The Dawn of Human Culture (4 page)
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BIPEDAL APES
Human evolution is still unfolding, and we don’t know if or how it may end. The beginning of the story is easier to construct, but it is still not entirely in focus. We know the setting—somewhere in equatorial Africa—and the time interval—sometime between 7 and 5
million years ago. It was then that the evolutionary line leading to humans separated from the line leading to the chimpanzees, our clos-est living relatives. The earliest representatives of the human line still looked and acted much like apes, and a casual observer might have mistaken them for a kind of chimpanzee. There was one essential difference, however: on the ground, they preferred to walk upright, on two legs. We know them today technically as the australopithecines, but in appearance and behavior, they could as well be called bipedal apes.
They are important to the dawn of human culture because they demonstrate humanity’s humble roots, and they show just how much we have 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 30
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changed in a remarkably short time. Measured against an individual human life span, the 5 to 7 million years of human history may seem unimaginably long, but it is very brief compared to the 3.5-billion-year history of life on Earth or even to the 25-million-year history of the monkeys and apes.
* * *
miles) southwest of Johannesburg (Figure 2.1). Dart later obtained two crates of fossil-bearing deposit from the same cave or one nearby. The deposit was a mix of sand and bone, cemented by limy glue into a rock type known as “breccia.” When Dart opened the crates, he saw breccia blocks from which numerous baboon fossils peeked out. But to his amazement and delight, one block also contained a natural cast from inside the skull of a more advanced primate. The cast was made of lime precipitated from water that once filled the skull. The lime replicated the skull’s interior, and the replica matched a depression in a second breccia block. When Dart looked inside the depression, he could see traces of bone.
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Hadar
Aramis
& Bouri
Omo Koobi Fora
West Turkana
Allia Bay
Kanapoi
Equator
ift
Tugen Hills
Rn
Olduvai Gorge
r e t
Laetoli
s e W
Atlantic
Eastern Rift
Ocean
Gladysvale, Drimolen,
Swartkrans, Kromdraai &
Sterkfontein
Makapansgat
Taung
Indian
Ocean
FIGURE 2.1
Locations of the australopith sites mentioned in the text.
Working with a hammer, chisels, and sharpened knitting needles, Dart set out to free the bone from its breccia prison. After a few weeks, he exposed the face and adjacent parts of the skull of a young ape-like creature (Figure 2.2). Its first molars were just erupting when it died, and the best current estimate is that it never reached its fourth birth-day. The individual was thus a child, but Dart estimated that if it had reached adulthood, its brain would have been only slightly bigger than 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 32
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natural cast
of the inside of the skull
deciduous
0
5 cm
canines
0
2 in
Taung
permanent first
molars (erupting)
FIGURE 2.2
The child’s skull from Taung, South Africa (drawn by Kathryn Cruz-Uribe from photographs and casts) (Copyright Kathryn Cruz-Uribe).
a chimpanzee’s and only a third the size of living person’s. At the same time, he saw that its milk (deciduous) canines were much smaller than those of a chimpanzee, and even more striking, that the foramen magnum or “large hole” on the base of the skull was in the human position.
The foramen magnum allows connections to pass between the spinal column and the brain, and in humans it is further forward and more downward facing than in apes. This is because in normal posture, only humans balance their heads directly on top of their spinal columns. On February 7, 1925, Dart described the child’s skull in the prestigious jour-02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 33
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nal
Nature,
and he assigned it to a previously unknown species that was
“intermediate between living anthropoids [apes] and man.” He called the new species
Australopithecus africanus,
or “African southern ape,”
but he regarded it as a human ancestor. Much later, others coined the name australopithecine for
africanus
and related species. The idea was to separate them formally from more advanced human species, but the separation has become blurred with time. Hence, we prefer the shortened, less formal term “australopith.”
Critics thought that Dart had been too hasty, and some suggested that the child might have become more ape-like if it had reached adulthood. Some also criticized Dart for inferring bipedalism from the skull and not from bones of the leg or foot, whose shapes are the best indication of how a creature walked or ran. Another objection stemmed partly from the Piltdown Hoax, a skull and lower jaw that had been deliberately altered to look ancient and then planted with genuinely ancient animal fossils at Piltdown, England in 1911–12. The hoax was finally exposed only in 1953, and in 1925, Piltdown implied that humans evolved their large brain early on, while
Australopithecus
africanus
suggested that the brain came late, after bipedalism had developed. In addition, some scientists discounted
africanus,
because they thought that fossils found in Java in 1891–92 proclaimed Asia, not Africa, as the cradle of humanity. The Javan fossils were genuine, but we now know that they are geologically much younger than
africanus,
and they are commonly assigned to the more advanced species,
Homo
erectus
. Finally, there was the problem that Dart could not estimate the geologic age of the Taung skull. Its antiquity remains uncertain to this day, but well-established dates for australopith fossils at other sites indicate that the Taung site formed at least 2 million years ago.
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Controversy over the Taung skull boiled on for more than a decade, and Dart was vindicated only through the efforts of his colleague and supporter, Robert Broom. Broom was a Scottish-born physician and authority on fossil reptiles, who had settled in Pretoria, about 90 kilometers (55 miles) north of Johannesburg. He was among the first scientists to examine the original Taung specimen, and he quickly accepted Dart’s diagnosis. Much more critically, however, he began a search for additional fossils, and in 1936 he was rewarded with a partial adult skull from breccia filling a cave on Sterkfontein farm, near the town of Krugersdorp, about 25 kilometers (15 miles) northwest of Johannesburg.
He subsequently recovered the knee end of a thigh bone (femur) at Sterkfontein and a second adult skull and a heel bone (talus or astra-galus) from cave breccia on the nearby farm of Kromdraai. By 1939, he had skulls which showed that adult australopiths were no more ape-like than the Taung child, and he had limb bones which demonstrated that they were bipedal. Their place in human evolution was established.
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The fossils from Taung, Sterkfontein, Gladysvale, and Makapansgat represent
Australopithecus africanus,
but those from Kromdraai, Swartkrans, and Drimolen come from a second species that specialists variously call
Australopithecus robustus
or
Paranthropus robustus
.
Paranthropus
was the name originally suggested by Broom for the fossils from Kromdraai. It means “alongside man,” and those who use it see a greater difference between
africanus
and
robustus
than those who don’t.
The South African caves contain no substance that can be reliably dated in years, and their geological antiquity must be judged from animal species that they share with dated sites in eastern Africa.
Application of such “faunal dating” shows that
africanus
lived in South Africa from about 3 million years ago until about 2.5 million years ago (Figure 2.3). It could have persisted until 2 million years ago, since no South African cave unequivocally records the interval between 2.5 and 2 million years ago. Based on faunal dating,
robustus
was present from about 2 million until shortly before 1 million years ago.
In many key respects,
africanus
and
robustus
were very similar, and both illustrate the basic nature of the australopiths, or bipedal apes.
By modern standards, individuals of both species were very small bodied.
The largest probably stood less than 1.5 meters (5 feet) tall, and the heav-iest probably did not exceed 50 kilograms (110 lbs.). Females tended to be especially small, and the sexual size difference, known as sexual dimorphism, far exceeded the difference in living humans. It was as great as or greater than in chimpanzees, and it suggests that
africanus
and
robustus
had a chimpanzee-like social organization in which males competed vig-orously for sexually receptive females. If so, like chimpanzees, they probably also had a social system in which males and females lived mainly separate lives, neither sharing food nor cooperating to raise the young.
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6
5
4
3
2
1
millions of
years ago
tugenensis
ramidus
anamensis
afarensis
platyops
africanus
aethiopicus
boisei
robustus
garhi
rudolfensis
habilis
ergaster
millions of
years ago
6
5
4
3
2
1
bipedalism
brain expansion
key
reduction of chewing teeth
anatomical
&
stone artifact manufacture
behavioral
archeological sites
traits
consumption of large mammals
FIGURE 2.3
Top: Time spans of the most commonly recognized human species that existed before 1 million years ago. Bottom: Time spans of some key anatomical and behavioral traits. Broken lines imply less secure or more speculative dating.
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Among other ape-like characteristics that
africanus
and
robustus
shared, the most conspicuous was their small brain size. In both species, adult brain volume averaged less than 500 cubic centimeters (cc). This compares to roughly 400 cc in chimpanzees and to 1400 cc in living people. Even when the averages for
africanus
and
robustus
are adjusted for small body size, their brains were less than half the size of ours. Both species also possessed very ape-like upper bodies with long, powerful arms that would have made them agile tree climbers. They differed from apes primarily in their lower bodies, which were shaped for habitual bipedal locomotion on the ground, and in their teeth.
The dental differences are important for two reasons. First, teeth and jaws strongly outnumber other fossil bones, because they are much more durable. They tell us we have australopiths even at sites where limb bones are not preserved. Second, teeth are a window on diet and other aspects of behavior. Chimpanzees and gorillas concentrate on soft foods like ripe fruit and fresh leaves that do not require heavy chewing.
Their molar teeth are thus relatively small, and they are encased in relatively thin enamel that soft foods are unlikely to wear away. In their chewing, they do not have to move the jaws from side to side with the mouth nearly closed, and they can therefore have large canines. These are particularly large in males who use them in threat displays and sometimes in violent conflict.
In contrast,
africanus
and
robustus
had greatly expanded molar teeth that were encased in thick enamel (Figure 2.4). The implication is that they often consumed tough, hard, gritty, or fibrous foods that required heavy chewing. Such foods probably included seeds that they found on the ground or bulbs and tubers for which they had to dig.
Members of both species also had small canines that would not have 02 Bipedal Apes.r.qxd 1/29/02 5:03 PM Page 38
