Abstract

We propose the hypothesis that genes of human intelligence are related with metabolism of thyroid and steroids hormones, which have a crucial role in brain development and function. First, there is evidence to support the idea that during hominid evolution small genetic differences were related with significant endocrine changes in thyroid and steroids hormones. Second, these neuroactive hormones are also related with unique features of human evolution such as body and brain size increase, penis and breast enlargement, pelvic sexual dimorphism, active sexuality, relative lack of hair and higher longevity. Besides underling many of the differences between humans and great apes, steroids hormones promote brain growth and development, are important in the myelination process, explain sexual dimorphisms in brain and intelligence and improve specific cognitive abilities in humans. Supporting our hypothesis, recent studies indicate differences in neuroactive hormones metabolism between humans and non-human primates. Furthermore, a link between X chromosome genes and sex steroids may explain why the frequency of genes affecting intelligence is so high on the X chromosome. This association suggests that, during hominid evolution, there was a positive feedback in both sexes on the same genes responsible for secondary sexual character development and intelligence. This interaction leads to acceleration of development of human brain and intelligence. Finally, we propose that neuroactive hormone therapy may provide significant improvement in some cognitive deficits in all stages of human life and in cases of neurodegenerative diseases. However, further investigation is needed, mainly in the enzymatic machinery, in order to understand the direct role of these hormones in intelligence.