Lone Survivors

by Chris Stringer

  As I have come to realize, we should not be looking for a single cause for Neanderthal extinction anyway; we need to take a wide view of this. The fascinating events that took place in western Europe some 35,000 years ago get most of the scientific and popular attention, but they were only the endpoints of hundreds of thousands of years of evolution and potential interaction between the lineages of modern humans and the Neanderthals (for example, ancestral populations could have been in contact intermittently in regions like western Asia). I am sure there were differences (many unknown to us) in appearance, communication, expression, and general behavior that would have impinged on how Neanderthals and moderns saw each other. So when the populations met, did they perceive each other as simply other people, enemies, alien, or even prey? And since the Neanderthals disappeared at different times across Asia and Europe, the reasons why they disappeared from Siberia might be different from why they became extinct in the Middle East, and again different from the factors at work in Gibraltar or Britain—and these factors may not always have included the presence of modern humans.

  This brings us back to one of the favored explanations for the extinction of people like the Neanderthals: behavior. I am one of those who have often invoked the behavioral superiority of modern humans over other human species as the main reason for our success and their failure, but reconstructing such behavior from the archaeological record, let alone deciding who is superior to whom, is no easy matter. In the next chapter we will look at new methods of unlocking evolutionary and behavioral insights from the fossils, and then in the succeeding two chapters we will consider what the archaeological record now seems to be telling us.

  3

  What Lies Beneath

  The fossil record of the early history of our species and our close relatives such as the Neanderthals has grown tremendously in the last twenty-five years. But what has developed at an even faster rate is our ability to unlock secrets from those fossils, secrets that tell us about the biology and the lives of those long-dead people. In this chapter I will show how new techniques let us look at the size and shape of ancient skulls, and reveal hidden structures such as the inner ear bones that can tell us about the posture, movement, and senses of vanished peoples. Now we can look at butchery marks microscopically to examine details of ancient human behavior, daily growth lines in fossil teeth to reconstruct how children grew up 1 million years ago, and we can use isotopes to reveal how ancient humans in different parts of the world exploited their environments, and what they ate. In the last twenty years traditional methods of recording the size and shape of fossil bones and teeth have been complemented and increasingly superseded by techniques that capture such information on a computer, through digitizing or scanning. The medical technology of computerized tomography (CT) X-raying has been particularly successful in extending work into anatomical structures that either are difficult to measure through traditional techniques (for example, the shape of a curved form like a brow ridge) or are otherwise inaccessible (for example, fossils hidden inside rocks or unerupted teeth in a jawbone). And the computational technique of geometric morphometrics (morphometrics simply means “measuring shape or form”) is allowing wider and more detailed comparisons of the size, shape, and even the growth patterns of fossil and recent samples.

  Most of these new techniques were not available when I began my research on human evolution, and they were still in their infancy when Recent African Origin models started to germinate in the 1980s. For example, when I made my four-month trip around Europe to measure about a hundred fossil skulls of archaic and modern humans in 1971, I carried a small suitcase full of metal measuring instruments, such as callipers, tapes, and protractors, and a camera to record the preservation and basic shape of the specimens I was studying. (A single well-preserved skull with a lower jaw might take up to half a day to record fully.) With no laptops or pocket calculators, all my data were slowly recorded by hand on paper sheets, and, without photocopiers, there was a great risk (unappreciated by me at first) that all the hard-won data on which my career depended could easily have been inadvertently lost or even stolen in the two thefts that I suffered from my car.

  When I returned to Bristol, I took months to laboriously transcribe my data onto punch cards and start the computational wheels turning on the single, massive (but in modern terms ridiculously underpowered) computer that served the whole of Bristol University. Now, a single researcher could (if he or she knew where to look) achieve the equivalent amount of data collection, which took me four months and about 5,000 miles of travel, in a few days sitting at a computer console, summoning up online measurements and CT scans. Far more sophisticated comparative analyses of skull shape than I achieved in two further years could probably be accomplished in a few more days! Still, I have no doubt that by directly studying the fossils, I did obtain insights into their nature that would not have been apparent had I been sitting at a remote workstation. Plus I had the thrill and honor of holding and studying firsthand such iconic fossils as the skulls from the Neander Valley and the Cro-Magnon rock shelter.

  The approach I used to measure and compare my samples of fossil and recent human skulls is now called conventional morphometrics, although in 1971 this was very much the standard approach and had been in use since before the time of Charles Darwin. The human skull has various points where bones meet, where muscle markings cross a bone, or where there are specific locations such as the external earhole or the widest breadth of the nasal opening. These “landmarks” are used as measuring points so that an instrument can, for example, be laid across the nose to record its breadth at its widest point, or can measure the total length of the braincase from the top of the nasal bones at the front to the farthest point away in the middle of the occipital bone at the rear. Measurements and their variations can then be directly compared between specimens, either singly or through the calculation of an index or angle, using two or more of the measurements. For example, the cephalic index (CI) was a much used ratio of the breadth of a skull to its length. This index was a basic measure of how long or broad-headed a skull was, and in some of the racist science of the last two centuries it was taken as a crude measure of “primitiveness,” on the assumption that the most backward “races” had the longest heads.

  My Ph.D. work made use of angles and indexes but also extended to the then relatively novel area of multivariate analysis, where a large number of measurements could be assessed together, with specimens compared in a computed space of many dimensions, or via a single distance statistic—a bit like a ratio or index but one calculated from many measurements combined, rather than just two. However, I realized even then that my measurements were not capturing the whole complex shape of a skull, particularly some of its curved surfaces, which were poorly marked by suitable landmarks. And it was evident when comparing small and large skulls even within a single population that they might change their relative proportions as they changed in size (the study of this is known as allometry), which was difficult to capture and visualize effectively with the techniques I had available in the 1970s.

  Today a new approach called geometric morphometrics allows far more effective visualization and manipulation of the shape of a complex three-dimensional object such as a skull. The whole shape is captured through scanning or digitizing, and virtual landmarks can be created by software at intervals across the surface of the object in question, such as a skull or jawbone. However, these secondary landmarks are still usually anchored to a network of primary points that correspond between the different objects to be compared, to provide a common frame of reference. A grid of points that reflect the overall shape (for example, of a Cro-Magnon skull) can be displayed on a screen, and a similar grid from another object (for example, a Neanderthal skull) can be compared side by side or overlaid. Geometric morphometric software can then reduce the skulls to the same overall size and measure the amount of shape distortion needed to change one to the other, and also quantify which areas of th
e skull change the least or the most in such comparisons. Thus research by anthropologists like Katerina Harvati has provided strong evidence that the shape differences between modern and Neanderthal skulls is certainly at the level of species differences in recent primates. These techniques can also be used to show how a series of skulls change as they mature and can even create theoretical evolutionary intermediates—say between a Homo erectus and a modern skull, which can then be compared with real examples, such as the skull of a Homo heidelbergensis. Geometric morphometrics has become particularly important when used in conjunction with that other technology recently applied to fossils: computerized tomography (CT).

  Wilhelm Röntgen, a German physicist, is usually credited with the discovery of X-rays, in 1895. He chose the name to reflect their then unknown nature, and his serendipitous find was to win him a Nobel Prize. He recognized their possible medical uses after he experimented by “photographing” his wife’s hand with the new discovery, clearly revealing the bones inside, and the technique was soon applied to new discoveries of fossils, such as the Neanderthals from Krapina in Croatia and the Homo heidelbergensis jawbone from Germany. X-rays found many uses in paleoanthropology over the succeeding century, but the flat form of the conventional X-ray image meant that structures could obscure each other, and they were not all correctly scaled in relation to each other (just as a shadow can be out of proportion to the original shape of an object).

  Not long after Röntgen’s discovery, an Italian radiologist, Alessandro Vallebona, proposed a method that provided a more focused single slice of X-ray imagery, which became known as tomography (from the Greek words tomos [slice] and graphein [to write]). This method found many uses in medicine, and about forty years ago Godfrey Hounsfield in the United Kingdom and Allan Cormack, working in the United States, independently came up with the development known as computerized tomography (CT), for which they also jointly won Nobel Prizes. CT scanners send several beams simultaneously from different angles, after which the relative strengths are measured, and a two-dimensional slice, or a three-dimensional whole structure, can be reconstructed from the data received. The computerized images reflect the density of the tissues or materials through which the beams have passed; for example, air spaces let through a strong signal, and teeth or fossil bone a much weaker signal. Moreover, the ability of CT to provide focused images means that much more detail is available than in conventional X-rays—so even the microstructures of bones and teeth can be examined.

  As CT technology and computing power have escalated, so has the impact on studies of human evolution. In some of the earliest applications in the 1980s, Javanese Homo erectus fossils were CT scanned, showing previously unseen inner ear structures, but the quality of the images was not really good enough to reveal their evolutionary patterns. But within ten years, things had moved on so far—in research pioneered by the paleoanthropologist Fred Spoor—that it was possible to image and compare the minute inner ear bones of several Neanderthal fossils, showing for the first time that they were distinct in shape from those of modern humans.

  Anatomically, our ears are divided into three parts: outer, middle, and inner. The outer ear gathers and transmits sound waves via the eardrum into the middle ear, where its tiny chain of bones converts the energy into mechanical vibrations. These middle ear bones—the malleus, incus, and stapes—are sometimes found in or next to the ear canal of fossil skulls, and so they have been studied in a few cases without the necessity of CT. Thus we know that early Neanderthal fossils from the Sima de los Huesos site at Atapuerca in Spain had middle ear bones shaped like ours, with the implication that the perception of sound in these early Neanderthals was already similar to our own today. The vibrations transferred via the middle ear bones then pass through the fluid and membranes of the cochlea of the inner ear to become nerve impulses, which are finally transmitted to the brain so that we can perceive the sounds. But our ears are not just for hearing; two other parts of the labyrinth of the inner ear also help us control our balance and head movement. The first consists of two fluid-filled chambers, which are lined by small hairs that sense the movement of tiny crystals of calcite, so that we can balance our head properly. The second consists of three fluid-filled loops that are arranged at ninety degrees to each other. These semicircular canals are also lined by hairs, which, through the motion of fluids on them, sense the movement and rotation of the head, and it is these canals that have proved so interesting when comparing the Neanderthals and all other humans. In particular we know that the size and shape of the semicircular canals are laid down before birth and remain unchanged as we grow up, and thus any differences that exist are likely to be genetic in origin and are largely unaffected by the environment during life.

  Nearly twenty Neanderthals have now been CT scanned to reveal their inner ear anatomy, and each semicircular canal is subtly distinct in size, shape, and orientation when compared with ours. What makes this discovery particularly intriguing is that the canals in the assumed ancestral species Homo erectus, and in early modern human fossils studied so far, are more like our own, so it seems to be the Neanderthals that are the odd ones out. But fossils in Europe that might be ancestors of the Neanderthals, such as the skulls from Steinheim and Reilingen in Germany, show an approach to the Neanderthal conformation, suggesting that the distinctive pattern could have evolved in Europe. But why?

  One possibility is that the shape of the semicircular canals is reflecting something else, such as overall skull or brain form, and it’s true that the Neanderthals do have some distinctive features in the shape of their temporal bones—the bone around the ear region on each side of the skull. Another possibility is that it reflects some form of adaptation—perhaps climatic—but against this, modern humans from cold climates show no significant differences from moderns who live in hot climates. The scientists who have conducted the most comprehensive studies, including Fred Spoor, argue that a plausible explanation lies in the essential function of the semicircular canals: to control the movement and rotation of the head. Although the exact mechanisms of interplay between the head and neck and the semicircular canal system are poorly understood, the Neanderthals had shorter but bulkier neck proportions than modern humans, which might have affected movements of the head, if it was more deeply buried within powerful shoulder and neck muscles. In addition, the Neanderthals had a more projecting back to the skull, a flatter base to the braincase, and a more projecting face, particularly around the nose, all of which might have made a difference to head movements all the way from less strenuous activities like walking through to highly energetic running or hunting.

  One of the first fossils to reveal these unusual Neanderthal inner ears forms part of the collections at the Natural History Museum in London. This is the rather fragmentary Devil’s Tower child’s skull (from its large size, probably a boy), found together with animal bones and stone tools beneath the sheer north face of the Rock of Gibraltar during excavations in 1926. It consists of three skull bones, half of an upper jaw, and most of a lower jaw, with a mixture of milk teeth and still-forming permanent teeth. In modern children, where the birth date is uncertain, or an unknown murder victim needs to be identified through forensics, the best way to estimate age is from their teeth. This method was applied to the Gibraltar fossil, and it was straightaway evident this was a child less than six years old in modern terms, as the first molar tooth was not yet ready to erupt. Studies of the fossil in 1928 suggested from the dental maturity that he was actually about five years old at death, but to judge from the voluminous skull bones, his brain size was already slightly larger than the modern average. Until 1982, everyone assumed that the bones belonged to one child, but in that year the anthropologist Anne-Marie Tillier suggested that while most of the bones did indeed represent a child of about five, the temporal bone was from a different and less mature child of about three years old at death.

  In the 1970s new microscopic techniques became available to study the microstr
ucture of teeth, and earlier suggestions that human tooth enamel contained daily “lines” of growth began to be studied as a means of estimating the length of time it took a tooth to develop, and hence of potentially gauging the age at death of a child. These daily lines are grouped and expressed on the surface of the front teeth as transverse ridges, or perikymata (from two Greek words meaning “around” and “a wave”), each of which represents about eight days of growth. In the 1980s, using a scanning electron microscope, I collaborated with the paleoanthropologists Tim Bromage and Christopher Dean, and subsequently also with the primatologist Bob Martin, to estimate the probable age of the Devil’s Tower child from its well-preserved upper central incisor, and to study its growth and development. By counting the perikymata and adding a few months to represent the small amount of root growth, we estimated the age at about four years. We also used a rare and important collection of human skeletons from the crypt of Christ Church at Spitalfields in the City of London, with actual age at death recorded on coffin plates or parish records, to test the perikymata method. We found that it worked well as an estimator of age in the children who had been buried there. In addition, I studied the temporal bones of the same children’s skulls to assess whether a temporal bone as immature as the one from Devil’s Tower could belong with the other bones and teeth of that child. The results were clear: both the teeth in the jaws and the temporal bone came from a child of about four years at death, and thus there was no reason to dissociate them on grounds of differential maturity. However, because the temporal bone was from the other side of the skull to the equivalent parietal bone, the two could not actually be directly articulated to prove they belonged together.