Pheromone Detection

Humans are part of the evolutionary lineage that derived the vomeronasal organ (VNO) for pheromone detection, however many humans lack the VNO. Despite the apparent lack of a functional VNO in hominids, behavioral MRI studies demonstrate the human ability to detect pheromones, indicating humans may be recapturing an earlier evolutionary ability to detect pheromones through the olfactory system.

While the VNO plays an important functional role in socio-sexual behavior in all New World primates studied to date, studies are conflicted as the presence or complete lack of existance of VNO in hominid species. It appears the functional and anatomical differences between chimpanzee and human VNO may be insignificant, and is likely that any differences between retained VNO anatomy in humans and non-humans are variations arising from relaxed selection pressure. Anatomic similarities in the VNO between humans and non-human primates are either due to the plesiomorphic nature of the VNO given a shared ancestry with old world primates, or synapomorphy from a derived difference occuring in a human and non-human common ancestor.

Although both old and new world primates can perceive pheromones, it is unclear how apes do so. Anatomical loss of the VNO appears to be ongoing in all ape species thus studied, thus hominid perception of pheromones may be through re-appropriated epithelial/olfactory systems. Any functional decline in pheromone detection probably occurred around the time old world primates split from new world primates and may be due to hominin reliance on other sensory systems. Interestingly, expression of the TRPC2 gene occurs only in the VNO, is essential for VNO function, and is a pseudogene in humans. By tracing the evolution of TRPC2 in 15 extant primate species, one study concluded the VNO became vestigal in a common ancestor of Old World monkeys and apes, indicating signalling by pheromones was replaced in importance by development of trichromatic vision.