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Eugene Dubois’s unearthing of a million-year-old Pithecanthropus erectus skullcap in Java in 1891 inspired generations of paleontologists and amateur rock hounds alike to find the “missing link” connecting apes and humans. A century and a half later the growing fossil record has enabled a general reconstruction of human evolution (Figure 0.1). Fossil remains have typically been grouped into evolutionary chronologies based on their provenance, and on characteristics such as their skeletal and cranial morphology, dentition, estimated brain size, and associated stone tool traditions.

While resultant phylogenetic trees vary somewhat from one another, most scholars propose that hominins⁵ evolved in Africa between 5 and 7 million years ago (ma) from among a heterogeneous stock of bipedal Pliocene apes. Potential candidates for the earliest primates on the human family tree include a diverse clade of tool-using apes known collectively as australopithecines, as well as more ancient and less robust forms, such as Ardipithecus ramidus. Members of the genus Homo, distinguished by their larger bodies and brains and by their fully omnivorous diet, are generally recognized as emerging around 2 ma. These first humans are exemplified by fossils such as Homo habilis and by multiple waves of Homo erectus populations that radiated throughout Eurasia over the succeeding millennia. A prevailing view is that advanced forms of H. erectus diverged around 400–500 thousand years ago (ka) into separate lines in Europe (H. heidelbergensis) and Africa (H. rhodesiensis). These populations ultimately led to the parallel emergence of Neanderthals (Homo sapiens neanderthalensis) and anatomically modern humans (Homo sapiens sapiens), respectively, by at least 200 ka.⁶ This progression of hominin types is associated with evolutionary milestones such as refinement of the infracranial skeleton, increasing encephalization, dietary reliance on animal flesh, the invention of fire, and increasingly complex stone tool technologies.

Figure 0.1. Overview of human evolution (copyright John A. J. Gowlett) from Gowlett and Dunbar (2008: 22). With permission of John Wiley and Sons, Inc.

Such evolutionary reconstructions provide the essential backdrop for current theories on human social origins. Recovered fossil and cultural remains have been utilized as windows on the subsistence activities, cognitive abilities, and social organization of ancestral humans through time. A cautionary note, however, is that our perspective on Pleistocene lifeways is based on material records that are still quite fragmentary, and that are subject to revision with the next great find. Recent discoveries suggest that ancient hominins were much more diverse than previously realized. Fossil specimens do not always fit into the tidy boxes of existing chronologies. In short, when looking back, it is important to remain open to new information and new interpretations—to know how much we don’t know. There are points in this book where the reader will be asked to entertain alternatives to popular theory where conclusions 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.