The VNO Debate: Vestigial or Functional?

The Core Debate

Is the human vomeronasal organ (VNO) a functional chemosensory system, or is it an evolutionary remnant with no current purpose?

This question has profound implications for understanding human pheromone detection.

Evidence Supporting Vestigiality

The majority of scientific evidence suggests the human VNO is non-functional:

1. No Sensory Neurons

“Among studies that use microanatomical methods, there is no reported evidence that human beings have active sensory neurons like those in other animals’ working vomeronasal systems.”

Adult human VNO tissue lacks the bipolar sensory neurons that characterize functional VNOs in other mammals.

2. No Neural Connections

The human VNO shows:

• No nerve fibers connecting to the brain
• No accessory olfactory bulb (the brain region that processes VNO signals in other mammals)
• No clear pathway for signal transmission

3. Pseudogene Receptors

Genomic studies have been particularly damning:

Gene Type	Human Status
V1R genes	~200 sequences, nearly all pseudogenes
V2R genes	Completely degenerated (only pseudogenes)
TRPC2	Pseudogene (critical ion channel)

A 2001 genomic study identified 56 BAC clones carrying 34 distinct VNO receptor sequences — all were non-functional pseudogenes.

4. Developmental Regression

The human VNO follows a clear pattern:

1. Develops normally in embryos
2. Supports GnRH neuron migration
3. Regresses after this critical period
4. Neural connections disappear

5. Evolutionary Timeline

“The massive V1R pseudogenization observed in the human genome started shortly before the separation of hominoids from Old World monkeys, apparently because of the reduced importance of vomeronasal pheromone communications.”

This suggests VNO function was lost 23-25 million years ago.

Evidence Suggesting Potential Function

Despite the weight of evidence above, some researchers argue for possible VNO function:

1. Monti-Bloch Experiments

The most cited evidence for VNO function comes from Monti-Bloch and colleagues:

• Applied putative pheromones (“vomeropherins”) to human VNO
• Recorded electrophysiological responses
• Observed changes in:
  • Autonomic function
  • Pulsatile LH/FSH release
  • EEG activity

Criticism: These studies have not been independently replicated, and the methodology has been questioned.

2. Structural Presence

Some argue that the persistent presence of VNO structures in adults suggests function:

• Why would evolution maintain a useless structure?
• Some VNO pits contain nerve-like structures
• Individual variation might mask functional cases

Counter-argument: Vestigial structures can persist for millions of years without function (e.g., human tailbone).

3. Researcher Skepticism of “Null Results”

Noam Sobel, a prominent olfaction researcher, has stated:

“The assertion that humans lack functional vomeronasal organs is based on ‘weak, null results.’”

He argues that proving non-function is methodologically difficult.

The Current Scientific Consensus

A comprehensive review by Tristram Wyatt concluded:

“Most in the field are sceptical about the likelihood of a functional VNO in adult human beings on current evidence.”

The consensus position:

1. The human VNO is most likely vestigial
2. This doesn’t rule out human pheromone communication
3. Other pathways (MOE, CN0) may detect pheromones

Alternative Detection Hypothesis

Even VNO skeptics acknowledge humans may detect pheromones through:

Main Olfactory Epithelium (MOE)

• Contains functional odorant receptors
• Some can bind steroid compounds
• Connected to limbic system via main olfactory bulb

Cranial Nerve Zero

• Projects directly to reproductive brain regions
• Contains GnRH neurons
• Doesn’t require functional VNO

Direct Absorption

Compounds might enter the bloodstream via:

• Nasal capillaries
• Cribriform plate diffusion
• Skin absorption

What This Means for Pheromones

If VNO is Vestigial

• Human pheromone detection differs from rodents
• Research based on rodent VNO may not apply
• Need to study alternative human pathways
• Products may work through non-VNO mechanisms

If VNO Has Some Function

• Could explain individual variation in pheromone response
• Might be a target for optimized delivery
• Would validate some classic pheromone research

The Practical Reality

Regardless of VNO status:

1. Humans do respond to chemical signals — Multiple studies confirm behavioral/physiological changes
2. The mechanism remains unclear — VNO, MOE, CN0, or combination
3. Products may work through any pathway — The subjective effect is what matters to users

Key Takeaways

1. Scientific consensus favors VNO vestigiality
2. Key evidence: no neurons, pseudogene receptors, no neural connections
3. Some electrophysiological studies suggest possible function
4. Alternative pathways likely compensate (MOE, CN0)
5. VNO debate doesn’t determine whether pheromones “work”

References

1. Meredith M. “Human vomeronasal organ function: a critical review of best and worst cases.” Chem Senses. 2001;26(4):433-45.

2. Trotier D, et al. “The vomeronasal cavity in adult humans.” Chem Senses. 2000;25(4):369-80.

3. Zhang J, Webb DM. “Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrhine primates.” PNAS. 2003;100(14):8337-41.

4. Monti-Bloch L, Bhashkar GS. “Electrooptogram in human vomeronasal organ.” J Steroid Biochem Mol Biol. 1996;58(3):259-65.