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The Modern Synthesis was not widely adopted by paleoanthropologists until the 1950s, but the perception has been that this event had important theoretical and methodological consequences for the study of hominid evolution. This paper presents a general historical overview of the state of evolutionary theory within paleoanthropology during the early twentieth century, the key events that led to the integration of the Modern Synthesis into paleoanthropology, and the major consequences this had. Among the most important effects were the rejection of Neo-Lamarckian and orthogenetic mechanisms to explain hominid evolution. The Modern Synthesis emphasized genetics, the centrality of natural selection as the driving force of evolution, the notion that populations are highly variable, and that evolution produced gradually evolving lineages where the boundary between ancestor and descendant species is fuzzy. The Modern Synthesis encouraged the reform of hominid taxonomy, which resulted in the dramatic reduction of hominid taxa, and influenced hominid phylogeny through such ideas as the single species hypothesis. However, historians of science and paleoanthropologists have raised questions regarding the specific influences of the Modern Synthesis and the extent to which major developments in paleoanthropological theory and practice since 1950 should be attributed to the Modern Synthesis or to a more complex range of developments.