The Dawn of Human Culture (6 page)
Conceivably, one or the other might even represent the last shared ancestor of australopiths and chimpanzees. Their status cannot be resolved without additional, more complete fossils, and in the meanwhile, the oldest widely accepted australopith comes from the site of Aramis, also in the Middle Awash Valley. We expand on Aramis here, for it nicely illustrates both the difficulties and rewards that fossil hunters can encounter in eastern Africa.
Aramis today is an inhospitable patch of sparse vegetation and ultrahigh temperatures. Ticks, vipers, and scorpions call it home, and at first glance, it looks like an unlikely place to seek fossils. Yet an international team of scientists who began working at the site in 1992
showed that when they look hard, sometimes crawling on their hands and knees, shoulder to shoulder, for days on end, they can recover fascinating traces of ancient life: seeds, fossilized wood, insect remains, and bones of birds, reptiles, and mammals. Potassium/argon analysis of volcanic ash shows that the fossils accumulated at Aramis about 4.4
million years ago.
The hard-won finds from Aramis reveal a far less forbidding ancient landscape. A dense forest lined the river. Acrobatic colobus monkeys clambered through the trees, and spiral-horned kudu antelopes browsed on leaves near ground level. Monkeys and kudus seem to have been the most common animals, but many other species were also present. These ranged in size from tiny rodents and bats to hippos, giraffes, rhinos, and elephants. The carnivores included large cats, hyenas, and other species that we would expect in Africa, and a bear that seems oddly out of place. The same bear occurs at other ancient African sites as far south as the Cape of Good Hope, and its presence underscores how 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 47
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much Aramis—and Africa—have changed over the past four million years.
The carnivores that hunted and scavenged near the river often chewed and crushed bones, and few specimens have survived intact.
Partial skeletons are particularly rare, with one prominent exception.
This represents a creature who, to the great fortune of paleontologists, died as floodwaters rose and covered its body with a layer of silt—a crucial step on the path to bone preservation.
In November 1994, University of California graduate student Yohannes Haile-Selassie was crawling across the surface at Aramis when he spotted some broken hand bones eroding from below. When he and his coworkers scraped the subsurface, more of the skeleton appeared: a tibia or shin bone, a heel bone, part of the pelvis, forearm bones, hand and wrist bones, and part of the skull. The bones were very fragile, and a careless touch could have turned them to powder, so the excavators softened the deposit with water, and they worked with surgical precision. Their painstaking efforts eventually retrieved more than one hundred pieces of the skeleton, including a nearly complete set of wrist bones and most of the finger bones from one hand. They recovered a lower jaw nearby.
The new skeleton proved to come from the same 4.4-million-year-old australopith species that Tim White, Gen Suwa, and Berhane Asfaw had described from other find spots at Aramis just two months before. Their description, in the journal
Nature,
was based on seventeen fossils, including a lower jaw, isolated teeth, pieces of skull, and three left arm bones. The species was roughly one-half million years older than any previously known australopith, and it was significantly more ape-like. To signal its position near the bottom of the human family 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 48
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tree, White and his colleagues named it
Australopithecus ramidus,
from
“ramid,” meaning “root” in the language of the local Afar people. Later, they concluded that it was so distinct that it deserved its own genus, and they renamed it
Ardipithecus ramidus
. Ardi means “ground or floor” in the Afar language, and the new name reinforced both the basal position of the species in human ancestry and the likelihood that it spent much of its time on the ground.
In the parts that have been described,
ramidus
was remarkably ape-like even for a bipedal ape. Its canines, for example, were exceptionally large relative to its molars, and its teeth were covered by thin enamel. It was also decidedly ape-like in the power of its arms, and it probably even possessed the ability to lock the elbow joint for added stability during climbing. If the teeth and arm bones were all we had, we might conclude that
ramidus
was only an ape, but a fragment from the base of the skull suggests that it carried its head in the human (bipedal) position. We will know just how bipedal when White and his colleagues describe the leg and foot bones of the partial skeleton.
Bipedalism is amply documented for the next youngest australopith, which Meave Leakey and her paleoanthropologist colleague Alan Walker described in 1995 from the sites of Kanapoi, southwest of Lake Turkana, and Allia Bay on the lake’s eastern margin. They named the species
Australopithecus anamensis,
from “anam,” meaning “lake” in the language of the local Turkana people. Potassium/argon dating shows that
anamensis
lived near Lake Turkana between 4.2 and 3.8 million years ago (Figure 2.3). Accompanying animal fossils show that the environs were wooded, but trees were probably sparser than they were at Aramis.
The bone sample of
anamensis
includes thirteen partial jaws, fifty isolated teeth, a piece of skull from around the ear region, two 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 49
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bones of the arm, a hand bone, a wrist bone, and the
pièce de résistance,
a tibia or shin bone. The jaws and teeth show that
anamensis
retained relatively large canines, but it also had the broadened molars and thickened enamel that mark virtually all later australopiths. The arm bones suggest that it preserved an ape-like ability to climb, but the tibia shows even more clearly that it was habitually bipedal on the ground. In people, in contrast to chimpanzees, the flat, articular surface at the knee end of the tibia is almost perpendicular to the shaft, and the shaft itself is heavily buttressed near both ends (Figure 2.6). These and other features allow people to shift their weight from one leg to the other during bipedal movement, and they are all present in the
anamensis
tibia.
Together, then, the teeth, the arm bones, and the tibia unequivocally finger
anamensis
as a bipedal ape.
On known parts,
anamensis
closely resembled
Australopithecus
afarensis,
which occurred in the immediately succeeding time period, and when
anamensis
becomes better known, it may turn out to be simply an early version of
afarensis
. Since
afarensis
was recognized first, its name would be applied to both species.
Afarensis
illustrates the bipedal ape character of the australopiths more clearly than any other species, because it is known from virtually every bone of the skeleton, often in multiple copies. That we know
afarensis
so well is due almost entirely to the efforts of Donald Johanson and his coworkers beginning in 1973 at Hadar, immediately north of Aramis in Ethiopia, and to the work of Mary Leakey between 1974 and 1979 at Laetoli, 45 kilometers (27 miles) south of Olduvai Gorge in northern Tanzania. At one small site, Johanson’s team recovered forty percent of the skeleton of a single individual (Figure 2.7), whom they immortalized as “Lucy,” from the lyrics of a Beatles tune 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 50
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nearly right angle
knee end
between the shaft of
the tibia and the
surface that articulates
with the femur
more
buttressing
oblique angle
less
more
buttressing
butressing
living human
chimpanzee
ankle end
Australopithecus
anamensis
(Kanapoi)
FIGURE 2.6
Front views of tibias (shin bones) of a chimpanzee,
Australopithecus anamensis,
and of a living human (redrawn after M. G. Leakey 1995,
National Geographic
190 (9), p. 45).
that was popular at the time. A partial skeleton is worth far more than the sum of its parts, because unlike isolated bones, it permits anthropologists to reconstruct bodily proportions, including, for example, the length of the arms relative to the length of the legs. At another small site, Johanson’s team found more than two hundred bones from at least nine adults and four juveniles who have been dubbed the “First Family.”
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“Lucy”
FIGURE 2.7
Left: The forty-percent-complete skeleton of “Lucy” (
Australopithecus afarensis
) from Hadar, Ethiopia (drawn from a photograph in M. H. Day 1986,
Guide to Fossil Man.
Chicago: University of Chicago Press, p. 250). Right: a reconstruction of the entire skeleton based on mirror-imag-ing and on other specimens of the same species (drawn after K. F. Weaver, 1985,
National
Geographic
168, p. 564).
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Together with fossils from other sites, they allow highly reliable estimates of variability within
afarensis,
including the degree of sexual dimorphism.
Based on the Hadar and Laetoli samples, Johanson, Tim White, and Yves Coppens defined
afarensis
in 1978, and they took the name from the Afar region of Ethiopia that includes Hadar, Aramis, and other key fossil sites. Potassium/argon analysis shows that Hadar fossils of
afarensis
accumulated between about 3.4 and 2.9 million years ago and that the Laetoli fossils are somewhat older. They take the species back to roughly 3.8 million years ago (Figure 2.3). Thus, even if
anamensis
is kept separate,
afarensis
spanned an interval of about a million years, and it changed little over this long span. At Laetoli, it occupied a dry environment, with few trees, but at Hadar it enjoyed generally moister, more wooded conditions. It was thus flexible in its environmental requirements.
Afarensis
had a small ape-sized brain that may have been even smaller on average than the brains of
africanus
or
robustus
. It shared their relatively small body size, but it was much more dimorphic. Males not only averaged perhaps fifty percent taller and heavier than females, they also had significantly larger canines. In both males and females, the jaws protruded farther forwards below the nose than in any other known member of the human family, and body proportions were intermediate between those of apes and later humans. Thus, the arms were very long relative to the legs, and the forearm was particularly long and powerful. Combined with ape-like curvature of the finger and toe bones (phalanges), the arms imply an ape-like agility in the trees.
At the same time, in all key respects, the pelvis, the leg, and the foot demonstrate bipedalism (Figure 2.8). The pelvis was shortened from 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 53
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short, broad
pelvis
femur
slanting
inwards
towards
the knee
valgus
angle
between
the femur
and the
tibia
non-opposable
big toe
modern human
Australopithecus
chimpanzee
afarensis
FIGURE 2.8
Lower limbs of a modern human, of
Australopithecus afarensis
, and of a chimpanzee (redrawn after D. C. Johanson & M. E. Edey 1981,
Lucy: The Beginnings of Humankind
. New York: Simon and Schuster, p. 157).
top to bottom and broadened from fore to rear to center the trunk over the hip joints and thereby reduce fatigue during upright, bipedal locomotion.
The femur slanted inwards towards the knee and formed a distinct (valgus) angle with the tibia so that the body could balance on one leg while the 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 54
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other was off the ground. And the foot had the expanded heel, upward arch, and non-divergent (non-opposable) big toe that are essential for human walking. In humans, each step involves a heel strike, followed by the planting of the foot over the arch, and finally by a pushing off from the big toe. If there were any doubt about this sequence in
afarensis,
Mary Leakey laid it to rest in what for anyone else would have been the discovery of a lifetime. In her excavations at Laetoli, her team uncovered a 27-meter (89-foot) long trail of footprints left by two
afarensis
individuals strolling together on a mushy surface that hardened about 3.6 million years ago. In heel strike, arch, and non-divergent big toe, the prints match ones that living humans make when they walk barefoot on a soft substrate.
If paleontologists wanted to construct a bipedal ape from scratch, they could probably not produce a more persuasive species than
Australopithecus afarensis,
and nothing could provide more compelling evidence that humans descend from apes. For opponents of this idea,
afarensis
is an even more formidable foil than the flamboyant Clarence Darrow, who found himself defending evolution in a Tennessee court half a century before
afarensis
was discovered.
* * *
