The face of human evolutionary studies that outsiders most commonly see is a nomenclatural thicket pruned by recurrent extinctions. Hypothetical rounds of species succession are so characteristic of paleoanthropology that they often are echoed in novels that use the evolutionary past as settings. Thus William Golding's The Inheritors represents Neanderthals while they are being exterminated by anatomically modern humans, as do Jean Auel's Clan of the Cave Bear (plus its imaginative sequels) and Björn Kurtén's Dance of the Tiger. Works of this sort add a lot of local color and speculative detail to conceptions of phylogeny that date back over a century.

As a result of these works, professional and popular, many nonspecialists believe that the central activity in paleoanthropology consists of argumentation about how many species existed, how many of them lived simultaneously during various time periods, and which ones emerged as survivors while their contemporaries passed into the oblivion of extinction.

These recurrent disagreements make the field appear to be so forbiddingly complex that even scientists in closely allied specialties can feel overwhelmed. This comment is based on my own experience over a period of years with a respected colleague, Paul T. Baker, who now has retired from his position as Professor of Anthropology at the Pennsylvania State University. Paul's area of specialization is the biology of human adaptability. In this realm he has been recognized internationally with various honors, including the Huxley Memorial Medal of the Royal Anthropological Institute.

Introduction

Following their spread from Africa to Eurasia a million or more years ago, hominids entered a prolonged phase of contact across these areas that allowed for the multidirectional transfer of two kinds of information – cultural and genetic. The cultural contact is documented, straightforwardly, by the association of human remains across the entire inhabited area with one or another manifestation of the Developed Oldowan/Acheulean tool complex. Tools of these related traditions are found at sites that arc from Kabwe in South Africa through Bodo in East Africa, westward through Europe at sites such as Arago in France and Steinheim in Germany, and eastward from Zuttiyeh in Israel through Narmada in India to numerous localities in China, including Dali and Maba. These continuities of material culture in themselves imply human contact and hence gene flow. This implication is supported further by the numerous broad phenotypic similarities among the human fossil remains discussed in greater detail below.

At the same time, other patterns delineate in situ adaptation to local and regional environments. There are contrasts in tool complexes between sites in the eastern and western ends of the Eurasian geographic continuum, of which just one major example is the lack of an Asian Acheulean (Pope & Keates, 1994; Schick, 1994). It is known that identifiable Levallois flakes and hand axes do occur at some Chinese sites, although infrequently (Pope, 1997).

Paleobiology has been referred to variously as a science, a discipline, and a paradigm. These terms all sound rather dauntingly formal for what might be characterized more modestly as an attitude toward the past. Logically, paleobiology is a subdivision of paleontology, since it derives its most direct evidence from the fossil record. But although paleontological evidence is itself static, comprising fossilized skeletal parts and associated remains for the most part, the paleobiological approach to this evidence is dynamic. Paleobiologists endeavor to reconstruct credible impressions of past populations and their members as they were in life: feeding; mating; giving birth to offspring and caring for them; avoiding predators; and enduring vagaries of weather, parasites, and diseases. The author of one paleobiological study covering an extinct group of tetrapods commonly referred to as ‘mammal-like reptiles’ noted that her aim was ‘to present the “hard facts” about dicynodonts and then go on to interpret these facts in physiological, behavioural and ecological terms…’ thereby ‘turning mere piles of bones into entities more approaching living animals’ (King, 1990).

Reviewing King's work, Rowe (1991) remarked that paleobiology is no longer the central paradigm that it had been in the 1960s and 1970s; instead other issues, particularly the reconstruction of phylogenies and the related question of evolutionary rates, have replaced paleobiology at the forefront of paleontological research and debate. Perhaps. Nonetheless, paleontological monographs dealing with fossil hominid remains continue to appear (e.g. Trinkaus, 1983; Rightmire, 1990; Madre-Dupouy, 1992; Höpfel, Platzer & Spindler, 1998) and many of these include valuable data that are pertinent to paleobiological reconstructions (Tobias, 1967, 1991).

Introduction

During the Miocene, our ancestors had been part of an enormous network of hominoid populations distributed widely across tropical forests and more open woodlands that covered much of the Eurasian and African continents. Possibly as early as ten million years ago, but in any case by six to eight million years ago, climates changed and forests across those regions contracted. Somewhere in Africa clusters of ape populations crossed a transitional adaptive zone, becoming distinguished at first by ecology and behavior, and only later by genes and morphology. Their evolution followed a mosaic pattern, with one of the earliest recognizable components of hominid status being the functional complex including upright posture and bipedal locomotion, followed by craniodental changes and brain expansion. As we saw in Chapter 8, hominids evolved for four or five million years, producing morphologically diverse and adaptively successful populations that spread over East and South Africa. From among those groups, about which much is known but more is uncertain, biologically and behaviorally advanced hominids later arose.

Foley (1995) refers to the populations resulting from the first transition to upright posture as hominids, and reserves the term human only for the much later populations (circa 100000 to 150000 years ago) characterized by distinctive patterns of anatomical structure and behavior. Between the diverse and fragmented populations of earlier hominids and later populations that are recognizably human lies another zone of transition occupied by what many paleoanthropologists believe are members of the genus Homo who have not yet achieved fully human status. The formal taxonomic designation that traditionally has been applied to these transitional hominids is Homo habilis, though as noted below, this taxon is subdivided by some investigators.

Introduction

Plio-Pleistocene hominid populations occupied a narrow genetic zone bounded by the genomes of extant chimpanzees and humans (but only approximately, because no living populations can be expected to have gene pools that have remained unchanged over millions of years). Judged from our present vantage, the outcome of hominid evolution represents a great anatomical step across this narrow genetic transition zone. The one percent DNA sequence difference between humans and chimpanzees, objectively small even now, must have been virtually indiscernible when the earliest hominid populations diverged onto their independent evolutionary pathway six to eight million years ago. At the phenotypic level, however, the evolution of hominids from ancestral apes was swift and pronounced, resulting in lasting morphological remodeling of the postcranial skeleton for upright posture and bipedal locomotion, along with other changes in cranial and dental characters.

Evolutionarily, these morphological changes establish that the adaptive capacities of early hominid populations were challenged and their gene pools restructured. Given the known extent of decoupling between molecular and morphological levels, it is probable that the first several million years of hominid evolution were characterized by discrepancies between phenostructure and zygostructure similar to those shown by the papionine populations spread across similar terrestrial ecosystems today. From the standpoint of adaptability, there is every reason to believe that in meeting these challenges, early hominids utilized capacities for flexible responses at several levels (behavioral, physiological, developmental, genetic) that we know are relied upon not only by humans, but also by papionines and to an even greater extent by chimpanzees, whose cultural capacities are increasingly well documented.

Introduction

This chapter starts with a view of the world as short in duration and long on certainty, and concludes with a world long in duration and short on certainty. Explaining how this transition in outlook came about requires some background to trace how the conceptual framework of human evolution came into being, how some of its present limitations came to be built in, and the ways in which it must be changed. In this brief survey, time – chiefly geological time, including perceptions about the age of the earth and life on it – is part of the inquiry. However, the major focus is on the ways in which ideas about constancy and change (which are embedded in temporal contexts) have shaped the perceptions that scientists have about the extent of variation within populations and the nature of relationships between populations.

Several sets of readers may wish to skip this chapter entirely. The first group comprises those who believe that paleoanthropology is in a period of ‘normal science,’ and that its practitioners should just get on with the pragmatic tasks of description, comparison, and classification. The second group is more theoretically inclined, but content with some of today's avant garde ideas (such as characterizing species as individuals) which provide justification for working assumptions about low levels of intraspecific variation and punctuational shifts among species. The third group is made up of people who share my concerns about what seem to be large inherent contradictions in representing continuous (but not necessarily gradual) processes in terms of discrete categories, and who do not need to be reminded again of the details.

Limitations of the evidence

The challenge of human paleobiology is to reconstruct the appearances and lifeways of our ancestors over the course of several million years from evidence that is, in the epic epithet most often adopted by critics of the field, ‘sparse and fragmentary.’ The evidence is limited, and most specimens are far from complete. But this situation poses a challenge that is neither insurmountable nor even especially unique. Parallels are found in many other disciplines. In her history of England's Lollard religious movement, Anne Hudson (1988: 8) acknowledged what others had termed the ‘fragility and discontinuity of the existing evidence.’ She then went on to state her own philosophical stance, ‘evidence does not cease to be evidence because it is not ideal; the rider to this is that the obligation lies upon the researcher to refine or modify his techniques to accommodate the difficulties and obtain results if … the picture without new sources is manifestly inadequate. But on the other hand, the problems of the sources must be squarely faced and clearly explained; no useful purpose will be served by offering new evidence which appears to be firm and unambiguous when it is questionable, however much it may complete the perspective.’

It would be presumptuous as well as incorrect to characterize the existing state of human evolutionary studies that has been created by paleoanthropologists as ‘manifestly inadequate.’ My preference is to borrow a colleague's far more optimistic characterization of the field as ‘infinitely perfectible.’ In this sense, human paleobiology has come a long way, but it has much further to go.

Introduction

The power of the adaptability approach for ordering data into frameworks for interpreting patterns of variation in living human populations was established in Chapter 5. As a consequence of its value in framing and testing hypotheses about population norms and variations, this research strategy generally has come to be accepted and used productively among human biologists who carry out investigations on the widely distributed populations of our species, particularly those living in extreme environments such as arctic, desert, and high altitude zones.

As shown in Chapter 6, the adaptability framework also can be extended from its original domain to that of living nonhuman primates. Aside from the behavioral realm, where language and elaborate technological mechanisms distinguish extant humans from other species, the modes by which populations adapt to their environments are shared broadly among mammals. Behavioral, physiological, and developmental, as well as genetic, factors come into play as extant primates, nonhuman and human, meet the challenges posed by the settings into which they have spread. An abundance of research, much of which is summarized by Foley (1987, 1995), clearly supports the inference that the same adaptive spectrum was available in various degrees to hominid populations of the past.

This chapter explores the patterns of phylogenetic relationships hypothesized for the hominids who lived and died over the last five or so million years. Studies of the morphological characters preserved in these fossil remains have led some paleoanthropologists to suggest greater species diversity than is supported by comparative molecular data derived from other groups of extant large mammals such as the bears, cats, and dogs.