Social DNA. M. Kay Martin

have been drawn based on only limited data or, alternatively, where new information compels us to perceive ancient social life in different ways.

      For example, modern humans are ensconced at the pinnacle of evolution—as the brightest and the best—while Neandertaloid, Homo erectus, and other archaic hominins have often been portrayed as brutish, dim-witted, and doomed to extinction at the hands of more intelligent or technologically advanced peoples. As will be argued later in this book, although hominin brains got bigger through time, so did hominin bodies. Relative brain size is being increasingly questioned as a reliable indicator of ancestral hominin intelligence. Recent neurobiological research suggests that the key to cognitive abilities may lie, instead, in the neuronal density, circuitry, and conduction velocity of gray matter, factors that are not discernible through external cranial measurements or endocasts of fossil specimens.

      Similarly, the chronological or cognitive pedigree of fossil hominins cannot be reliably equated with or pigeonholed by the type of lithic traditions with which they made their living. Simple pebble tools and flakes, for example, were utilized for millions of years alongside or in the absence of stone tools fashioned by more refined knapping techniques. A rule of thumb is perhaps that ancient peoples used tool types that “worked” in their ecological niche, regardless of their antiquity or point of origin. Moreover, nonlithic traditions based in perishable organic materials such as wood and fiber are underrepresented in the archaeological record, but may have provided the basis for alternative ecological adaptations. Thus, it is less useful to characterize ancient populations by their tool types—as Oldowan, Acheulean, or Mousterian “peoples”—than it is to examine the range of adaptations potentially associated with these and other material technologies.

      The tendency for chronologies to put ancient hominins into discrete boxes has a long tradition in paleontology, where each new fossil discovery is frequently celebrated with a unique phylogenetic designation as a distinct species. Historically, such specimens have been grouped into evolutionary stages that connote a linear progression of increasing complexity over time. Passage from one stage to the next is often perceived through the lens of replacement, with lesser species absorbed or overrun by more advanced species through mass migration, technoeconomic prowess, or superior intelligence.

      An alternative theory proposed early on by Ernst Mayr (1950), and more recently adopted by Wolpoff (1989) and Finlayson (2014), is that there was never more than one species of Homo at any one time in our evolutionary past.⁷ In other words, once the threshold to the genus had been crossed, subsequent evolutionary changes were largely a matter of degree rather than kind. In this model, Homo sapiens erectus represents a single polytypic species that established the gene-culture foundation for all subsequent members of the genus, occupying the entire breadth of temperate latitudes in Africa and Eurasia (a region Finlayson refers to as the “Middle Earth”) for about 1.5 million years. The single-species theory recognizes that separation of hominin clades by geographic barriers and by climate change events led to the periodic isolation of gene pools, the proliferation of distinct lineages, and periodic extinctions. It differs from more traditional models, however, by arguing that such separations were insufficient to produce speciation. In other words, significant gene flow occurred throughout the Pleistocene, allowing members of diverse Homo lineages to periodically reconnect, mate, and produce fertile offspring.

      A major criticism of the one-species theory lies in the degree of diversity observed in the accumulating hominin fossil record. A wide variety of hominins appear to have lived contemporaneously throughout the Pleistocene, alternating periods of migration and inbreeding with periods of relative isolation and genetic bottlenecks. Paleontologists disagree on where to draw species boundaries among disparate lineages of ancient hominins, but are increasingly reticent to view the course of human evolution as a linear progression of forms—as a single main line flanked by a number of side branches or dead ends. Rather, our evolutionary pathway may more closely approximate a braided stream, the various branches of which periodically diverged, crossed over, and reconnected throughout the Pleistocene.

      Boaz and Ciochon (2004: 166) have suggested that it may be more appropriate to replace the concept of gene pool with gene sea, across which genes flowed subject to the currents, waves, and eddies created by climate change and natural selection. Adaptive changes have taken place gradually among closely related populations, creating clinal gradients over time and space. Homo sapiens is now, and has always been, a polytypic species. The recent sequencing of Neanderthal and modern human DNA lends credence to the notion of ancient gene exchange and of population replacement through hybridization. Resources and opportunities permitting, Pleistocene hominins made love, not war. The braided stream model of gene flow and speciation will be favored in this book.

      Water Drives Ecology

      The course of human evolution is inextricably linked to water. Daily access to adequate drinking water is a biological imperative and the primary limiting factor that has shaped how and where ancient peoples lived and prospered. Discussion among paleoanthropologists has been focused in two principal areas: (1) water as a critical element of the early hominin habitat that accompanied arboreal abandonment and expansion into the savannas; and (2) the impact that climatic events and associated changes in water distribution and availability had on the demography and lifeways of Pleistocene populations.

      The literature abounds with theories on the abandonment of arboreal life. There is general agreement, however, that the first hominins to venture from the warm, moist environment of the forest canopy did so on the margins of transitional habitats that were defined by water sources. Finlayson (2014: 22–24) surveyed the habitat elements common to known fossil sites in order to better characterize what he termed the essentials of early hominin real estate. The vast majority were found to be associated with settings that combined “shallow water, trees, and open (treeless) spaces.” Terrestrialism expanded the range and types of potential food sources for initiates, gradually converting them from tacit fruitarians into omnivores. This bipedal vanguard, commonly associated with a wide variety of Pliocene apes, is assumed in popular theories to have been followed by more advanced proto-humans that ventured farther into the surrounding countryside to procure sources of animal flesh—an accomplishment often credited with leapfrogging early hominins into the genus Homo.

      Water, or the absence thereof, is seen by Boaz (1997) and Finlayson (2014) as a driver of increasing sophistication in early humans. Pleistocene climatic swings were accompanied by dramatic ecological changes, including the retreat of tropical forests, increasing aridity, and desertification. Such conditions are proposed to have selected for complex reasoning and strategies necessary for survival in the face of diminishing water supplies. Prevailing theories about life in the Pleistocene argue that early hominins, tethered as they were to sources of water, were forced to increase their mobility and geographical range, limit their group size, adopt new technologies, and/or embark on migrations to more favorable habitats for sustenance.

      Such theories acknowledge the critical importance of water sources for quenching hominin thirst. But notably, they are generally silent about the extent to which rivers, lakes, marshes, and marine shorelines also contained the necessary food sources to fill their bellies. Such well-watered habitats, which support a wide variety of flora and lipid-rich fauna, were variably distributed throughout Africa and Eurasia even during the waxing and waning of glacial events. These premium habitats were theoretically capable of supporting sizeable populations on a seasonal or year-round basis. Not coincidentally, this is precisely where hominin fossils are most frequently found. This book will examine recent paleoecological research and archaeological evidence that underscores the importance of mosaic habitats and aquatic resources in early human adaptations.

      The implications of well-watered habitats and resource diversity for early human social life are profound. Periods of extreme aridity did occur during the Pleistocene. Lakes and rivers in some regions did dry up. But surviving populations were necessarily opportunistic. It is equally probable that, as an alternative to settling for a Spartan existence on the arid savanna, enterprising hominins simply followed existing waterways and coastal highways to more favorable habitats, or shadowed the advance and retreat of glacial ice seeking the “ecological release” of untapped environments. This book will re-examine