We are very glad to read of Dr. Previc’s enthusiasm for our recent findings on thyroid hormone differences between humans and great apes (Gagneux et al., 2001). We are responding herewith to some of the interesting issues that he has raised. Our paper presented human-ape comparative results on plasma thyroid hormone levels and levels of transthyretin (TTR) in plasma and cerebrospinal fluid (CSF). Given the crucial role of thyroid hormone in brain development, we suggested that developmental studies of humans and great ape thyroid hormone levels are warranted, especially for the CSF/brain side of the blood brain barrier. We did not hypothesize about how such hormonal differences originally came about. Extrapolating from currentday human-ape differences to the series of events that brought about these changes sometime during our evolutionary past is fraught with difficulty. However, we are glad if our results provide potential support to other existing hypotheses. Increased consumption of animal foods and frequent strenuous exercise may indeed have played a crucial role in resetting dopaminergic levels in hominoids, as proposed by Dr. Previc’s extensively researched paper on this subject (Previc, 1999). However, it is also evident that present-day humans can achieve aboveaverage cognitive capacities in the absence of meatrich diets or increased physical activity levels. As far