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Authors: Ian Tattersall

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One big difference between apes and humans is that apes develop to maturity much faster than we do, depriving them of the extended childhood that provides us with so much opportunity to learn. On her own, Selam cannot do much to demonstrate how quickly individuals of
A. afarensis
developed; but the expectation would certainly be that she lay on the apish side of the curve. The fossil has yet to be fully freed from the hard rocky matrix that envelops it, but as far as we can tell, her upper body structure confirms the generally arboreal features found in adult
A. afarensis.
Not only does Selam's scapula show a shoulder joint that was oriented largely upward, as a good climber needs for holding the arms above the head, but her hand also shows features associated with climbing. Ape and human hands may look superficially similar, but they are in fact constructed very differently. Apes have thumbs that are short in relation to the fingers, and the hand is long, with its major axis in line with the arm. The ape hand is more the hand of a powerful grasper than of a dexterous manipulator; and it is the kind of hand that you want if you are going to spend most of your time clambering around in
the
branches of trees. In contrast, the major axis of the modern human hand goes across the palm, and the thumb is long and can be opposed precisely to any of the other, shortened, digits. Selam's digits appear to have been long and curved.

The skull of “Selam,” the infant skeleton found at Dikika, Ethiopia. Despite its tender age of only three years, this tiny 3.3 million-year-old skeleton has yielded a wealth of information about the structure and development of the species
Australopithecus afarensis.
Courtesy of Zeresenay Alemseged.

To complete this image of a hominid that was not completely wedded to the ground, CT scans of Selam's ear region suggest that the semicircular canals of her inner ear resembled those of apes and other australopiths. These canals are important organs of balance, and their orientation reflects not only the way the head is habitually held, but how well it is insulated from movements of the spine on which it is poised. Selam's semicircular canals are reported to resemble those of apes and other early bipeds, suggesting that although her species may have been an upright walker, it was not suited for fast running—an activity in
which
it is important to maintain a fairly constant head position despite the gyrations of the body below.

But Selam is not the only surprise from Dikika. In mid-2010 the research group there came up with something that was even more remarkable. Strata dated to just under 3.4 million years ago yielded four superficially unimpressive fragments of mammal bone that proved, on close inspection using a scanning electron microscope, to bear markings that archaeologists suggested were of the kind produced only by stone tools. To understand the significance of this finding, you need to bear in mind that 3.4 million years ago is
800 thousand years
before we have any evidence of stone tools themselves. The very first stone tools we know of are reported from not very far away along the Awash Valley, but are a mere 2.6 million years old. Yet stone tools are very durable things—carnivores don't chew on them, and under most circumstances they preserve in the record indefinitely. If ancient hominids at Dikika were clobbering one rock with another to produce sharp cutting flakes so they could butcher carcasses, where are those flakes? And where are the “cores” with the flakes removed from them? It's not as if paleoanthropologists haven't thoroughly scoured the Dikika and Hadar landscapes for interesting objects of this age.

There are several possibilities as to why no stone tools have been found in these regions. One is that the paleontologists had the wrong “search image,” and simply were not finding such implements this far back in time. But even the most primitive deliberately made tools show distinctive signs of manufacture, and it's unlikely that over many years experienced searchers would totally miss pieces of stone with obvious modifications. Alternatively, the Dikika researchers suggest that the very early history of stone tool-making showed very low “intensity”: i.e., just one flake was removed per core, so that each core would show little sign of modification while the flakes themselves were rare. Another possibility is that the scratches are in fact “trampling marks” produced by the sharp hooves of grazing mammals that had stepped on the bones. But, perhaps most likely, the australopiths had simply used naturally broken stones for butchery. Experimental archaeologists have now shown that it is indeed possible to dismember a mammal carcass using stones of the kind that are routinely fractured against each other while being swept
down
rivers. Such pieces don't have the razor-sharp edges of deliberately manufactured stone tools, but they can nonetheless do the job.

Still, whatever exactly happened, under the microscope two of these slivers of bone (a piece of rib and a fragment of femur, one from an animal the size of a cow, the other the size of a goat) not only show “cut-marks” of the kind that are produced by slashing with a sharp tool while the bone is still fresh, but they also bear scratches and pits such as those made when a piece of fresh bone is scraped or bashed with a hard and pointy object. So here is a strong suggestion that early hominids, presumably
Australopithecus afarensis,
were indeed butchering the carcasses of large animals out there on the bushy Dikika landscape, some 3.4 million years ago—even if they weren't, strictly speaking, stone toolmakers.

UP THE RIVER

Dikika presumably has plenty more surprises in store for paleoanthropologists. Doubtless so also has an area, some distance upriver, that is known as the Middle Awash Valley. This region occupies a unique position in paleoanthropology. It has not produced fossils as lavishly as Hadar has, but it has yielded hominid remains that range from
Ardipithecus kadabba
at 5.8 million years ago, to the very earliest days of our own species
Homo sapiens
a mere 160 thousand years ago. No other place in the entire world registers events in hominid evolution over such an enormous span of time. The 4.12-million-year-old Ethiopian
Australopithecus anamensis
fossils are from the Middle Awash, and from relatively close by comes a geologically younger partial hominid skeleton assigned to
Australopithecus afarensis,
from a 3.58-million-year-old locality known as Woranso-Mille.

The Woranso-Mille bones are substantially earlier than any of the
A. afarensis
fossils from Hadar, and they are comparable in age to the Laetoli scraps. Unfortunately the skeleton lacks a skull or teeth, but the skeleton is said to be broadly similar in preserved portions to the later and smaller-bodied Lucy. A scapula is quite well preserved, and although its shoulder joint portion does seem to have been quite upwardly oriented, it is unlike its counterpart in the Dikika child in bearing no particular resemblances
to
its equivalent in any African ape. As for the lower part of the body, the Middle Awash researchers think that that their bigger-bodied specimen is more relevant than the diminutive Lucy for assessing exactly how
A. afarensis
walked, as smaller subjects weigh less and therefore require fewer specializations to support their body weight. This is actually pretty arguable, for one thing because it's hard to imagine a pelvis and leg structure more suggestive of bipedality than Lucy's. But it is certainly good to have a larger counterpart skeleton. The Woranso-Mille individual sadly lacks a complete leg, but by the team's estimate this hind limb had been relatively a bit longer than Lucy's. If this is indeed the case, then the Woranso-Mille fossil might fit a bit better than the Hadar materials with the sort of hominid that made the more or less contemporaneous Laetoli prints. Frustratingly, it has no foot bones, leaving lots of room for speculation on this point.

Moving up the Middle Awash geological section, some jaws from about 3.4 million years ago have also been attributed to
Australopithecus afarensis.
These are interesting, but don't tell us a lot more than we already knew. The picture becomes a lot more exciting about a million years later—which is well after
A. afarensis
had disappeared from the record at Hadar. Fossils from a 2.5 million year-old site called Bouri have been given the name
Australopithecus garhi
(“garhi” apparently meaning “surprise” in the local language). And although the specimens concerned are not that impressive—consisting mainly of some cranial fragments that include the brow and a reasonably complete upper jaw with teeth, plus a few postcranial bones—they certainly did come with a surprise attached. The Middle Awash team claimed that the arm and leg bones, combined with a slightly longer hind limb than Lucy's (as in the as-yet-undiscovered Woranso-Mille skeleton), indicated powerful limbs. And while they did not directly associate the limb bones with the skull parts, the reconstructed skeletal proportions may have been a factor in their claim that they had found a new and “advanced” form of
Australopithecus
that was directly antecedent to our own genus
Homo
(even though the teeth preserved in the palate are biggish, and closely resemble comparable specimens from Hadar). However, the official reason for this conclusion was that the Bouri fossils were “in the right place, at the right time” to play the role of ancestor for
Homo,
regardless of any
anatomical
particularities that might have tied the Bouri form in with any of its proposed descendants—or with
A. afarensis.

All of this may sound as if the Middle Awash area has somehow tantalizingly preserved the record of a steady progression of hominids at regular intervals in time. But you need to know that all interpretation of the Middle Awash materials by their discoverers has been conditioned by the underlying belief that the story of human evolution has essentially been a linear one. The idea is that a single central lineage gradually transformed under natural selection from one species into the next, until the primitive
Ardipithecus
had been transmuted into the finely burnished
Homo sapiens.
This is a perspective that gives greater significance to the age of each fossil concerned than to its anatomy. And while there is a certain logic here, it is a logic that only applies if you think of evolution as a steady chain of species running through time. You will have already gathered that this is not the only way of viewing either the evolutionary process itself, or the shape of the human evolutionary drama that resulted from it; but, as we'll see in some detail later, it is a viewpoint that has lingered with particular tenacity among paleoanthropologists.

Whatever the underlying process, it wasn't the Bouri fossils themselves that were the real surprise. An accompanying article described some mammal bones from the Bouri deposits that clearly bore cut-marks made by sharp stone flakes—and remember, this was a decade before the comparable finds downriver at Dikika. During the second half of the twentieth century, the notion of “Man the Toolmaker” had exerted a powerful attraction upon paleoanthropologists. Making stone tools, it was thought, was the key behavior that had sent humans off on their unique path, and was thus the defining attribute of humankind. Very early stone tools, over two million years old, had been reported since the 1970s from sites in Kenya and the Omo Basin of southern Ethiopia; and tools as much as 2.6 million years old were shortly to be announced from the nearby Middle Awash locality of Gona. But at the time, the Bouri cut-marked bones represented the earliest evidence of tool use by any ancient hominid—and the only hominid that could be implicated in this behavior, even though the association was not definitive, was
Australopithecus garhi—
a bipedal ape. Once it had been realized that stone tools were already being made over two million years ago, the hunt had
of
course been on for early
Homo
in the same time range, and some fossil fragments arguably attributed to early members of our genus had duly turned up. But the anatomically primitive nature of the Bouri find demanded a rethink.

No actual stone tools were found at Bouri, but those from Gona resembled the simple “Oldowan” tools (named for Tanzania's Olduvai Gorge, at which such implements were first identified) that were already known from later sites. These were small cobbles of mainly volcanic fine-grained rock that had been hit with a “hammer” stone to detach one or more sharp flakes that could be used for cutting. Often the cores themselves show signs of being used for pounding, an activity that has been associated with the characteristic torsional (twisted) fractures that occur when long bones are bashed with hard objects to extract their marrow.

Crude as Oldowan utensils might appear, they are remarkably efficient, as archaeologists have shown by butchering entire elephants using flakes an inch or two long made using Oldowan techniques. What's more, the effectiveness of these simple implements is also evident in the fact that stone tool kits barely changed for a million years after Gona times (and almost two million after Dikika), even as new kinds
of
hominids came and went. Clearly, this was a highly successful technology that did everything that was demanded of it.

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