Phylogenetic analyses reveal that at least three distinct olfactory subsystems are broadly consistent in vertebrates, and a fourth accessory system (vomeronasal) solely arose in tetrapods.
Serving as living descendants that thrived almost half a billion years ago, the study of OR genes in lampreys also provides deep insight into the origins of vertebrate olfaction.
One such species, Latimeria chalumnae, holds particular interest since they contain Class II OR genes that are present in mammals and amphibians but are absent in fish.
[26] The skull of the extinct Hadrocodium wui, which are considered reptiles that evolved into the first mammals, has unveiled significant implications of the reptilian olfactory transition.
[34] The nasal concha, or turbinates, is composed of little bones and soft tissue that provide structure to the nose and aid in the perception of smell.
[34] The skull of Brasilitherium held more promising results, as it contained a secondary palate separating the nose from the mouth, thereby enhancing skeletal durability and preservation of the turbinate structure.
It was inferred that these structures found in Brasilitherium performed the same roles in modern mammals; specifically, the anterior overlying tissue warmed incoming odorants and the posterior portion was responsible for picking up scent.
In most species with a rhinarium, the organ takes the form of a firm pad with dual nostrils, allowing for the processing of both olfactory and tactile information.
[23] Smell is often viewed as a mitigated special sense during the emergence of higher neural function, and correspondingly, olfaction has been increasingly reduced throughout the course of primate evolution.
The disparity is explained by the development of acute vision in Catarrhini (apes and Old World monkeys) 40 million years ago, namely during the period when the Earth became cooler.
Trichromatic vision was evolved to enhance long-distance perception and foraging for ripe fruit, reducing the selective advantage of possessing a large OR gene repertoire.
[23] Contrarily, New World howler monkeys (Alouatta) have independently evolved routine trichromatic vision, yet they still perform pheromone communication and do not exhibit reduced olfactory capabilities.
It is suggested that although enhanced vision relaxed the selection of sensitive smell, it did not render olfaction as an unneeded trait, and may have been advantageous in a jungle habitat.
[23] Ample evidence suggests that olfactory social behaviours, such as sniffing and scent marking, are heavily involved in communicative interactions among primate species.
[23] This theory was further elaborated by Le Gros Clark, asserting the reduction of olfactory structures was attributed to the diminished need for smell in arboreal environments.
[38] This theory was later challenged with the existence of arboreal mammals that do not exhibit primate traits that are considered "adaptive" (specifically, reduced olfaction), yet they are still successful within their respective environment.
[citation needed] Aside from an external nose structure, some primates contain a vomeronasal organ to detect odorants of higher molecular weight.
[23] Genomic analysis has asserted that vomeronasal organ receptors became impaired approximately 23 million years ago in primate evolution, before the advent separation of Old World monkeys and hominoids.
Emergence of such a structure mainly derives from respiratory needs in varying climates; for instance, a large nasal cavity in Neanderthals adjusted for the cold environment and low humidity of that epoch.
As such, the human nose displays reduced innervations of the olfactory mucous membrane, decreased snout length, and an overall reduction of complexity of the nasal concha.
[citation needed] Humans have a high interindividual variation in pseudogenes and OR genes which research attributes to geographical and cultural separation.
[47] A weak positive selection acting on human nucleotide diversity is proposed because of a report that observed genomic segments in a 450kb cluster of olfactory genes found on chromosome 17.
These differences are proposed to help with future genotype-phenotype studies such as evaluating the effect of genetic drift on these populations and finding greater functionality in pygmyolfactory receptor genes and pseudogenization.
[1] Assuming that various OR subfamilies bind to different odorant classes, it is likely that humans are able to detect a wide range of smell similarly to mice.
[49] Olfactory senses are generally more heightened for a terrestrial species than for an aquatic, as airborne volatiles are more important to detect than water-soluble scents in land animals.
Support for this theory is exemplified through the fish-to-tetrapod transition, where animals began to populate terrestrial niches and a tremendous expansion of the olfactory system can be observed.
Adaptation allows for the olfactory system to appropriately respond to the appearance of novel scents or changes, yet it also maintains equilibrium with odorant concentrations in ambient environments.
[53] Support for the participation of both central and peripheral systems derives from experiments showing that monorhinal stimulation results in both ipsilateral and contralateral adaptation.
[51] Psychophysical support for this theory draws from studies that have reported relatively small decreases in peripheral response after repeated stimulation despite significant reductions in behaviorally perceived intensity.
Consequently, the degree of adaptation may rely on differences in odorant clearance among species, which would include properties of nasal mucociliary, submucosal blood flow, and expiratory desorption.