and a half million years ago also coincided with that of a number of finds in eastern Africa that their discoverers, presumably a bit worried about the increasing disarray of Homo habilis (and indeed, of Homo rudolfensis), diplomatically preferred to allocate simply to “early Homo.” Before the remarkable finding of those cut-marked bone fragments at Dikika, the 2.5-million-year date of the oldest of these fossils coincided pretty closely with the earliest evidence for the use of stone tools, and this was something that fed back powerfully into both the “early Homo” and the “Man the Toolmaker” ideas. Still, putting any of these fossils in our genus is a bit of a stretch, based solely on their preserved anatomy; and, as new evidence accumulates, this coincidence is beginning to look less like a red herring than as the basis for a self-fulfilling prophecy that sent paleoanthropologists up a blind alley.
Fortunately, we don’t have to wait long after the two-million-year point to start finding fossils that really do merit inclusion in Homo because of all the features they specifically share with us. We’ll discuss them in a moment; but first I should point out that, unsettlingly, it is hard to know right now where those strikingly new and innovative fossil relatives came from. There is little to connect them directly to any of the “archaic Homo” fossils we’ve just been talking about; and while we know a large array of australopiths—and there can be little doubt that ultimately it was one branch of these early bipedal apes that gave rise to Homo—it is really hard to pinpoint where among these diverse creatures the origin of our genus lay. To put the situation in a nutshell, there is n
ot one fossil among all those known in the period before about two million years ago that presents itself as a compelling candidate for the position of direct progenitor of the new hominids to come. All we can say right now is that the period between about 2.5 and two million years ago was clearly a time of continuing evolutionary ferment among members of the hominid family. The ongoing experimentation with the hominid potential that went on in this period is expressed in an intriguing diversity among the fossil hominids we know; but, to add to the uncertainty, it is a diversity that we still glimpse only dimly.
This uncertainty is partly due to the fragmentary nature of the evidence, but there is a good chance the glimpse is also dim because of a general reluctance among paleoanthropologists to accept even in principle that such diversity is indeed out there. One reason for this is that it is difficult to make sense of the abundant but frustratingly incomplete evidence that we have at our disposal. Sorting out species structure within a sample of fossils is the most basic of tasks a paleontologist takes on, but even at the best of times it is also often one of the hardest. The simplest default hypothesis at which you can arrive when you’re poring over a table covered in fossil fragments is that everything you’re looking at belongs to the same variable species. As such, you don’t have to decide just where any possible demarcations lie. But this is only one factor; and to a large extent the reluctance to perceive diversity also stems from an underlying expectation about evolutionary pattern. It will require just a little bit more history to understand why, in recent decades, paleoanthropologists have tended to take such an extraordinarily inclusive approach to membership in our genus.
Masters of the Planet Page 11