Lone Survivors

by Chris Stringer

  Nevertheless that “fit” was demonstrated a few years later when the CT experts Christoph Zollikofer and Marcia Ponce de León used the technique to reveal further anatomical data and to produce a three-dimensional reconstruction of the whole skull, showing that the temporal bone undoubtedly belonged with the other remains. They not only mirror-imaged the missing parts from the preserved portions but were also able to complete a hypothetical whole skull by “importing” elements from other Neanderthal children who had the appropriate parts preserved, adjusting their size virtually to complete the fit. To test the method, the researchers also virtually disarticulated a modern child’s skull of comparable maturity and demonstrated that they could re-create it very accurately, using only the portions preserved in the Gibraltar child.

  Having re-created the Devil’s Tower child’s skull digitally and on-screen, they could also remake it physically, using a technique called stereolithography. This technique was developed for industrial purposes to test the fit of parts with each other, and rather than carving out or molding a shape, objects are built up via the consecutive solidification of thin layers of a light-sensitive liquid resin. It is magical to watch the process—an ultraviolet laser beam, guided by the digital CT data, gradually materializing a solid object out of a pool of transparent resin. A skull or jaw can be re-created, one thin layer at a time, as the beam flickers across the resin, causing the liquid to progressively set. This replication method has many advantages over conventional molding and casting: it causes no damage to the surface of valuable fossils since it is noninvasive, it is remarkably accurate and nondistorting, and internal structures such as air spaces and unerupted teeth can be replicated and made visible if the transparent resin is left uncolored.

  But this was not all that was revealed. The boy’s teeth (including those still unerupted in the jaws) were also studied in great detail, and a feature that had been noted in previous research was given special attention. The front teeth in the two halves of a lower jaw are usually mirror images of each other in terms of their positions and orientations, but in the Devil’s Tower specimen, some on the right side seemed out of place. The CT images clearly showed that this boy had suffered a fracture of the lower jaw earlier in life but had survived, allowing the injury to heal quite well, and so this was unlikely to have been the cause of his early death. As already mentioned, he was large-brained, and the CT reconstruction also allowed an accurate estimate of his brain size, which would have been between 1,370 and 1,420 cubic centimeters, with a little more growth to come—a volume already comparable with those of European men of today.

  There has been much discussion about how Neanderthals grew up—whether they matured at the same pace as we do today—and the Devil’s Tower child has become an important part of the discussion. Apes have rapid brain growth before birth and relatively slower growth in the years immediately after, while we have rapid brain growth both before and after birth. At birth, allowing for body size differences, human babies already have brains that are one third larger, relatively, than those of apes, but by adulthood our brains are three times larger. The fact that we must grow our brains so much after birth is largely dictated by the limits imposed by the size and shape of the birth canal of the human pelvis, and it’s likely that there is a limiting threshold of about five hundred cubic centimeters, after which a substantial period of postbirth growth in brain volume would be required.

  This threshold must have been reached during the time of Homo erectus, which means that erectus babies probably had extended periods of immaturity compared with apes, during which the brain could continue to grow at a fast rate. For example, estimates suggest that compared with our landmarks for the average eruption age for the first, second, and third molars of about six, twelve, and eighteen years, erectus may have had a timing of about five, nine, and fifteen years respectively. But that eruption sequence marking important stages in childhood, adolescence, and the beginning of adulthood would still have been far more prolonged than in the chimpanzee, whose molar eruption ages are about three, six, and ten years.

  Essentially apes have an infancy of about five years, after which they have about seven years of adolescence and are then projected into adulthood, whereas modern humans have two extra phases inserted between infancy and adolescence: childhood (about three to seven years) and a juvenile phase (between about seven and ten years). In these phases the child is still dependent on support from its mother and older kin, for protection, for learning, and for food to grow and fuel an energetically demanding large brain. The fact that our children grow so slowly spreads out the energetic costs of rearing them and may be an important contributing factor in the greater number of children that Homo sapiens parents can sustain, compared with apes. And recent studies have shown that although adult human brain size is essentially achieved by the age of eight, the brain continues to wire up its connections and cross-connections right through adolescence, when in humans there is still much to learn culturally and socially. In addition, we mature much later than the other apes, with an adolescent phase lasting between the ages of about ten and eighteen. Neanderthals with their large brains must have had long childhoods too, although, as we shall see, there is some evidence that they reached adulthood earlier than the average for humans today—not surprising, and perhaps even essential, if most adults were likely to die before they reached forty (see chapter 6). So their learning processes would have been extended too, even if not quite to the extent we find in our species, and their brains may have had to grow to their large size at a slightly faster rate and over a shorter period—which perhaps explains some aspects of their diet. Whether their large brains endowed them with an intelligence like ours is another fascinating question.

  The brain and head size of Homo sapiens at birth are right at the limit of what is practicable for the human birth canal to withstand, and medical science may be required to assist in difficult deliveries, taking over the role of midwives in traditional societies. There are a few poignant Cro-Magnon burials of women with seemingly newborn babies, attesting to the difficulties of birth 30,000 years ago. A notebook account of the 1932 excavation of the much more ancient Tabun Neanderthal woman’s burial, in what was then Palestine, mentions the skeleton of a fetus tucked against the side of her body. Sadly, these enigmatic remains were never described, and we do not know whether this was a mistaken identification or if the material was too fragile to recover from the hard cave sediments. But the woman’s skeleton did survive and is curated at the Natural History Museum, representing the most complete female Neanderthal skeleton yet described (others from the Sima de las Palomas in Spain are in the process of study and publication by Erik Trinkaus and colleagues).

  The CT experts who worked on the reconstruction of the Devil’s Tower child’s skull also worked on reconstructing the pelvis of the Tabun woman. In the absence of the putative remains of the baby that was found with it, they instead reconstructed the fragile skeleton of a newborn Neanderthal child buried at Mezmaiskaya in Crimea, and in a spectacular demonstration of the power of CT technology, they combined the two in order to investigate Neanderthal obstetrics. They discovered that the child’s brain size was about four hundred cubic centimeters, typical of a newborn today, but the skeleton was already much more strongly built. In testing the birth process, it was apparent that the slightly wider pelvis of the Tabun woman should have eased labor. However, the baby’s skull was already Neanderthal-shaped, with a longer head and a more projecting face, suggesting that the birth process would have been as difficult for the Neanderthals as for us, involving the same unique (to humans) twisting of the baby’s body during delivery.

  In another CT study of the Tabun woman’s pelvis, this time without a direct newborn comparison, the paleoanthropologists Tim Weaver and Jean-Jacques Hublin came to rather different conclusions, arguing that the Neanderthal birth process would not have been the same as ours. The modern birth canal is widest across its breadth higher up, but then alter
s downward to become widest front to back, which is why our babies generally change position as they descend. However, in their reconstruction Tim and Jean-Jacques found that the Tabun birth canal was wide across its breadth throughout, and thus the Neanderthal birth process would have been simpler than ours, without the need for additional rotations, and perhaps less dangerous. We Homo sapiens have narrower pelvises than either our Neanderthal cousins or our African predecessors, for reasons that are still unclear, but this evidence suggests that the change in our hips led to new evolutionary demands, probably requiring both biological changes in the process of delivery and social changes in the level of support needed for modern human mothers giving birth.

  As we saw, teeth are a valuable resource in studies of our evolution, and because they are already highly mineralized, they preserve very well as fossils. Their size and shape are largely under genetic control (identical twins have similar teeth), and the form of the tooth crown has proved particularly useful in comparing fossil and recent humans. Distinctive patterns of tooth cusps and wrinkles characterize different populations today; a set of unworn teeth can be assigned to a region of the world with a fair degree of confidence using forensics. The anthropologist Christy Turner used this variation to propose an “Out of Asia” scenario for recent human evolution twenty years ago, based on the fact that the “average” dental morphology today can be found in the aboriginal peoples of southeast Asia. He argued that these populations were closest to the original modern human dental pattern, and that this indicated the location of the original source area for H. sapiens.

  However, Christy’s approach could not account for the undoubted similarity between recent Australian and African dental patterns, and I, together with my colleagues Tim Compton and Louise Humphrey, added fossil teeth from Europe to the mix, showing that an African origin for our present dental variation was still the most likely. That conclusion has been further strengthened by Christy’s former students Joel Irish and Shara Bailey, who have added many other fossil teeth to their analyses. This kind of work has also been important in studies of earlier human evolution, for example, in showing that the Atapuerca Sima de los Huesos fossils are clearly related to later Neanderthals, and that the Skhul and Qafzeh early moderns from Israel have “African” traits in their teeth.

  The famous British archaeological site of Boxgrove, near Chichester, has produced over four hundred beautifully made flint handaxes from levels also rich with the remains of interglacial mammals such as horse, red deer, elephant, and rhino. The fact that even the rhino bones showed extensive evidence of butchery led to a reevaluation of the capabilities of hunter-gatherers 500,000 years ago, in terms of their primary access to such resources. These people were not merely scavenging; they were apparently also highly capable hunters. They could secure the carcasses of large mammals for the extraction of the maximum nutritional benefit in a landscape populated by dangerous competitors such as lions, wolves, and large hyenas.

  Most of the four hundred handaxes from Boxgrove, with the British Museum curator Claire Fisher.

  The importance of Boxgrove was heightened by the 1993 discovery of a human shinbone attributed to Homo heidelbergensis, and two years later, two lower incisor teeth from another individual were discovered. Work with conventional light microscopes and scanning electron microscopes revealed a great deal of the evidence of animal bone butchery and showed that the Boxgrove tibia had been gnawed at one end by a medium-sized carnivore such as a wolf or a hyena. Microscopic studies also showed that the front surfaces of the incisors were covered in a mass of scratches and pits, suggesting that stone tools were being used as part of food processing, and the teeth were probably being marked accidentally during such activities. Together with Mark Roberts and Simon Parfitt, the leaders of the excavations, and the anthropologists Simon Hillson and Silvia Bello, I was involved in further research on the incisors using a sophisticated imaging microscope, the Alicona.

  These studies revealed other, perhaps less routine, activities. The teeth were certainly heavily worn on their crowns, suggesting that this was a middle-aged adult at death, but immediately apparent just below the crowns, much of the roots were coated in the sort of hard plaque that your dental hygienist is likely to remove during checks. This deposition indicates that the roots of these teeth must have been partly exposed above the gums during life, indicating receding gums or, more likely, that the front teeth were being strongly rocked back and forth, probably as they clenched something between them. For many years it has been argued from the strong and rounded wear on their front teeth that Neanderthals indulged in such behavior, and that food, fibrous materials, or skins were being softened or otherwise processed, with clenched teeth acting as a third hand, or a vice.

  So it certainly looks like this activity has a much deeper antiquity in Europe, and that many of the cuts and scratches on the Boxgrove incisors were made unintentionally, when a flint tool cut through material being held in the mouth. But the Alicona revealed something else: there was also an unusual series of relatively fresh, deep, and semicircular scratches on the front surfaces of both incisors, evidently made near the time of death with much greater force than the other scratches, and in a completely different direction and action. The roots were also marked by heavy cuts, indicating that these too were made near the time of death. This suggests the possibility that these more violent actions were part of the butchery of this Boxgrove individual around (and we hope for his or her sake after) the time of death.

  The famous skeleton discovered in the Neander Valley, Germany, in 1856.

  As well as carrying such scars of life and perhaps death, our teeth, as I already explained, contain important signals of our life history in their incremental lines, the dental equivalent of tree rings, which are laid down daily, rather than yearly. These lines have been studied microscopically through their surface expressions—such as the perikymata—but they can also be examined internally through broken surfaces or sections of teeth. I worked in a collaboration with anthropologists including Chris Dean, Meave Leakey, and Alan Walker to examine the growth lines in the molars of several fossil humans, including the Tabun Neanderthal from Israel, when a chip of enamel was briefly removed to apply electron spin resonance dating (see chapter 2) to it. We found that, unlike the pattern in tooth fragments of Homo erectus, this Neanderthal did overlap with the fastest developmental rates that we could find in modern molars. In 2007 another team, including Tanya Smith and Jean-Jacques Hublin, studied several teeth from a Neanderthal child from Scladina Cave in Belgium, which in terms of modern human dental development should have been nearly eleven years old when it died. However, their study determined that its actual age at death was only about eight, and the second molar was erupting significantly earlier in the Neanderthal than in modern children, thus signifying a shorter childhood and faster growth than ours.

  It was unclear whether these different results regarding Neanderthal maturation were due to inaccuracies in the different methods, to variation between individuals, or perhaps even to evolutionary changes among Neanderthals in their growth patterns. What was most needed to resolve these questions were larger and wider-ranging samples from the fossils. However, as long as microscopic techniques were dependent on having naturally broken teeth or, even less likely, a museum curator willing to have his or her precious fossils sliced up, it seemed unlikely that such samples would materialize. And in terms of nondestructive techniques, only the very finest CT scans—microCT—could even begin to reveal the minute hidden details of incremental lines; so anthropologists have been very fortunate indeed to have yet another technology available to them: the synchrotron.

  Many people have heard of the Large Hadron Collider, the world’s largest and highest-energy particle accelerator, buried in a tunnel near Geneva, in Switzerland. This is a massive example of a synchrotron, a circular chamber that progressively accelerates atomic and subatomic particles such as electrons or protons, using electrical and magnetic fo
rces. Not far away in Grenoble, France, is a smaller device that is occasionally diverted from the problems of high-energy physics to send its expensive electrons through precious fossils. The 52 kiloelectron-volt synchrotron X-ray beam has already revealed new species of beetles and ants from the time of the dinosaurs, entombed in opaque amber, and even tiny embryos of the dinosaurs themselves, within their mother’s eggs. But the technology is now also being applied to hominin fossils such as the skull of Sahelanthropus, perhaps an ancestor from over 6 million years ago, and to more recent erectus and Neanderthal fossils. Resolution can be fourfold better than the best CT scanners, down to the width of a single cell, and researchers are now queuing up to submit their fossils to the magic of the Grenoble synchrotron.

  In one of the first really significant uses of the synchrotron for modern human origins research, some of the team who announced that the Scladina Neanderthal had matured faster than we do were joined by the synchrotron researcher Paul Tafforeau, applying the technology to an early Homo sapiens child’s jaw. This was from the Moroccan site Jebel Irhoud, and, as I previously described, one of the adult skulls from there was important in my realization that Africa could have been a key region for modern human origins. The Irhoud material is currently dated to about 160,000 years, and could be even older, but opinions differ about the classification of the specimens. In my view, overall, they still lie beyond the range of modern human anatomy and are farther away than specimens of a similar age from African sites like Omo Kibish and Herto.