Paleoanthropology and Evolutionary Theory

Research TOC PALEOANTHROPOLOGY AND EVOLUTIONARY THEORY Ian Tattersall othing in recent years has affected the interpretation of the human fossil record more profoundly than have theoreti- Ncal notions of how the evolutionary process works. Paleoanthropologists generally learn these fundamental concepts early in their careers and rarely, if ever, reexamine them thereafter, even as they pass them along to their students—a practice that manifests itself in the extraordinary parochialism that our particu- lar branch of paleontology continues to exhibit. The last couple of decades have been a time of extraordinary ferment in the fields of evolutionary theory and phylogenetic reconstruction—ferment that has been fully reflected in the literature of other fields of verte- brate paleontology. From reading the bulk of the paleoanthropo- logical literature, however, you’d never guess it. Let’s start at the beginning and look at how evolutionary theory developed, before examining a couple of examples from the human fossil record. Back in the mid-nineteenth century, Charles Darwin defined evolution as “descent with modification.” This formulation recognizes the two fundamental aspects of evolutionary process. The first of these is that life has a genealogical history (phylogeny), through which all organisms alive today are interre- lated by descent from a single common ancestor that existed in the remote past. The second aspect—modification—involves physical change over time. It is the interaction of these two elements that has resulted in the pattern we see in nature: All the species of organisms are groupable on the basis of their morphological (anatomical) characteristics into a vast hierarchical series of sets within sets. This pattern had been noted long before Darwin wrote his great book On the Origin of Species. Indeed, a hundred years before Darwin’s 1859 publication of his evolutionary views, the great Swedish naturalist Linnaeus had used the hierarchy of living things as the basis for the system of classifying living things that we use today. In the Linnaean system species are grouped into genera, genera into families, families into orders, orders into classes, and so forth. It was not until Darwin’s time, however, that it was realized that this pattern resulted from common descent. Members of the same order, for instance, share a single common ancestor—members of the same family share a more recent common ancestor, and members of the same genus share an even more recent common ancestor. Darwin based his notion of evolution on two fundamental observations: first, that in every species individuals vary among themselves in heritable characteristics and, second, that in every generation more individuals of each species are born than ever sur- vive to reproduce themselves. From this emerged the concept of 3 4 RESEARCH FRONTIERS “natural selection,” by which the individuals best adapted to their environments multiply more successfully than those more disad- vantaged—and pass their favorable characteristics on to their off- spring, so that these favorable traits will become more common in each succeeding generation. In essence, natural selection is nothing more than differential reproductive success, mediated by the envi- ronment; and though the differences between individuals of the same species are normally slight, over the generations the continual pressure of natural selection will lead to the accumulation of substantial change. Variation will always be there, though, so that if cir- cumstances change, the blind forces of natural selection can shift direction and drive the lineage along another adaptive pathway. Although Darwin titled his book On the Origin of Species, he actually paid scant attention to the implied question of how the natural diversity of species originates. In large part this may have been due to the fact that, in Victorian England, he faced a substantial problem in getting his evolutionary notions accepted. This problem lay in the entrenched belief, grounded in Christian theology, that species are static and unchanging, remaining forever as designed by the Creator. In order to destroy this notion of immutability, Darwin concentrated on showing that species responded instead to the pressures of natural selection by changing over time—changing so much, indeed, that in the end they evolved themselves out of exis- tence, becoming new species in the process. While in a sense this idea did explain how new species might arise, it did nothing to address the question of the multiplication of species, that is, the origin of the diversity of species that is such a conspicuous feature of the living world. Paleoanthropologists, obsessed with tracing the history of the single species Homo sapiens back into the distant past, have, however, continued to be entranced by the notion of gradual change. Darwin formulated his evolutionary theories in the absence of any understanding of the processes of heredity. For his purposes, the simple fact that physical variations are inherited was enough. But with the rapid development of the science of genetics in the early years of the twentieth century, notions of how the evolutionary process worked began to multiply. Darwin’s ideas of evolution revolved around the gradual accretion of tiny changes; but the Dutch botanist Hugo deVries, for example, discovered that large- scale changes could occur in evening primroses. This suggested to him that new species could arise by sudden major disruptions of the hereditary material in a single individual, and hence it was PALEOANTHROPOLOGY AND EVOLUTIONARY THEORY 5 unnecessary to invoke natural selection in the origin of species. By the 1930s, however, a general agreement on the mechanics of evolution emerged under the rubric of the “Modern Synthesis,” expounded most importantly in three books by the geneticist Theodosius Dobzhansky (1937), the systematist Ernst Mayr (1942) and the paleontologist George Simpson (1944).1 This formulation had the elegant simplicity of all great ideas, essentially reducing the entire spectrum of evolutionary phenomena to the working of natural selection on the inherited variation that occurs within populations. Dobzhansky identified three levels at which the evolutionary process acts: (1) in the origin of genetic novelties; (2) in the ordering of those novelties in “molding the genetic structures of populations into new shapes”; and (3) in the “fixation of the diversity already attained on the preceding two levels.”2 The first level is accounted for by genetic mutations and recombinations. Where “normal” variation (as occurs among viable, nonmalformed individuals) is involved, such genetic changes usually result in rather minor differences, on the order of those with which Darwin was concerned. Natural selection then acts on these at the second level. Together these two processes explain the phenomena of “microevolution,” the genetic and physical changes that occur in populations. This much was pure Darwinism, integrated with the new knowledge of genetics. Dobzhansky, though, was also concerned with discontinuities in nature, which is where his third level of the evolutionary process enters in. Larger-scale differences among organisms—the phenomenon of “macroevolution”—were also to be accounted for. How? Ultimately, Dobzhansky and his colleagues decided that macroevolution and microevolution are essentially the same thing: that large-scale evolutionary changes, at the species level and higher, consist simply of the accumulation of tiny generation- by-generation changes over long periods of time. Simpson, the paleontologist, explained this by borrowing the geneticist Sewall Wright’s metaphor of the “adaptive landscape,” whereby populations balance atop peaks of optimal adaptation, while natural selection prunes off individuals who fall into the valleys between them.3 By noting that environments change, and altering the metaphor to that of a “choppy sea,” Simpson visualized peaks splitting apart and carrying fragments of the population away from each other on separate adaptive courses. Under such circum- stances, relatively rapid change and macroevolutionary innova- tion might be expected. 6 RESEARCH FRONTIERS Before long the architects of the Synthesis were ready to share their insights with the paleoanthropologists, who had stood very much to one side while all this rethinking was going on. Basic to the Synthesis was the realization that species do not consist of individuals all of whom conform more or less to a basic archetype, but rather are made up of clusters of variable individuals and populations bound together by reproductive continuity. The forays of the synthesists into paleoanthropology reflected this fundamental observation. As early as 1944 Dobzhansky weighed in with the suggestion that in the light of population variability, Asian Homo erectus should be viewed as belonging to our own species Homo sapiens.4 Six years later Mayr argued in the same vein that the aus- tralopithecines should be included in the genus Homo.5 Neither Dobzhansky nor Mayr was at all familiar at first hand with ranges of morphological variation in primate species; but their sugges- tions came as a breath of fresh air to paleoanthropologists, bur- dened as they were at the time by a plethora of genus and species names that almost matched the available fossils in number. The upshot was that, while few followed Dobzhansky and Mayr all the way in adopting these sweeping conclusions, paleoanthropology fell

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