The Quaternary period marks the transition from environments populated with fabled, extraordinary creatures such as saber-toothed cats, giant ground sloths, dire wolves, cave bears, woolly rhinos, and mammoths to the more familiar landscapes of modern time. Anatomically modern humans appeared during the late Quaternary. The Quaternary is composed of the Pleistocene epoch, or Ice Ages, which lasted from about 1.7 Ma to 10 ka, and the Holocene epoch, or present interglacial, which begins where the Pleistocene ends and extends to the present. The Quaternary was characterized by dramatic geographic shifts of continental glaciers, accompanied by major transgressions and regressions of sea level. During the height of the last major North American glacial period, the Wisconsinan, much of the continental shelf was exposed and probably represented a vast lowland savannah. Fishermen have recovered proboscidean teeth and bones from the shelf at depths of more than 90 m (Whitmore et al., 1967). Within this dynamic context the geographic ranges of many species of animals and plants expanded and contracted, and a concomitant pattern of originations and extinctions developed.

To evolutionary biologists the Quaternary record represents a vast laboratory, full of natural experiments, in which hypotheses of evolutionary tempo and mode may be examined. The benefits of working in this laboratory are manifold. Modern species are often represented, and their past distributions in some cases often provide important, independent climatic signals in their own right, and in other cases can be tested against independent climate data to ascertain the relative importance of physical perturbations in causing evolutionary events.