This paper presents a thermal model for the prehistoric origin and development of clothing. A distinction is drawn between simple and complex forms of clothing, with broad implications for the interpretation of paleolithic technological transitions and the emergence of modern human behavior. Physiological principles and paleoenvironmental data are harnessed to identify conditions requiring simple, loosely draped garments and the more challenging conditions that demanded additional protection in the form of complex garment assemblages. No actual clothing survives from the Pleistocene, yet the archaeological record yields evidence for technological and other correlates of clothing-more evidence than is generally supposed. Major innovations and trends in the distributions and relative frequencies of lithic and other tool forms may reflect the changing need for portable insulation in the context of fluctuating ice age climates. Moreover, the nonthermal repercussions of complex clothing can be connected with archaeological signatures of modern human behavior, notably adornment. Alternative models are less parsimonious in accounting for the geographical and temporal variability of prominent technological and other behavioral patterns in association with environmental change.