more parabolic dental arcade

Homo species indeterminate (Hadar Specimen A.L. 666-1)

Australopithecus afarensis (Hadar Specimen A.L. 200-1a)

Homo species indeterminate (Hadar Specimen A.L. 666-1)

Figure 3.8

Upper jaws of Australopithecus afarensis and early Homo from Hadar, Ethiopia (drawn by Kathryn Cruz-Uribe from photographs).

jaw, a Hadar team led by William Kimbel found three Oldowan choppers and seventeen flakes that had eroded from the same deposit, and when they excavated they recovered another core tool and thirteen more flakes. They also found fragments of animal bones, including one that bore a possible stone tool mark. So far, the artifacts are the oldest to have been recovered in direct association with a human fossil.

Neither the Hadar jaw nor the other two fossils that may represent Homo before 2 million years ago inform on brain size, but if stone flaking and brain expansion were closely linked, then brain expansion must have begun by 2.5 million years ago. Future discoveries may confirm this—or they may not. The Australopithecus garhi skull from Bouri, Ethiopia, which we described in the last chapter, provides fodder for doubters. This is because it anticipates Homo in its dentition, but not in the enclosure for the brain, which was no larger than in australopiths. The Bouri deposits have not yielded any stone artifacts, but they have provided animal bones that were cut and broken with stone tools. Unlike nearby Gona, Bouri lacked cobbles or other rock fragments that were suitable for flaking, and when the tool makers visited, they may have carefully conserved their implements until they could return to a locality like Gona. If so, they were thinking ahead in a way that is decidedly human. The bones they damaged include an antelope tibia shaft that was repeatedly cut, bashed, and chopped to get at the marrow, the femur of a three-toed horse that was cut when it was separated from adjacent bones and stripped of flesh, and an antelope lower jaw that was cut on the inner surface when the tongue was removed.

The implication may be, as Time Magazine suggested in April 1999, that garhi was "the first butcher." Tim White, whose team found the garhi skull and the tool-marked bones, is more cautious: "It's circumstantial evidence, and not as strong as it might be. It's possible that some [other] hominid came by and left the tools. Then a year later, a carnivore dropped the carcass of a different kind of hominid [garhi] in the same place." He continues: "What it tells you, though, is that there was a hominid in these habitats with stone tools [who was] engaged in large mammal carcass processing. That's very important. The behavior is, in some ways, more important than whether it was garhi engaging in the behavior." White calls the bone-processor a "superomnivore" to distinguish it from its predecessors who were probably more ape-like in both diet and behavior.

White and his team have scoured all the Bouri exposures for fossils and artifacts, and it will be many years, perhaps decades or centuries, before fresh erosion at Bouri provides new clues. Still, there are other like-aged east African sites to explore, and one may yet provide a larger-brained species dated to 2.5 million years ago. The discovery will satisfy those who believe that brain enlargement and stone flaking originated in an evolutionary feedback loop. If garhi, however, coexisted with a larger-brained companion, then there must have been at least three distinct human types by 2.5 million years ago—an early robust australopith, garhi, and the putative larger-brained species. We could even argue for four types, if we accept, as seems increasingly likely, that Australopithecus africanus was restricted to South Africa and disappeared there without issue before 2 million years ago.

In short, the proper metaphor for human evolution between 3 and 2 million years ago may turn out to be a bush, and the high degree of variability in habilis/rudolfensis between 1.9 and 1.6 million years ago may actually represent the tips of multiple branches that the fossil record will eventually reveal. If there was such a bush, though, natural selection had severely pruned it by 1.6 million years ago, and thereafter only two branches survived—the robust australopiths and the line that ultimately led to ourselves (Figure 3.5). By 1.7 million years ago, this line had produced a species that departed sharply from the australopiths in anatomy, behavior, and ecology, and there is no question about its assignment to Homo. Its members have been called the first "true humans," and we explore next the important step they represent on the long road to human culture.

The First True Humans

We have suggested that human evolution was characterized by a series of short, abrupt steps or punctuations, separated by long periods with little or no change. So far, we have described a possible first punctuation, which occurred between 7 and 5 million years ago and produced bipedal apes, and a better-evidenced second event, which occurred between 3 million and 2 million years ago and produced the first stone tool makers. The abruptness of each step is debatable, but the stability that followed is patent. Thus, the anatomy of the bipedal apes changed little over intervals that lasted a million years or more. The anatomy of the earliest tool makers is poorly known, but they were probably equally conservative, judging by a remarkable lack of change in the tools they produced. They may have had larger brains than the bipedal apes, but they may also have retained an ape-like upper body form and a high degree of size difference between the sexes. If so, it's probable that they continued to rely heavily on trees for food and refuge and that they had an ape-like social organization that involved little or no cooperation between the sexes. When we know them better, we may decide that for all effects and purposes, they were "technological apes."

We turn now to a third step that occurred about 1.8 to 1.7 million years ago. It is more fully documented than its forerunners, and it was at least as momentous, for it produced a species that anticipated living people in anatomy, behavior, and ecology, save mainly for its smaller brain. With this caveat in mind, its members can reasonably be labeled the first "true humans," and this is how we will refer to them here. Early on, the first true humans authored a major advance in stone flaking technology, but thereafter, both their anatomy and their artifacts appear to have remained remarkably stable for a million years or more. In this respect, they were marching to the same drummer as their predecessors.

In late August 1984, Kamoya Kimeu was prospecting for fossils along the south bank of the Nariokotome River, west of Lake Turkana in northern Kenya. Kimeu had long assisted Richard and Meave Leakey in their quest for ancient human bones, and before his retirement in 1993, he had probably found more than anyone else. On this occasion, his team had been in the field for two weeks, but their extensive fossil haul included no human specimens. They planned to move camp the next day, but while others rested or did chores, Kimeu continued the hunt. He picked a difficult, unpromising spot, a slight rise protected by an acacia tree within a sun-baked gully. The surface was littered with black lava pebbles, and any fossils that had eroded out were likely to have been trampled by local herds of goats and camels. Kimeu's chances seemed slim, but he had overcome such odds before, and he did so again now. He soon spotted a match-book-sized piece of black bone, hardly distinguishable from the surrounding pebbles, and when he picked it up, he knew that it came from the forehead of an extinct kind of human.

Kimeu's assessment drew the Leakeys and their paleoanthropo-logical colleague, Alan Walker, to the find spot, and over the next four years, they led parties that meticulously excavated the deposits nearby. In the end, they not only managed to piece together a complete skull, they also recovered most of the skeleton that went with it. The skeleton turned out to represent an adolescent male, whom his discoverers affectionately dubbed the "Turkana Boy." Analysis of the enclosing sediments showed that the boy had died and been rapidly buried on the edge of a marsh about 1.5 million years ago. His skeleton was even more complete than Lucy's, found a decade earlier in deposits that were 1.8 million years older, and it is still the most complete skeleton from any human who lived before 120,000 years ago. Its significance matches that of Lucy, for if she left no doubt that her kind were bipedal apes, the Turkana Boy showed just as clearly that his kind were true humans.

Recall that Lucy was tiny—probably only about one meter (3' 3") tall, and she had very long arms relative to her legs. She also had an ape-like cone- or funnel-shaped trunk, which narrowed upwards from her pelvis to her shoulders (Figure 4.1). From a distance, a modern observer might have mistaken her for a kind of chimpanzee. The Turkana Boy was tall—about 1.62 meters (5'4") at time of death and

"Turkana Boy" (Homo ergaster)

"Lucy" (Australopithecus afarensis)

"Lucy" (Australopithecus afarensis)

2 ft

"Lucy" (Australopithecus afarensis) scaled to the stature of the "Turkana Boy"

2 ft

"Lucy" (Australopithecus afarensis) scaled to the stature of the "Turkana Boy"

Figure 4.1

Stature and body proportions in the "Turkana Boy" and in "Lucy" (redrawn after C. B. Ruff 1993, Evolutionary Anthropology 2, p. 55).

destined to reach 1.82 meters (6') or more if he had survived to adulthood. His arms were no longer, relative to his legs, than in living people, and he had a barrel-shaped chest over narrowed hips. From a distance our time-traveler might have confused him for one of the lanky Turkana herders who live around Nariokotome today.

Close up, our observer would soon realize his error, for the Turkana Boy had a skull and face that would startle any living human (Figure 4.2). His brain was nearly full grown, but its volume was a mere

880 cubic centimeters (cc), only 130 cc greater than the maximum in Homo habilis (including all of its possible constituents) and 450 to 500 cc below the average in living people. The size increase from habilis all but melts away when the Turkana Boy's larger body size is considered. His braincase—the part of the skull that enclosed his brain-was long and low, and the skull walls were exceptionally thick. It was the thickness of the forehead fragment that first alerted Kimeu to the kind of human he had found. The boy's forehead was flat and receding, and it descended to merge at an angle with a bony visor or browridge over his eyes. His nose was typically human in its forward projection and downwardly oriented nostrils, and in this he differed from the australopiths and habilis who had ape-like noses that were flush against the face. The nose aside, however, his face was striking for its great length from top to bottom, and his jaws projected far to

5 cm

2 in

5 cm flat, receding forehead browridge face and jaws projecting strongly forwards no chin long, low braincase flat, receding forehead long, low braincase browridge face and jaws projecting strongly forwards no chin

"Turkana Boy"

2 in

"Turkana Boy"

Figure 4.2

The skull of the "Turkana Boy" (drawn by Kathryn Cruz-Uribe from photographs and casts) (Copyright Kathryn Cruz-Uribe).

the front. His lower and upper jaws were massive, and they contained chewing teeth that were significantly larger than our own, even if they were smaller than the average in habilis or the australopiths. The bone below his lower front teeth slanted sharply backwards, meaning that he was completely chinless.

On reflection, contemplating a seemingly improbable combination of modern body and primitive head, our hypothetical observer might wonder if his companion was a visitor from an alternative universe or perhaps the product of some strange genetic experiment. In a sense, he was both, but the alternative universe was our own world long ago, and the experimenter was nature.

The Turkana Boy's skeleton provided unique insight into the body structure of his people, but in the early and middle 1970s, teams from the Kenya National Museum had already recovered two skulls, nine partial lower jaws, a much less complete skeleton, and some isolated limb bones that all closely resembled his. The specimens came from deposits dated between 1.8 and 1.6 million years ago at Koobi Fora on the eastern shore of Lake Turkana, and from the time of their discovery, they were likened to east Asian fossils that are assigned to the primitive human species Homo erectus. The antiquity of the Asian specimens is disputed for reasons we discuss below, but most if not all are probably younger than a million years. If then, as many authorities believe, the Koobi Fora, Nariokotome, and east Asian specimens should be placed in the same species, erectus would have an African origin.

The similarities between the east African and Asian fossils are unquestionable, but some specialists have also pointed to subtle and potentially significant differences. Thus, on average, the African skulls tend to be somewhat higher-domed and thinner-walled than their east Asian counterparts, and they have less massive faces and browridges. In these respects and others, they are more primitive or less specialized, and they may tentatively be assigned to a separate species for which the name Homo ergaster has been proposed. The name translates roughly as "working man," and it was first applied to some of the Koobi Fora fossils that came from deposits that also contained flaked stone tools.

The removal of the east African fossils from erectus to ergaster would be trivial if we accepted the once common notion that erectus was directly ancestral to Homo sapiens, for ergaster would then be simply an early stage of erectus. Fossils that date from after 500,000 years ago, however, now indicate that sapiens evolved in Africa while erectus continued on largely unchanged in eastern Asia (Figure 4.3). In form and geologic age, ergaster is well positioned to be the ancestor not only of erectus but also of sapiens, and this is the view we adopt here.

The ancestry of ergaster is murky, but it may have originated suddenly from habilis (or from one of the variants into which habilis may eventually be split) in adaptive response to a sharp increase in aridity and rainfall seasonality that occurred across eastern Africa about 1.7 million years ago. Alternatively, at the end of the last chapter, we noted that future research may reveal a bush of human species between 3 million and 2 million years ago, in which case ergaster could represent a branch totally separate from the variants of habilis.

millions of years ago

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