ABSTRACT. Iodine is essential for modern humans and may have been essential for Neandertals as well. Today about 30 percent of the world's population is at risk of iodine deficiency disorders (idd), 750 million people suffer from goiter, 43 million have idd-related brain damage and mental retardation, and 5.7 million are afflicted by cretinism, the most severe form of idd. Distinctive Neandertal skeletal traits are identical to those of modern humans who suffer from cretinism. Cro-Magnon Venus figurines also exhibit distinctive traits associated with cretinism among modern humans. This new evidence, coupled with recent mitochondrial dna findings, suggests that a single genetic alteration, which improved the ability of the thyroid gland to extract and utilize iodine, may account for differences between Neandertals and modern humans. Late Pleistocene human evolution, consequently, may be explained by several alternative interpretations involving iodine pathology and/or biological adaptation. Speciation may have resulted from the geographical isolation of inland populations.