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The broad area of science known as paleoanthropology, which seeks to understand the biological and cultural evolution of Homo sapiens, takes a largely multidisciplinary approach, with numerous experts—from geochronologists to archeologists and a host of others—involved in field research and the study of the fossil record.
Paleoanthropology is the broad field of science devoted to understanding the biological and cultural evolution of our own species, Homo sapiens, and of the zoological family Hominidae to which it belongs. (Hominidae is the family of primates that contains the living Homo sapiens and its close fossil relatives—those that are not more closely related by descent to the living great apes: the chimpanzees, bonobos, gorillas, and orangutans.) Central to the practice of paleoanthropology are paleontologists, scientists who study the fossil record that comprises the direct biological evidence of humanity’s past; but the field is fleshed out by scientists of many different kinds, including stratigraphers, geochronologists, taphonomists, functional and comparative anatomists, systematists, molecular and population geneticists, archaeologists, evolutionary biologists of various kinds, and a host of others. The notion of paleoanthropology as an essentially collaborative area of science dates back to the early 1960s, when Louis Leakey (earlier in his career the very exemplar of the traditional “lone paleontologist”) and his wife Mary brought together specialists of various kinds to investigate the hominid-bearing deposits of Olduvai Gorge, in Tanzania. Clark Howell, then of the University of Chicago, soon thereafter made the multidisciplinary approach definitive in his explorations of the fossil-rich sediments of the Omo basin in southern Ethiopia. Since that time virtually all intensive human evolutionary field investigations have included a diversity of experts and have involved collaborations with many outside specialists.
The Nature of the Evidence
The archive of human biological evolution is the fossil record. Fossils consist of the remains of dead creatures that have been preserved in the accumulating geological record. Almost invariably, such remains consist of bones and teeth, since these are the hardest elements of the body and best resist decay and other forms of destruction. The destructive processes themselves are the specialty of taphonomists, who study how remains are scattered and broken postmortem, and how fossil assemblages are accumulated. Fossils of land creatures are preserved within the sedimentary rocks that result from the deposition of eroded particles, principally by water in lakes and rivers. Such sediments are laid down in sequences within which later deposits overlie earlier ones, a process that is reconstructed by the geologists known as stratigraphers. The place of fossils in a rock sequence can determine their relative age (older than this, younger than that), but the determination of absolute ages (in years) is the province of geochronologists. In their efforts to quantify the time that has elapsed since a particular event (the cooling at the surface of a lava flow, for instance), geochronologists most frequently take advantage of the fact that unstable, “radioactive,” forms of elements contained in rocks “decay” to stable states at known and constant rates. For earlier periods, volcanic rocks (which may be interlayered in sedimentary sequences, and hence give an estimate of the age of fossil-bearing sediments above and below them) are favorite objects of geochemical dating; in more recent times, actual fossils (e.g., by radiocarbon) or sometimes even artifacts (e.g., by thermoluminescence) can often be assigned dates in years.
The fossils themselves provide information of many kinds. First, they tell us about the variety of hominids that existed in the past. Based on their anatomical similarities and differences, paleontologists first classify fossils into species, which are both the basic kinds of organisms and the building blocks of ecosystems. Elementary as it may sound, the sorting of fossils into species is one of the most difficult processes in paleoanthropology, and one of the most contentious. In recent years new methods such as scanning electron microscopy and computerized tomography have helped to enlarge the range of morphologies that can be brought to bear on such problems. Once species are recognized, their genealogical relationships (those due to ancestry and descent) may be inferred. Again, such inferences are normally based on their morphologies and are similarly subject to a variety of algorithms and approaches. Over the past few decades molecular comparisons among extant species have helped solidify our notions of the relationships among extant species, and hence of the framework within which hominid fossils have to be fitted. In the hominid arena, molecular evidence has served to substantiate the species difference between Homo sapiens and Homo neanderthalensis, following the recent success of molecular geneticists in extracting short sections of mitochondrial DNA from fossils of the latter.
Further anatomical scrutiny can reveal a great deal about how extinct species may have lived. Particularly when functional comparisons can be made with the structures of extant forms whose behaviors are known, such study allows tentative reconstruction of the anatomically limited (in the hominid case, most importantly locomotor) behaviors of the species concerned. In rare cases, such as the famous 3.5-million-year-old footprints preserved at Tanzania’s Laetoli that directly document upright bipedalism at that great remove in time, the inferences made from bony anatomy may be independently confirmed. Our understanding of what our predecessors ate is enhanced by analyses of teeth and of how they wear, and the analysis of stable isotope ratios in fossil bone can further augment our understanding of ancient diets. Further, the examination of associated fossil faunas and floras, and of the geological evidence for the circumstances in which sediments enclosing particular fossils were deposited, can reveal a great deal about the environments in which the creatures of interest had lived and behaved.
In the hominid case, our knowledge of ancient behaviors is vastly enhanced by the archaeological record, which begins around 2.5 million years ago with the invention of the first stone tools. Archaeology, sometimes defined as “the study of ancient garbage,” focuses upon the traces—of any and all kinds—of their activities left behind by ancient humans. It is not confined simply to the study of ancient artifacts, but also extends to the ways in which those artifacts were accumulated at particular sites, and to how such sites are located within the landscapes in which they are found. By combining analyses at all these levels, much can be determined about how now-extinct humans interacted with the environment around them and to a certain extent with each other, though it has to be admitted that even a rich archaeological record is but an indirect reflection of the complex social, economic, and material lives that were led by earlier hominids.
The Human Evolutionary Record
The reconstruction of the human evolutionary past has been greatly influenced by views of the evolutionary process itself. In the mid-twentieth century, most Anglophone paleoanthropologists fell under the sway of the “Evolutionary Synthesis,” a view of the evolutionary process that ultimately ascribed virtually all evolutionary phenomena to gradual generation-by-generation change of gene frequencies in populations, under the guiding hand of natural selection (whereby in each generation those individuals with favorable heritable adaptations reproduce more successfully than those less favorably endowed). This perspective led to an essentially linear view of human evolution, which was seen as involving a slow, dogged slog from primitiveness to our current burnished perfection.
Subsequently, an enlarging hominid fossil record forced the realization that the evolutionary process is more complicated than this, and is subject to a host of external influences, many of which are entirely random with respect to adaptation. The resulting picture of the hominid record is one of diversity, of evolutionary experimentation whereby many hominid species have emerged and done battle in the ecological arena, trying out the many ways there evidently are to be hominid. The story of our family is one of many species originations and many species extinctions. It may seem natural to us today, since this is what we are familiar with, that Homo sapiens is the lone hominid in the world; but in fact it is a highly atypical situation, and one that strongly hints that there is something very unusual indeed about our species.
The First Upright Bipeds
The earliest fossils that have been claimed to lie somewhere in our ancestry, but not in that of the apes as well, come from African sites in the period between 7–6 and 4.4 million years ago. The genera Sahelanthropus, Orrorin, and Ardipithecus are largely known from different parts of the skeleton, and all of them have been disputed as hominids in one way or another—reflecting the fact that as yet we have no clear idea of what the earliest hominid ought to look like. What they all have in common, however, is that each genus has been claimed on one slender basis or another to have been an upright biped. In the period between about 10 and 7 million years ago the ancient African forests began to fragment as the climate became less humid and more seasonal, and this clearly provided new ecological opportunities for terrestrial bipeds. Small wonder, then, that this adaptation has become the de facto criterion for membership in Hominidae. However, it still remains possible, even likely, that upright bipedality evolved more than once within the ancestral group from which both living apes and humans are descended.
The best-documented early bipedal hominid species is Australopithecus afarensis, known from sites in eastern Africa dating between about 3.8 and 3.0 million years ago. Exemplified by the famous “Lucy” skeleton, this species was small-bodied, standing between about 31?2 and 41?2 feet tall. Such creatures retained a variety of features useful in climbing, though they show a variety of pelvic and hind-limb specializations for bipedality, and would certainly have moved bipedally when on the ground. Above the neck, however, the proportions of A. afarensis were apelike, with a large face hafted onto a small, apesized braincase—which is why paleoanthropologists often characterize these early hominids as “bipedal chimpanzees.”
This combination of features was a remarkably successful one, remaining essentially stable as a whole variety of species of such “archaic” hominids came and went over the period preceding about 2 million years ago. Living on the fringes of the forests and in the newly expanding woodlands, hominids like A. afarensis probably subsisted primarily upon plant foods, although they may have scavenged the remains of dead animals and hunted small mammals much as some chimpanzees do today.
It was presumably a hominid of this archaic, small-brained kind that first began to manufacture stone tools around 2.5 million years ago. Consisting of small sharp flakes struck from one river cobble using another, these tools were crude but remarkably effective, and must have had a profound effect on the lives of their makers, allowing them, for instance, to detach parts of carcasses and carry them to safer places for consumption.
Early Hominids with Modern Body Proportions
Interestingly, no technological change marked the emergence at just under 2 million years ago of the first hominid species with body proportions essentially like our own. Built for life out on the broiling tropical savanna, far from the safety of the trees, it was most likely the unprecedented mobility of this striding biped, often known as Homo ergaster, that led to its almost immediate spread beyond the confines of Africa, best exemplified at the 1.8 million-year-old site of Dmanisi, in the Caucasus.
Technological innovation, in the form of the deliberately shaped “handaxe,” appeared later, also in Africa, at about 1.5 million years ago. During this period there also began a trend toward hominid brain-size increase, although the exact pattern of that increase will have to await better understanding of hominid diversity through this period.
By about 1 million years ago hominids had penetrated Europe and had begun to diversify there in a process that ultimately culminated in Homo neanderthalensis. This hominid had a brain as large as our own, albeit housed in a very differently structured skull. Meanwhile, the lineage leading to Homo sapiens was evolving in Africa, although this stage in human evolution is poorly—albeit tantalizingly—documented by fossils. Both molecular and fossil evidence suggests that anatomically modern Homo sapiens had emerged in Africa by a little under 200,000 years ago, and by around 100,000 years ago such hominids had reached Israel, which was also at least sporadically occupied by Neanderthals around this time. Interestingly, in the period of coexistence between about 100,000 and 50,000 years ago, the Neanderthals and moderns of the eastern Mediterranean region shared essentially the same technology. During this time, though, we find the first stirrings in Africa of the symbolic behavior patterns that characterize Homo sapiens worldwide today.
Shortly after a more sophisticated Stone Age technology was developed in Israel, presumably by Homo sapiens whose ultimate origins lay in Africa, the local Neanderthals disappeared, and in short order Europe was invaded by modern peoples, at about 40,000 years ago. These “Cro-Magnons” left behind them an amazing record of virtually the entire panoply of modern symbolic behaviors, including representational and geometric art in various media, music, notation, bodily ornamentation, elaborate burial, and so forth. At the same time, technologies became enormously elaborated and embarked upon a pattern of constant innovation and change. Earlier hominids had adapted old tools to new purposes as environments changed; but cognitively modern Homo sapiens typically accommodated to such change by inventing new technologies.
The sequence of events just summarized strongly suggests that, with the emergence of Homo sapiens in Africa, an unanticipated ability for symbolic cognition was born. This new cognitive potential was evidently acquired in an emergent event, in which a chance coincidence of biological acquisitions resulted in something entirely new. Made possible by a long evolutionary history, but not an inevitable result of it, the expression of this unprecedented potential (the biological underpinnings of which were presumably acquired in the reorganization that led to modern anatomy) had to await behavioral discovery, much as ancestral birds had feathers for millions of years before discovering they could use them to fly. Most plausibly, the behavioral releasing agent concerned was the invention of language, an activity that is intimately tied up with symbolic thought.
Once the transition to symbolic thought had been made, Homo sapiens was positioned to eliminate its hominid competitors such as the Neanderthals and to embark on an aggressive demographic expansion. At this initial stage all human societies were economically based on hunting and gathering; rapidly, however, sedentary lifestyles relying on the domestication of plants and animals were independently adopted in various parts of the world. Sedentarism then led to further population expansion, urbanization, economic specialization, and the development of complex societies. And it also led to a redefinition of the relationship of Homo sapiens to the rest of the world: to what the paleontologist Niles Eldredge (1995, 101) has characterized as a “declaration of independence” from our surrounding ecosystems.
Implications of Paleoanthropology
The study of paleoanthropology teaches us above all that the process leading to the arrival on Earth of Homo sapiens was not one of constant fine-tuning over the eons. Rather, our cognitively unique species appeared in a short-term event that was emergent in nature, rather than representing the culmination of any preexisting trend. This entirely unprecedented event witnessed the replacement of a history of highly sporadic hominid innovation, in the context of very low population sizes, by patterns of demographic expansion and restless local invention in both the social and technological realms—even as the inherent limitations of human experience, together with an apparent reluctance to learn from such experience, guaranteed that similar patterns would tend to recur over and again.
Further, the prehistory of sedentary societies almost everywhere shows that technological overintensification has repeatedly combined with climatic vagaries to ensure eventual economic collapse—for reasons that are often due as much to innate human proclivities as to the external proximate causes. Despite its extraordinary ratiocinative abilities, Homo sapiens is not an entirely rational creature, and its history worldwide reflects that fact. Evolutionary psychology and other reductionist approaches to the contrary, we cannot understand our own history as that of a creature that is biologically perfected for—or even broadly adapted to—any particular way of behaving.
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