The Dawn of Human Culture (17 page)
Late Acheulean hand axes from southern England (redrawn after J. J. Wymer 1968,
Lower
Palaeolithic Archaeology in Britain
. London: John Baker, p. 147).
edge view (Figure 5.4). Later Acheulean people also produced more refined flake tools that are indistinguishable from those of their successors. The greater technological sophistication of later Acheulean people may have been crucial to their successful colonization of Europe.
University of Colorado archeologist Thomas Wynn has stressed that early Acheulean ability to impose even crude, two-dimensional 05 Humanity Branches Out.r.qxd 1/29/02 5:05 PM Page 143
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symmetry on a hand axe probably signals a cognitive advance over preceding Oldowan tool-makers. If so, then the wonderful three-dimensional symmetry of many late Acheulean hand axes may mark an equally important advance that now allowed the people to rotate the final tool in their minds while it was still encased in the raw rock. The nature and timing of the shift from the early to the late Acheulean remain to be firmly established, but if the transition turns out to have occurred abruptly about 600,000 years ago, it could have coincided closely with a rapid expansion in brain size that biological anthropologists Chris Ruff, Erik Trinkaus, and Trent Holliday have detected. Their analysis suggests that between 1.8 million and 600,000 years ago, brain size remained remarkably stable at roughly sixty-five percent of the modern average, but not long afterwards it increased to about ninety percent of the modern value. If a spurt in brain size and associated changes in skull form sparked the appearance of
heidelbergensis,
its emergence 600,000 years ago would signal a punctuational event like the one that we previously proposed for
ergaster
more than a million years earlier. The analogy would be especially apt if future research confirms a link between
heidelbergensis
and late Acheulean technology to parallel the one that we have postulated between
ergaster
and the origin of the Acheulean Tradition.
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to a much smaller degree, and the result is not only that living people have large brains, but also that they are highly encephalized, that is, they have brains that are exceptionally large for their body mass.
Mammals are generally more encephalized than other kinds of animals, and even the earliest mammals had brains that were about four times larger than those of like-sized reptiles. Much of the difference in size came from the development of the cerebral cortex, the folded mantle of gray matter that we think of first when we visualize a human brain.
The original mammals were probably mainly nocturnal, and their enlarged brains may have functioned to process information from multiple senses—smell, touch, and hearing, as well as sight—as they sought food and safety. Mammalian brains continued to evolve, but in most groups, encephalization—the ratio of brain size to body mass—
plateaued early on. The most conspicuous exception to this generaliza-tion concerns the Primates, which have routinely spawned more encephalized forms during their entire history, spanning the last 65 million years or so. People are of course Primates, and in this light, their extraordinary encephalization can be seen as the culmination of a long-standing evolutionary trend.
UCLA neuroscientist Harry Jerison notes that the human brain is roughly six times larger than we would predict from the relationship between brain size and body size in other mammals. Even if we restrict the survey to monkeys and apes and scale them to human body size, human brains are about three times larger than we would expect. The fossil record suggests that whenever encephalization has occurred, it occurred rapidly, and the human brain illustrates the point especially well. It may actually have been the most rapidly evolving organ in the history of the vertebrates.
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The benefits of a larger brain are obvious, but there are also costs. In modern humans, the brain accounts for only about two percent of body weight, but it consumes roughly twenty percent of the body’s metabolic resources. In addition, large brains and the constraints imposed on the birth canal by bipedalism vastly complicate birthing. A survey of other mammals suggests that human brains should be even larger at birth, or more precisely, that the human gestation period should be perhaps three months longer. Restricting it to nine months increases the likelihood that the fetus will make it out, but it also means that newborn human infants are more helpless than those of apes and other mammal species, and this imposes a further cost, mostly on mothers. Obviously the brain got bigger anyway, so the pros must have out-weighed the cons, and Jerison proposes that the most general benefit was the ability to accumulate novel behaviors, such as those we detect through time in the archeological record. Jerison also notes that a major function of the brain, and more particularly of the cerebral cortex, is to build a mental image or model of the “real world,” which in his words is “the brain’s way of handling an otherwise impossible load of information and is the biological basis for mind.” Brain expansion after 600,000 years ago presumably increased the amount of data that the human brain could process, and this in turn allowed the development of more sophisticated mental models. “Brains are, after all, information-processing organs,” notes Jerison, “and [natural] selection for brain size must have been selection for increased or improved information-processing capacity.”
Humans before 600,000 years ago surely had sophisticated mental models of their world, but rapid brain expansion about this time may have enhanced their ability to communicate these models to 05 Humanity Branches Out.r.qxd 1/29/02 5:05 PM Page 146
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others, that is, it may mark a major step in the development of human language. No topic is more intriguing and more difficult to address concretely than the evolution of language, but as Jerison points out, language is almost a kind of sixth sense, since it allows people to supplement their five primary senses with information drawn from the primary senses of others. Seen in this light, language becomes a kind of
“knowledge sense” that promotes the construction of extraordinarily complex mental models, and language alone may have provided sufficient benefit to override the costs of brain expansion.
We suggest below that the development of fully modern behavior about 50,000 years ago—“the dawn of human culture” to which the title of this book refers—may mark the development of fully modern language and that this development may have been rooted in yet another neurological shift. We emphasize the “may,” because the human brain reached its nearly modern size not long after 600,000 years ago, and if a neurological change occurred 50,000 years ago, it was confined to brain structure. Unfortunately, fossil skulls, even ones that are much differently shaped than our own, reveal little about brain structure, and arguments for neurologically driven behavioral change after 600,000
years ago cannot be tested independently of the behavioral (archeological) evidence that suggests them.
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modern humans were evolving in Africa. Occasional proto-Neanderthal fossils have long been known from sites like Swanscombe, England, and Steinheim, Germany, that are between 400,000 and 200,000 years old, but the certainty with which we can now reconstruct Neanderthal roots stems mainly from one site—the extraordinary Sima de los Huesos at Atapuerca, often abbreviated for simplicity as the “Sima” (or “Pit”).
Unlike its sister site, the Gran Dolina, the Sima was never exposed by a railway trench or any other commercial activity, and its original entrance long ago collapsed. It is a tiny chamber with a floor area of about 17 square meters (185 square feet) that can be reached today only via a 13-meter (43-foot) vertical shaft located about one-half kilometer (one third of a mile) from the entrance to the cave system. The chamber would probably be unknown to science if young men from nearby Burgos had not long been interested in exploring underground cave systems with torches and ropes. Graffiti show that they had entered the Sima system by the late thirteenth century A.D., and in the mid-1970s an exploratory group told a paleontology student that the Sima abounded in bear bones. The bones were so striking and abundant that the Sima was named for them.
The first human fossil—a lower jaw—turned up in 1976 in a jum-ble of bear bones and rocks on the cave floor. The jaw intrigued Spanish paleoanthropologists, but the Sima seemed like such a miserable place to work that they directed their attention to other nearby caves. In 1982, they returned for another brief look. “We didn’t expect to find any other human fossils,” recalls Juan-Luis Arsuaga. “We thought we were lucky with the discovery of the mandible.” But after minimal searching, the team found two human teeth and they decided to see what other trea-sures the Sima might hold. Since 1984, a handful of excavators have 05 Humanity Branches Out.r.qxd 1/29/02 5:05 PM Page 148
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descended a ladder into the cave for about a month each summer to work hunched over in tight quarters with limited oxygen. To begin with, the lack of oxygen limited work time to half-hour stretches. Rubble littered the floor of the cave, and the human bones came from deposits below a layer packed with cave bear bones. It took the team five years to remove the rubble and cave bear bones, one backpack at a time. Only then could they begin the interesting work of excavating fossil human bones.
In 1989, lights were installed and a ventilation hole was punched from the surface into an adjacent chamber. The excavators could now stay in the cave for three hours at a time. They lay atop wooden planks, and they used spatulas to carve layers of wet clay away from individual human bones, behaving more like sculptors than fossil hunters. Arsuaga likens the site to an operating room, since the surface is now entirely covered in plastic, except for the small area under excavation. The analogy goes further, for the fossils are very fragile until they have been removed and allowed to dry in the open air. Hands must move with surgical precision to avoid destroying precious specimens.
“Every season, we excavate only about 1 square meter (11 square feet) to a depth of just 20 centimeters (8 inches),” says Arsuaga, “but we find two or three hundred human fossils in that small space.”
It still took a few years of cold and cramped work to demonstrate the Sima’s potential to the paleoanthropological community.
Early on, the team recovered tiny bones from the finger tips, and Arsuaga says, “We knew there were complete skeletons in the Sima de los Huesos, but nobody believed us. Now the scientific community is interested, but in the 80s nobody was interested in that damn site.”
1992 provided the turning point, for in that year, Arsuaga and his colleagues uncovered the first human skulls.
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They first exposed a portion of forehead with a prominent browridge. Patient follow-up showed that the browridge was connected to a braincase. The excavators were elated, and they paused from their work to sip champagne in a roomier, adjacent chamber. Continuing, they turned up a large upper canine tooth and then a second braincase.
Returning to the cave for a final time before closing up for a year, crew member Ignacio Martínez insisted on digging a little more. Within half an hour, the team had recovered a face that fit onto the second braincase. A year later, they found a matching lower jaw, and the fossil skull became one of the most complete on record. In the same season, they excavated yet another skull, for a total of three.
Attempts to date the Sima are ongoing, but the best available estimates place the layer with human fossils near 300,000 years ago, about halfway between
heidelbergensis
as we define it here and the full-blown Neanderthals whom we discuss in the next chapter. The Sima people were also intermediate between
heidelbergensis
and the Neanderthals in key anatomical respects. Neanderthal skulls were remarkably large, with an average internal skull volume, or endocranial capacity, of about 1520 cc. This compares to perhaps 1400 cc in living humans. Two of the Sima skulls are relatively small, with endocranial capacities of 1125 and 1220 cc, but the third has a capacity of 1390 cc, which is comfortably within the Neanderthal range. It is in fact the largest skull yet recovered from any site older than 150,000 years. Even more striking, the Sima skulls combine widely shared primitive skull characters with ones that are distinctively Neanderthal (Figure 5.5).
Thus, like virtually everyone but the Neanderthals, they had large mastoid processes (a downward facing bony bump behind and below the ear), while unlike everyone but the Neanderthals, they had faces that 05 Humanity Branches Out.r.qxd 1/29/02 5:05 PM Page 150
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Cranium 4
Cranium 5
Cranium 6
large
forward projection of
mastoid
the face along the
process
midline
0
5 cm
