I included additional details on olfaction, the MHC, and mate-choice in a recent review. Simply put, our sense of smell is the only sense that directly, albeit unconsciously, influences us to choose both for sexually dimorphic (male/female) traits, and for genetic similarity/diversity. Culturally restricted female choice, or lack of restriction ("free" choice), explains some aspects of choice for genetic similarity/diversity when human populations are compared. More technically and across species:

“Barran and colleagues (2005) have reviewed the evolutionary significance of the GnRH [Gonadotropin releasing hormone] molecule and receptor signaling from protochordates to vertebrates. Though its evolutionary origin is in unicellular organisms, GnRH became the biological core of vertebrate reproduction. The mammalian form of GnRH, which is found in humans, has a wide distribution in vertebrate species. This form of GnRH can be detected in primitive bony fish, but not in species that are thought to have evolved earlier. Thus, the human form of GnRH arose some 400 million years ago (Sherwood, Lovejoy, & Coe, 1993), although its evolutionary origins date back much further, to unicellular organisms like yeast.”

“Concisely put, the across-species conservation of a [GnRH-directed] neuroendocrine chemical communication system and an immune chemical communication system [facilitated by diversification of the GnRH receptor] enable genetic diversity in many organisms that functionally distinguish between self and non-self. This duality in the conservation of neuroendocrine system and immune system chemical communication enables mammals to respond to immunological non-self signals with a GnRH-directed LH (i.e., neuroendocrine) response, which facilitates mammalian sexual reproduction [including human sexual reproduction].” (Kohl, 2006)


Kohl, J. V. (2006). The Mind's Eyes: Human Pheromones, Neuroscience, and Male Sexual Preferences. Journal of Psychology & Human Sexuality, 18(4), 313-369.