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Figure 3.6

Thumb metacarpals of a modern human, Paranthropus robustus, Australopithecus afarensis, and a chimpanzee (redrawn after R. L. Susman 1994, Science 265, fig. 3).

with a broader thumb tip, it provides attachment for three muscles that chimpanzees lack and that promote precision grasping in humans.

No tools are associated with Australopithecus afarensis, and Susman's criterion suggests that none were to be expected, since afarensis had a chimpanzee-like thumb metacarpal. Tools abound with much later Homo erectus and Homo neanderthalensis, and again in keeping with advance expectations, both had typically human thumb metacarpals. Tools also occur at robust australopith sites, but in this instance, no prediction is possible because the same sites usually contain bones of early Homo. And therein lies a dilemma. Early Homo and robust australopiths can be cleanly separated only on their teeth and skulls. The isolated limb bones that occur at most sites could come from either. At Swartkrans, these bones include a thumb metacarpal that Susman assigns to robustus, because unequivocal robustus teeth and skull parts heavily outnumber those of Homo. In form, the metacarpal is typically human, and if Susman's assignment is accepted, it could imply that robustus made some or all of the Swartkrans stone tools. The problem is that the metacarpal could represent Homo, and this might even seem likely, since it strongly resembles thumb metacarpals in much later humans. In short, thumb metacarpal form does not unequivocally finger robustus as a stone tool maker.

It remains possible, of course, that both robustus and early Homo made stone tools, but if so, we might expect two distinct tool traditions between 2.5 million years ago and the time when robustus and its east African relative, boisei, became extinct, at or shortly before 1 million years ago. Oldowan tools may be too crude to reveal separate traditions, but tools of the Acheulean Industry or Culture that replaced the Oldowan 1.7 to 1.6 million years ago were more formal, and they suggest only one evolving tradition. No one questions that Homo alone produced the Acheulean tradition, since it persisted long after the robust australopithecines had disappeared.

This is not to say that robustus made no tools, and it could be responsible for some polished bone fragments found at Swartkrans and at nearby Drimolen Cave. Experiments with modern replicas indicate that the polish formed when someone used the fragments to open termite nests. Chimpanzees savor termites, and in some groups, individuals routinely probe nests with modified branches. If robustus had developed a more aggressive twist on this basic strategy, its success could explain a peculiarity in the carbon composition of its dental enamel. Carbon comes in two naturally occurring non-radioactive forms (isotopes)-carbon 12 (12C) and carbon 13 (13C)—and in tropical or subtropical environments like those that robustus inhabited, grasses tend to be significantly richer in 13C than leaves, tubers, fruits, or nuts. The ratio of 13C to 12C in the tooth enamel of an animal reflects the ratio in its preferred foods, and a team of geochemists led by Julia Lee-Thorp of the University of Cape Town has shown that robustus enamel is relatively enriched in 13C. Robustus individuals must thus have been feeding fairly heavily on grasses or on grass-eating animals. Grass-eating itself can be ruled out, because grasses contain small, hard particles (phytoliths) that score teeth in a distinctive way, and robustus teeth lack the signature scratches. Feeding on grass-eating antelopes or other mammals cannot be dismissed, but focusing on grass-eating termites or other invertebrates would have been far less risky.

If we eliminate robust australopiths, it may seem a simple matter to determine who made Oldowan tools. Unfortunately, it is not, and to explain why, we have to back up a little and expand on the history of the Leakeys' research at Olduvai Gorge. Recall that their first human fossil represented the robust australopith, Paranthropus boisei. They found it in 1959 at site FLKI near the very bottom of the Gorge, where it was accompanied by numerous Oldowan tools and fragmentary animal bones. Understandably, they assumed that boisei made the tools and collected the bones. (They initially spoke of Zinjanthropus boisei or "Boise's east African man" in honor of one of their financial sponsors. The species was subsequently reassigned to Paranthropus, but Zinjanthropus or "Zinj" lives on in the vernacular, and FLKI is often known alternatively as FLK-Zinj.) In 1961, the paleoanthropological world was electrified when Louis, Jack Evernden, and Garniss Curtis, two pioneers in potassium/argon dating from the University of California at Berkeley, announced that "Zinj" and his tools were 1.75 million years old. The date itself stirred a revolution, since to that point, many authorities, Louis Leakey included, assumed that human evolution might have spanned no more than a million years. Suddenly there was a lot more time to accommodate both biological and behavioral change.

The discovery of "Zinj" enabled the Leakeys to obtain funding to excavate other 1.8- to 1.6-million-year-old Olduvai sites, and they soon recovered remains of a second, larger-brained, smaller-toothed, bipedal species. Louis and his anatomist colleagues Phillip Tobias and John Napier formally described it in Nature in 1964, and they dubbed it Homo habilis, or "handy man" to signal their belief that it—and not Zinj—was the Oldowan tool maker. They and others reasoned that brain enlargement fostered tool-making and that tools to process food fostered smaller chewing teeth. In reducing "Zinj" to non-technological status, they anticipated the position we have taken here. However, the years have not been kind to habilis, and there is now reason to question its status as a species and as a tool maker.

In a nutshell, the difficulty for habilis comes down to this. Between 1969 and 1975, a team led by the Leakeys' son Richard recovered numerous skulls, jaws, and other bones from deposits dated between 1.9 and 1.6 million years ago at Koobi Fora on the eastern margin of Lake Turkana in northern Kenya. The time interval was the same one the Leakeys had established for boisei and habilis at Olduvai. Some of the Koobi Fora specimens clearly represented boisei, and for present purposes, they can be placed aside. Others come from something more Homo-like, but if they are lumped with the Olduvai habilis sample, habilis becomes extremely variable. Some individuals (from Koobi Fora) had relatively large skulls and large australopith-size teeth, while others (from both Koobi Fora and Olduvai) had small australop-ith-size skulls and small Homo-sized teeth (Figure 3.7). Brain volume, estimated from eight Olduvai and Koobi Fora skulls, averaged 630 cubic centimeters (cc), but it ranged from a low of 510 cc to a high of 750 cc. The smallest and largest skulls both come from Koobi Fora, and limb bones in the same deposits imply equally large differences in body size. To some specialists, the differences suggest a persistence of the high degree of sexual dimorphism that characterized the australopiths, but to others they indicate that habilis actually confounds two species. The smaller-brained, smaller-toothed species could still be called habilis, since it more closely matches the definition that Louis Leakey and his colleagues offered in 1964. Its larger-brained, larger-toothed contemporary would require a new name, for which advocates have proposed Homo rudolfensis, based on "Rudolf," the now obsolete colonial name for Lake Turkana.

If we accept two species, only one could be ancestral to later humans including ourselves, and the choice is not easy. If brain expansion is emphasized, then rudolfensis is the clear winner, but if dental and facial reduction are accentuated, then habilis is the better candidate. Limb bones may favor rudolfensis, if we assume that some larger isolated thigh bones (femurs) represent this species. In size and shape,

Figure 3.7

Reconstructed skulls of Homo habilis from deposits east of Lake Turkana (formerly Lake Rudolf), northern Kenya (redrawn after F. C. Howell 1978, in Evolution of African Mammals, Harvard University Press: Cambridge, MA, fig. 10.9). Specialists who want to divide Homo habilis between two species would keep the skull on the left within Homo habilis, but they would assign the skull on the right to a new (second) species, Homo rudolfensis.

Figure 3.7

Reconstructed skulls of Homo habilis from deposits east of Lake Turkana (formerly Lake Rudolf), northern Kenya (redrawn after F. C. Howell 1978, in Evolution of African Mammals, Harvard University Press: Cambridge, MA, fig. 10.9). Specialists who want to divide Homo habilis between two species would keep the skull on the left within Homo habilis, but they would assign the skull on the right to a new (second) species, Homo rudolfensis.

they closely resemble the thigh bones of later humans, and they suggest that rudolfensis was significantly larger than any known australopith. In contrast, two highly fragmentary partial skeletons that are thought to represent habilis in the strict sense suggest tiny bodies (one individual may have been no more than 1 meter or 3'3" tall) and arms that may have been remarkably long compared to the legs. Given the australopith-like dentition of rudolfensis and the australopith-like body and small brain of habilis, some authorities have suggested that they should both be removed from Homo and placed in Australopithecus. This is ultimately a matter of definition, and an answer won't help us to decide whether habilis, rudolfensis, or both produced the Oldowan tools that occur in the same deposits at Olduvai and Koobi Fora. Unfortunately, for the moment, there is no way to tell, and if they actually were separate species, we can only speculate on how they differed behaviorally and ecologically.

The habilis/rudolfensis conundrum might be resolved if fossil hunters could recover enough additional bones to determine conclusively how many anatomical or size modes existed 1.9 to 1.6 million years ago. If future discoveries confirmed that there were only two, the implication would be for a single species marked by an extraordinary degree of sexual dimorphism. If new discoveries suggested four modes, we might conclude that there were two species, each predictably with two sexes. The problem might also be resolved if field workers were to recover additional, more complete skeletons to confirm body size and proportions in one or both species. But these are big "ifs," and the pace of fossil discovery suggests that they are unlikely to be satisfied soon.

It's obviously also crucial to know the history of habilis/ rudolfensis before 2 million years ago. In some features of face and brow, rudolfensis recalls 3.5-million-year-old Kenyanthropus platyops, and if the resemblance implies an ancestor-descendant relationship, rudolfensis could be removed from Homo to Kenyanthropus. This would reduce the puzzling variability in early Homo, but there are no fossils between 3.5 and 1.9 million years ago to link platyops and rudolfensis, and the differences between them in brain size, tooth size, and other aspects are profound. For the moment then, it seems wise to withhold judgment on a possible connection. What is certain is that the line (or lines) that produced habilis/rudolfensis were distinct by 2.5 million years ago, because the collateral robust australopith lineage had already emerged by this time.

Unfortunately, platyops aside, so far, there are only three fossils that may document habilis/rudolfensis before 2 million years ago. These are a skull fragment from Chemeron, Kenya, a lower jaw from Uraha, Malawi, and an upper jaw from Hadar, Ethiopia (Figure 3.8). The Hadar jaw is the most important, because it is more obviously from Homo than the Chemeron skull fragment and it is more firmly dated than the Uraha jaw. Potassium/argon analysis of overlying volcanic ash places the Hadar jaw just before 2.33 million years ago, and it resembles Homo in multiple features, including its narrowed molars, its limited forward projection (prognathism) below the nasal opening, and the parabolic shape of its dental arcade—the path that the tongue describes as it passes over each tooth beginning with the third molar on one side and ending with the third molar on the other. In the australopiths, the molars tend to be significantly broader, the upper jaw protrudes further forwards below the nose, and the dental arcade is more U-shaped. On the ground near the more U-shaped dental arcade more U-shaped dental arcade

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