Humans versus apes, can the difference reside in their hormones?

Big apes share more than 90% of our genome and still we are undoubtedly very different. So what is it that gives us our unique “humanness” and higher intelligence ? In an article about to be published in the ’journal Medical Hypotheses’ a group of Portuguese researchers propose that the differences which separate apes and humans, such as brain size and intellect, can be explained by differences in thyroid and steroids hormones.

It is now well accepted that intelligence is dependent on brain size, and in fact the brain of an ape is approximately a third of the size a human’s brain. This means “factors” known to affect brain size might be involved in the different intelligence observed, not only between human and non-human primates, but also throughout human evolution. And that is the premise for the work of H.R. Correia, S.C. Balseiro and M.L. de Areia, a group pf Portuguese academics from the Department of Anthropology, University of Coimbra, Coimbra, Portugal.

The team of researchers propose that the differences observed between apes and humans, and also between different hominids, result from distinct metabolisms in thyroid and sex steroids hormones, such as testosterone (the hormone responsible for male secondary sex characteristics) and estrogens (responsible for female secondary sex characteristics).

Firstly, thyroid and sex steroids are known neuroactive hormones, meaning that they influence the development and function of the brain and so also intelligence. For example, lack of iodine – a crucial precursor for the production of thyroid hormone – results in mental retardation by affecting dopamine, one of the most important neurotransmitters in the organism and known to be crucial for abstract intelligence. On the other hand, sex steroid hormones not only affect every stage of brain growth and development, but also control several central nervous functions throughout life. Estrogens, for example, are known to protect against verbal memory decline and reduce the risk of Alzheimer’s in women after the menopause. Receptors for this hormone and progesterone (the male equivalent) are found throughout the central nervous system emphasizing the importance of sex hormones in the function of this organ.

Secondly, thyroid and sex steroids can also explain the physical changes occurring concomitantly with brain enlargement during human evolution. Both these type hormones affect body growth – and we know that body increase is an hallmark of human evolution – while sex steroids seem to be responsible for many of the physical changes acquired during this period, such as penis and breast enlargement, development of more sexual differentiated bodies, longer life span and full-time sexual receptivity. These traits also differentiate Homo sapiens from apes. Furthermore, the development of sex-differentiated brains, with women and men presenting different intellectual capabilities, such as man’s propensity to high visual-spatial ability versus women’s superior verbal skills superiority found in modern humans seems to be also the result of sex steroids.

Finally, several independent studies have shown that thyroid and sex steroids metabolism differ in humans and apes further supporting Correia, Balseiro and Areia’s proposal.

Correia, Balseiro and Areia’s work is extremely pertinent at a time when the development of subjects, such as genetics and neurophysiology, together with the recent news that chimpanzees share a striking 96% of our genome, raises yet again the question of what is this strange “Humanness” of us.

Correia, Balseiro and Areia’s hypothesis is very interesting but, as the team of researchers say itself, needs further investigation to be understood and thoroughly proved. But, if the theory is substantiated, not only will we be closer to understanding what we really are, but we will be also able to use exogenous hormones to treat several mental problems. In fact, whatever cognitive dysfunctions or neuro-degenerative diseases, neuroactive hormones might prove to be a revolutionary treatment.

Piece researched and written by: Catarina Amorim (catarina.amorim@linacre.ox.ac.uk)