Once, a very long time ago, before primates began to diversify themselves into their own unique phylogenetic branch, they too used pheromones much like all other terrestrial vertebrates. However, somewhere along the line, selective pressure decided that most of these creatures wouldn't find as much of a need for pheromones as they did in the past. Here I will walk you though a brief history of where, when, and why primates began to lose this ability.
Primates: Prosimians, New World, Old World, and Apes
First, a quick introduction to primates. There are four major classifications:
Prosimians are the first of this group and are considered the most primitive classification within the order. They include animals such as lemurs, lorises, Aye-ayes, bushbabies and tarsiers. Due to their primitive nature, they are believed to have retained many characteristics to the common ancestor of primates.
New World monkeys, also known as platyrrhines, are slightly more advanced than prosimians, but divergent from higher primate lineage. This classification encompasses howler monkeys, squirrel monkeys, marmosets, owl monkeys, ands uakaris. In general they are small to mid-sized primates, ranging from 14 to 70 centimeters in height and 1.2 to 15 kilograms. Their easiest distinguishing markers are their noses and their tails. New World monkeys are the only ones that have prehensile tails, and they all have flat noses with side-facing nostrils. Another important characteristic is their lack of color vision, which, as will be discussed later, may play a role in the evolution of pheromones.
Old World primates, the catarrhines, are the some of the more familiar species, and include the baboon, macaque, colobus, and the langur. They are typically larger than the New World monkeys, ranging from 0.7 to 50 kilograms in weight and 34 to 70 centimeters in length. Most Old World monkeys still have their tails, distinguishing them from apes, but do not have the same prehensile abilities that New World monkeys do.
The last group is the apes, the most advanced group within the primate classification. However, this group branches further into two subgroups: the lesser apes, which include the gibbon and the siamang, and the great apes, which are the orangutans, gorillas, chimpanzees, and, as you may well guess, humans. The lesser apes are all medium sized, the largest weighing up to 14 kilograms. Their main distinctive characteristic is their long arms, which are excellent for swinging through trees. Large apes are much heavier, ranging from 40 kilograms to whatever ridiculous sizes that humans manage to gorge themselves to. Apes are subjects of many cognition studies as they are generally highly intelligent species.
The Vomeronasal Organ (VNO)
This is the structure that is responsible for the perception of pheromones in mammals. This structure is nestled in right next to the nasal cavity, though for the most part it is cut off in its own separate area by bone or cartilage, with a small opening to allow the pheromones to enter. Though it is present in most mammals, its presence in humans is currently under great debate; though there is evidence of a VNO-type structure in fetal development, it quickly degenerates and becomes vestigial by adulthood. It is also vestigial in other Old World primates, and there is no thick sensory epithelium that one would expect to see for pheromone detection. New World primates, however, do still have a VNO.
There are two types of neuronal receptors in the VNO: V1R and V2R. These are transmembrane structures connected to G proteins, which translate a chemical signal from outside of the cell into a physical change inside. These structures respond to the presence of pheromones and, ultimately, will cause sexual or social behavior changes once the signal reaches the brain. These receptors have been the primary location for genetic research concerning the role and structure of pheromone systems, as obstruction of these receptors in mice severely decreases sexual and social behaviors.
Genetic differences account for the different roles that the VNO plays within separate species. These differences come about due to genetic mutations and selective pressure. There is evidence for two different types of genetic mutations within the VNO: purifying selection (the selective deletions of genetic code) and positive selection (increasing the frequency of certain genetic coding).
Purifying selection is what one would expect to see in the case where a gene is being kept a system, and unnecessary data is being eliminated so the function of the gene can be maintained. Most codons (genetic code which are used to create proteins) are actually under purifying selection most of the time due to the many mutations that are constantly occurring; these mutations, which are usually deleterious, are thusly removed.
Positive selection occurs when a new mutation that is more advantageous than the current trait comes into the genetic coding and is favored immediately, increasing the frequency at which it occurs in DNA. Some researchers have found cases where there is positive selection within the VNO in humans, which implies that there may still be pheromone activity within primates that we currently do not understand.
Selective pressure is what most people understand evolution to be. If a certain mutation is beneficial to a species, then it will be more likely to be passed on. A trait can be selected above the others as it is giving more benefits, or it can be ignored as it isn't doing much for the animal anymore.
In the next few sections I will walk you through some of the research that has been done on the primate VNO and explain the theories that have been proposed surrounding this research. Nothing here is absolutely conclusive yet as researchers are still attempting to collect data. Not much is known about the VNO or its role within primates as of yet, and researchers are still trying to understand the full genetic sequence that is involved in VNO function.
Independent Evolution of the V1R Genes
Mundy and Cook examined the V1RL genes (those that are associated with the five putative receptors in humans) in13 species of non-human primates and determined that the gene formation occurred independently along the different evolutionary lines of between the great apes and New World monkeys. Six of the tested species were found to have an open reading frame in the tested sequence, meaning that this gene may still be functional in these animals. These species are gorillas, pygmy marmosets, three species of howler monkey, and humans. The other eight species clearly had pseudogenes, as genetic data was deleted or missing in the gene frame, or there were extra stop codons added into the sequence.
In an attempt to learn more about the evolution of the V1R receptor family, Giorgi and Rouquier focused on the marmoset. The marmosets are New World monkeys and do have an intact VNO. The social effects of pheromones, including recognition of group members and their reproductive status, have been well documented. However, when compared to the rodent V1R genetic code, there were only five pseudogenes identified and no actual functional genes.
Giorgi and Rouquier used the same techniques to examine the variants of this gene in other primate species. However, they found that they were only able to amplify the sequences in the gorilla, chimpanzee and the orangutan, which means that the other species tested were probably too different in their genetic structure to be analyzed using the same procedures.
Within their study Mundy and Cook found evidence for both purifying and positive selection on the V1RL genes. When examining the V1RL open reading frame, they found that there is evidence for purifying selection in primate V1RLs when all receptor sites are treated equally. However, it appears that recently (in evolutionary terms) there has been a relaxation of selective pressure, implying that in some species of New World monkeys may only have a vestigial VNO. Yet there is also evidence for positive selection in certain sections of the primate V1RL. For example, one codon position has a greater than 95% chance of being under positive selection. Its location is particularly interesting, as well, as it is predicted that it could interact with odorant ligands (which bind with receptor sites on cells to start a chemical signal). This would be consistent with the ability to detect scents as pheromones; the presence of positive selection on this codon means that there could, in fact, still be latent abilities within primates to detect species-specific pheromones. There is also a chance that the V1RL1 function in humans could be diversifying, as there is significant polymorphism on this gene.
This is interesting when considering how closely related some of these species are, yet how different the role of V1RL receptors in each system. For example, the V1R gene is the only gene that has known expression in humans, yet in the chimpanzee, our closest relative, it is a pseudogene, and it has an open reading frame in the gorilla. This implies that each species went through separate evolution for pheromones and that they all faced different selective pressure.
Despite the fact that humans have an open reading frame, implying that there may be some lingering function that we currently can't detect, it is generally accepted as unlikely that we would have retained this function while the chimpanzees and orangutans lost this function completely. Instead, it is hypothesized that the V1R receptor set was originally used in the VNO but has since evolved to be a purely odorant receptor, perhaps still maintaining pheromone-like ligand properties. This is supported by the concurrent regression of the VNO and the evolution of pseudogenes; both seem to have happened around the same time.
However, it is also possible that the genetic sequences that are being used as a comparison, the mouse V1R genes, are too different from those exhibited within primates due to the different selective pressures each species faces. We know that the marmoset uses pheromones in a species-specific manner, yet with the current methods researchers were unable to detect any VR genes. Therefore there may be pheromone receptors in primates that we haven't found yet simply because we're looking for the wrong genetic code. There is much greater likelihood of overlapping in genetic coding of species that are closely related than those that are very different, so perhaps when we find the genetic sequencing for a primate species such as the marmoset, we will be able to detect genes (whether functional or not) in humans and other primates.
While considering TRP2, an ion channel that has been proven to be particularly important in the VNO pathway, researchers recently discovered premature genetic code that causes the close of genetic translation (known as a codon). This same code is found in all hominids and Old Word monkeys (except for the orangutan). It has since been nicknamed "Adam" as it is speculated to be the first mutation within the gene that caused it to become obsolete. Since then many mutations have followed, but this one allowed all of the others to follow along.
Adam first appeared 23 million years ago in a common ancestor of both hominids and Old World primates. Adam is probably the only stop codon that appeared in the genetic code before hominids and Old World monkeys diverged, signaling the beginning of the elimination of the VNO. After, several species-specific mutations occurred, accumulating for over 23 million years. However, since Adam is lacking in the orangutan genetic code, it is speculated that the mutation was lost within this species, most likely after it separated from the African Apes. Instead, the orangutans have their own stop codon just a little farther up on the gene.
Rhesus Monkey Sex Attractants
Michael, Keverne, and Bonsall were able to determine that, to some extent, reproductive behavior in Rhesus monkeys (Old World monkeys) is influenced by olfaction. They found that when vaginally dosed with a small amount of estrogen, ovariectomized rhesus monkeys were more attractive to their male partners (as determined by the frequency of mounting behavior). However, when the males had their noses plugged, making it so that they could not smell, they no longer showed the same level of attraction to the females, implying that olfaction definitely plays a part in the sexual and social interactions of Rhesus monkeys.
In order to isolate the chemical attributes of the secretions produced by the female, the researchers compared fluids collected from five untreated ovariectomized females to that of five ovariectomized females treated with estradiol, the predominant sex hormone in females. They found that the amount of volatile components were eight times greater in the females who had been treated as compared to those who hadn't. After confining a section of this fragrance from donor females and applying it to four other females, the researchers noticed and instant increase in the amount of sexual behaviors exhibited by the male partners, even though the females were unreceptive to their advances.
Further examining this volatile section of the secretions, they were able to identify several components of the fluids and the relative amounts. From this they created a synthetic mixture, designed to match in concentration and composition. When applied to the females in the same manner as the donor secretions, they found the same behavioral patterns in the males, which suggests that Michael, Keverne and Bonsall have found the composition of pheromones in Rhesus monkeys.
Possible Reasons for Pheromone Elimination
One popular speculative reason for the reduction of the role of pheromones in primate social interactions is the introduction of tri-color vision. With this development, males could now see the red sexual swelling of the female's genetalia and could better sense her sexual readiness. If this method of detection were more successful than pheromones, then this system would be preferred, dominant, and become widespread. As most of us enjoy color vision today, it's apparent that this method of sexual recognition is a very effective one. And, when something better comes along, it's typical to try and replace it; in this case, the VNO has been replaced by color vision. The evolution of tri-color vision in primates occurred at approximately the same time as the genetic mutations that began to disintegrate the VNO.